[PATCH] dma-buf: heaps: Add Coherent heap to dmabuf heaps

[PATCH] dma-buf: heaps: Add Coherent heap to dmabuf heaps

[Albert Esteve]

Add a dma-buf heap for DT coherent reserved-memory
(i.e., 'shared-dma-pool' without 'reusable' property),
exposing one heap per region for userspace buffers.

The heap binds a synthetic platform device to each region
so coherent allocations use the correct dev->dma_mem,
and it defers registration until late_initcall when
normal allocator are available.

This patch includes charging of the coherent heap
allocator to the dmem cgroup.

Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
This patch introduces a new driver to expose DT coherent reserved-memory
regions as dma-buf heaps, allowing userspace buffers to be created.

Since these regions are device-dependent, we bind a synthetic platform
device to each region so coherent allocations use the correct dev->dma_mem.

Following Eric’s [1] and Maxime’s [2] work on charging DMA buffers
allocated from userspace to cgroups (dmem), this patch adds the same
charging pattern used by CMA heaps patch. Charging is done only through
the dma-buf heap interface so it can be attributed to a userspace allocator.

This allows each device-specific reserved-memory region to enforce its
own limits.

[1] https://lore.kernel.org/all/20260218-dmabuf-heap-cma-dmem-v2-0-b249886fb7b2@redhat.com/
[2] https://lore.kernel.org/all/20250310-dmem-cgroups-v1-0-2984c1bc9312@kernel.org/
---
 drivers/dma-buf/heaps/Kconfig         |  17 ++
 drivers/dma-buf/heaps/Makefile        |   1 +
 drivers/dma-buf/heaps/coherent_heap.c | 485 ++++++++++++++++++++++++++++++++++
 include/linux/dma-heap.h              |  11 +
 kernel/dma/coherent.c                 |   9 +
 5 files changed, 523 insertions(+)

diff --git a/drivers/dma-buf/heaps/Kconfig b/drivers/dma-buf/heaps/Kconfig
index a5eef06c42264..93765dca164e3 100644
--- a/drivers/dma-buf/heaps/Kconfig
+++ b/drivers/dma-buf/heaps/Kconfig
@@ -12,3 +12,20 @@ config DMABUF_HEAPS_CMA
 	  Choose this option to enable dma-buf CMA heap. This heap is backed
 	  by the Contiguous Memory Allocator (CMA). If your system has these
 	  regions, you should say Y here.
+
+config DMABUF_HEAPS_COHERENT
+	bool "DMA-BUF Coherent Reserved-Memory Heap"
+	depends on DMABUF_HEAPS && OF_RESERVED_MEM && DMA_DECLARE_COHERENT
+	help
+	  Choose this option to enable coherent reserved-memory dma-buf heaps.
+	  This heap is backed by non-reusable DT "shared-dma-pool" regions.
+	  If your system defines coherent reserved-memory regions, you should
+	  say Y here.
+
+config COHERENT_AREAS_DEFERRED
+	int "Max deferred coherent reserved-memory regions"
+	depends on DMABUF_HEAPS_COHERENT
+	default 16
+	help
+	  Maximum number of coherent reserved-memory regions that can be
+	  deferred for later registration during early boot.
diff --git a/drivers/dma-buf/heaps/Makefile b/drivers/dma-buf/heaps/Makefile
index 974467791032f..96bda7a65f041 100644
--- a/drivers/dma-buf/heaps/Makefile
+++ b/drivers/dma-buf/heaps/Makefile
@@ -1,3 +1,4 @@
 # SPDX-License-Identifier: GPL-2.0
 obj-$(CONFIG_DMABUF_HEAPS_SYSTEM)	+= system_heap.o
 obj-$(CONFIG_DMABUF_HEAPS_CMA)		+= cma_heap.o
+obj-$(CONFIG_DMABUF_HEAPS_COHERENT)	+= coherent_heap.o
diff --git a/drivers/dma-buf/heaps/coherent_heap.c b/drivers/dma-buf/heaps/coherent_heap.c
new file mode 100644
index 0000000000000..870b2b89aefcb
--- /dev/null
+++ b/drivers/dma-buf/heaps/coherent_heap.c
@@ -0,0 +1,485 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * DMABUF heap for coherent reserved-memory regions
+ *
+ * Copyright (C) 2026 Red Hat, Inc.
+ * Author: Albert Esteve <aesteve@redhat.com>
+ *
+ */
+
+#include <linux/cgroup_dmem.h>
+#include <linux/dma-heap.h>
+#include <linux/dma-buf.h>
+#include <linux/dma-mapping.h>
+#include <linux/err.h>
+#include <linux/highmem.h>
+#include <linux/iosys-map.h>
+#include <linux/of_reserved_mem.h>
+#include <linux/platform_device.h>
+#include <linux/scatterlist.h>
+#include <linux/slab.h>
+#include <linux/vmalloc.h>
+
+#define DEFERRED_AREAS_MAX CONFIG_COHERENT_AREAS_DEFERRED
+
+/*
+ * Early init can't use normal memory management yet (memblock is used
+ * instead), so keep a small deferred list and retry at late_initcall.
+ */
+static struct reserved_mem *rmem_areas_deferred[DEFERRED_AREAS_MAX];
+static unsigned int rmem_areas_deferred_num;
+
+static int coherent_heap_add_deferred(struct reserved_mem *rmem)
+{
+	if (rmem_areas_deferred_num >= DEFERRED_AREAS_MAX) {
+		pr_warn("Deferred heap areas list full, dropping %s\n",
+			rmem->name ? rmem->name : "unknown");
+		return -EINVAL;
+	}
+	rmem_areas_deferred[rmem_areas_deferred_num++] = rmem;
+	return 0;
+}
+
+struct coherent_heap {
+	struct dma_heap *heap;
+	struct reserved_mem *rmem;
+	char *name;
+	struct device *dev;
+	struct platform_device *pdev;
+#if IS_ENABLED(CONFIG_CGROUP_DMEM)
+	struct dmem_cgroup_region *cg;
+#endif
+};
+
+struct coherent_heap_buffer {
+	struct coherent_heap *heap;
+	struct list_head attachments;
+	struct mutex lock;
+	unsigned long len;
+	dma_addr_t dma_addr;
+	void *alloc_vaddr;
+	struct page **pages;
+	pgoff_t pagecount;
+	int vmap_cnt;
+	void *vaddr;
+#if IS_ENABLED(CONFIG_CGROUP_DMEM)
+	struct dmem_cgroup_pool_state *pool;
+#endif
+};
+
+struct dma_heap_attachment {
+	struct device *dev;
+	struct sg_table table;
+	struct list_head list;
+	bool mapped;
+};
+
+static int coherent_heap_attach(struct dma_buf *dmabuf,
+				struct dma_buf_attachment *attachment)
+{
+	struct coherent_heap_buffer *buffer = dmabuf->priv;
+	struct dma_heap_attachment *a;
+	int ret;
+
+	a = kzalloc_obj(*a);
+	if (!a)
+		return -ENOMEM;
+
+	ret = sg_alloc_table_from_pages(&a->table, buffer->pages,
+					buffer->pagecount, 0,
+					buffer->pagecount << PAGE_SHIFT,
+					GFP_KERNEL);
+	if (ret) {
+		kfree(a);
+		return ret;
+	}
+
+	a->dev = attachment->dev;
+	INIT_LIST_HEAD(&a->list);
+	a->mapped = false;
+
+	attachment->priv = a;
+
+	mutex_lock(&buffer->lock);
+	list_add(&a->list, &buffer->attachments);
+	mutex_unlock(&buffer->lock);
+
+	return 0;
+}
+
+static void coherent_heap_detach(struct dma_buf *dmabuf,
+				 struct dma_buf_attachment *attachment)
+{
+	struct coherent_heap_buffer *buffer = dmabuf->priv;
+	struct dma_heap_attachment *a = attachment->priv;
+
+	mutex_lock(&buffer->lock);
+	list_del(&a->list);
+	mutex_unlock(&buffer->lock);
+
+	sg_free_table(&a->table);
+	kfree(a);
+}
+
+static struct sg_table *coherent_heap_map_dma_buf(struct dma_buf_attachment *attachment,
+						  enum dma_data_direction direction)
+{
+	struct dma_heap_attachment *a = attachment->priv;
+	struct sg_table *table = &a->table;
+	int ret;
+
+	ret = dma_map_sgtable(attachment->dev, table, direction, 0);
+	if (ret)
+		return ERR_PTR(-ENOMEM);
+	a->mapped = true;
+
+	return table;
+}
+
+static void coherent_heap_unmap_dma_buf(struct dma_buf_attachment *attachment,
+					struct sg_table *table,
+					enum dma_data_direction direction)
+{
+	struct dma_heap_attachment *a = attachment->priv;
+
+	a->mapped = false;
+	dma_unmap_sgtable(attachment->dev, table, direction, 0);
+}
+
+static int coherent_heap_dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
+						  enum dma_data_direction direction)
+{
+	struct coherent_heap_buffer *buffer = dmabuf->priv;
+	struct dma_heap_attachment *a;
+
+	mutex_lock(&buffer->lock);
+	if (buffer->vmap_cnt)
+		invalidate_kernel_vmap_range(buffer->vaddr, buffer->len);
+
+	list_for_each_entry(a, &buffer->attachments, list) {
+		if (!a->mapped)
+			continue;
+		dma_sync_sgtable_for_cpu(a->dev, &a->table, direction);
+	}
+	mutex_unlock(&buffer->lock);
+
+	return 0;
+}
+
+static int coherent_heap_dma_buf_end_cpu_access(struct dma_buf *dmabuf,
+						enum dma_data_direction direction)
+{
+	struct coherent_heap_buffer *buffer = dmabuf->priv;
+	struct dma_heap_attachment *a;
+
+	mutex_lock(&buffer->lock);
+	if (buffer->vmap_cnt)
+		flush_kernel_vmap_range(buffer->vaddr, buffer->len);
+
+	list_for_each_entry(a, &buffer->attachments, list) {
+		if (!a->mapped)
+			continue;
+		dma_sync_sgtable_for_device(a->dev, &a->table, direction);
+	}
+	mutex_unlock(&buffer->lock);
+
+	return 0;
+}
+
+static int coherent_heap_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma)
+{
+	struct coherent_heap_buffer *buffer = dmabuf->priv;
+	struct coherent_heap *coh_heap = buffer->heap;
+
+	return dma_mmap_coherent(coh_heap->dev, vma, buffer->alloc_vaddr,
+				 buffer->dma_addr, buffer->len);
+}
+
+static void *coherent_heap_do_vmap(struct coherent_heap_buffer *buffer)
+{
+	void *vaddr;
+
+	vaddr = vmap(buffer->pages, buffer->pagecount, VM_MAP, PAGE_KERNEL);
+	if (!vaddr)
+		return ERR_PTR(-ENOMEM);
+
+	return vaddr;
+}
+
+static int coherent_heap_vmap(struct dma_buf *dmabuf, struct iosys_map *map)
+{
+	struct coherent_heap_buffer *buffer = dmabuf->priv;
+	void *vaddr;
+	int ret = 0;
+
+	mutex_lock(&buffer->lock);
+	if (buffer->vmap_cnt) {
+		buffer->vmap_cnt++;
+		iosys_map_set_vaddr(map, buffer->vaddr);
+		goto out;
+	}
+
+	vaddr = coherent_heap_do_vmap(buffer);
+	if (IS_ERR(vaddr)) {
+		ret = PTR_ERR(vaddr);
+		goto out;
+	}
+
+	buffer->vaddr = vaddr;
+	buffer->vmap_cnt++;
+	iosys_map_set_vaddr(map, buffer->vaddr);
+out:
+	mutex_unlock(&buffer->lock);
+
+	return ret;
+}
+
+static void coherent_heap_vunmap(struct dma_buf *dmabuf, struct iosys_map *map)
+{
+	struct coherent_heap_buffer *buffer = dmabuf->priv;
+
+	mutex_lock(&buffer->lock);
+	if (!--buffer->vmap_cnt) {
+		vunmap(buffer->vaddr);
+		buffer->vaddr = NULL;
+	}
+	mutex_unlock(&buffer->lock);
+	iosys_map_clear(map);
+}
+
+static void coherent_heap_dma_buf_release(struct dma_buf *dmabuf)
+{
+	struct coherent_heap_buffer *buffer = dmabuf->priv;
+	struct coherent_heap *coh_heap = buffer->heap;
+
+	if (buffer->vmap_cnt > 0) {
+		WARN(1, "%s: buffer still mapped in the kernel\n", __func__);
+		vunmap(buffer->vaddr);
+		buffer->vaddr = NULL;
+		buffer->vmap_cnt = 0;
+	}
+
+	if (buffer->alloc_vaddr)
+		dma_free_coherent(coh_heap->dev, buffer->len, buffer->alloc_vaddr,
+			       buffer->dma_addr);
+	kfree(buffer->pages);
+#if IS_ENABLED(CONFIG_CGROUP_DMEM)
+	dmem_cgroup_uncharge(buffer->pool, buffer->len);
+#endif
+	kfree(buffer);
+}
+
+static const struct dma_buf_ops coherent_heap_buf_ops = {
+	.attach = coherent_heap_attach,
+	.detach = coherent_heap_detach,
+	.map_dma_buf = coherent_heap_map_dma_buf,
+	.unmap_dma_buf = coherent_heap_unmap_dma_buf,
+	.begin_cpu_access = coherent_heap_dma_buf_begin_cpu_access,
+	.end_cpu_access = coherent_heap_dma_buf_end_cpu_access,
+	.mmap = coherent_heap_mmap,
+	.vmap = coherent_heap_vmap,
+	.vunmap = coherent_heap_vunmap,
+	.release = coherent_heap_dma_buf_release,
+};
+
+static struct dma_buf *coherent_heap_allocate(struct dma_heap *heap,
+					      unsigned long len,
+					      u32 fd_flags,
+					      u64 heap_flags)
+{
+	struct coherent_heap *coh_heap;
+	struct coherent_heap_buffer *buffer;
+	DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
+	size_t size = PAGE_ALIGN(len);
+	pgoff_t pagecount = size >> PAGE_SHIFT;
+	struct dma_buf *dmabuf;
+	int ret = -ENOMEM;
+	pgoff_t pg;
+
+	coh_heap = dma_heap_get_drvdata(heap);
+	if (!coh_heap)
+		return ERR_PTR(-EINVAL);
+	if (!coh_heap->dev)
+		return ERR_PTR(-ENODEV);
+
+	buffer = kzalloc_obj(*buffer);
+	if (!buffer)
+		return ERR_PTR(-ENOMEM);
+
+	INIT_LIST_HEAD(&buffer->attachments);
+	mutex_init(&buffer->lock);
+	buffer->len = size;
+	buffer->heap = coh_heap;
+	buffer->pagecount = pagecount;
+
+#if IS_ENABLED(CONFIG_CGROUP_DMEM)
+	if (mem_accounting) {
+		ret = dmem_cgroup_try_charge(coh_heap->cg, size,
+					     &buffer->pool, NULL);
+		if (ret)
+			goto free_buffer;
+	}
+#endif
+
+	buffer->alloc_vaddr = dma_alloc_coherent(coh_heap->dev, buffer->len,
+						 &buffer->dma_addr, GFP_KERNEL);
+	if (!buffer->alloc_vaddr) {
+		ret = -ENOMEM;
+#if IS_ENABLED(CONFIG_CGROUP_DMEM)
+		goto uncharge_cgroup;
+#else
+		goto free_buffer;
+#endif
+	}
+
+	buffer->pages = kmalloc_array(pagecount, sizeof(*buffer->pages),
+				      GFP_KERNEL);
+	if (!buffer->pages) {
+		ret = -ENOMEM;
+		goto free_dma;
+	}
+
+	for (pg = 0; pg < pagecount; pg++)
+		buffer->pages[pg] = virt_to_page((char *)buffer->alloc_vaddr +
+						 (pg * PAGE_SIZE));
+
+	/* create the dmabuf */
+	exp_info.exp_name = dma_heap_get_name(heap);
+	exp_info.ops = &coherent_heap_buf_ops;
+	exp_info.size = buffer->len;
+	exp_info.flags = fd_flags;
+	exp_info.priv = buffer;
+	dmabuf = dma_buf_export(&exp_info);
+	if (IS_ERR(dmabuf)) {
+		ret = PTR_ERR(dmabuf);
+		goto free_pages;
+	}
+	return dmabuf;
+
+free_pages:
+	kfree(buffer->pages);
+free_dma:
+	dma_free_coherent(coh_heap->dev, buffer->len, buffer->alloc_vaddr,
+			  buffer->dma_addr);
+#if IS_ENABLED(CONFIG_CGROUP_DMEM)
+uncharge_cgroup:
+	dmem_cgroup_uncharge(buffer->pool, size);
+#endif
+free_buffer:
+	kfree(buffer);
+	return ERR_PTR(ret);
+}
+
+static const struct dma_heap_ops coherent_heap_ops = {
+	.allocate = coherent_heap_allocate,
+};
+
+static int __coherent_heap_register(struct reserved_mem *rmem)
+{
+	struct dma_heap_export_info exp_info;
+	struct coherent_heap *coh_heap;
+#if IS_ENABLED(CONFIG_CGROUP_DMEM)
+	struct dmem_cgroup_region *region;
+#endif
+	const char *rmem_name;
+	int ret;
+
+	if (!rmem)
+		return -EINVAL;
+
+	rmem_name = rmem->name ? rmem->name : "unknown";
+
+	coh_heap = kzalloc_obj(*coh_heap);
+	if (!coh_heap)
+		return -ENOMEM;
+
+	coh_heap->name = kasprintf(GFP_KERNEL, "coherent_%s", rmem_name);
+	if (!coh_heap->name) {
+		ret = -ENOMEM;
+		goto free_coherent_heap;
+	}
+
+	coh_heap->rmem = rmem;
+
+	/* create a platform device per rmem and bind it */
+	coh_heap->pdev = platform_device_register_simple("coherent-heap",
+							 PLATFORM_DEVID_AUTO,
+							 NULL, 0);
+	if (IS_ERR(coh_heap->pdev)) {
+		ret = PTR_ERR(coh_heap->pdev);
+		goto free_name;
+	}
+
+	if (rmem->ops && rmem->ops->device_init) {
+		ret = rmem->ops->device_init(rmem, &coh_heap->pdev->dev);
+		if (ret)
+			goto pdev_unregister;
+	}
+
+	coh_heap->dev = &coh_heap->pdev->dev;
+#if IS_ENABLED(CONFIG_CGROUP_DMEM)
+	region = dmem_cgroup_register_region(rmem->size, "coh/%s", rmem_name);
+	if (IS_ERR(region)) {
+		ret = PTR_ERR(region);
+		goto pdev_unregister;
+	}
+	coh_heap->cg = region;
+#endif
+
+	exp_info.name = coh_heap->name;
+	exp_info.ops = &coherent_heap_ops;
+	exp_info.priv = coh_heap;
+
+	coh_heap->heap = dma_heap_add(&exp_info);
+	if (IS_ERR(coh_heap->heap)) {
+		ret = PTR_ERR(coh_heap->heap);
+		goto cg_unregister;
+	}
+
+	return 0;
+
+cg_unregister:
+#if IS_ENABLED(CONFIG_CGROUP_DMEM)
+	dmem_cgroup_unregister_region(coh_heap->cg);
+#endif
+pdev_unregister:
+	platform_device_unregister(coh_heap->pdev);
+	coh_heap->pdev = NULL;
+free_name:
+	kfree(coh_heap->name);
+free_coherent_heap:
+	kfree(coh_heap);
+
+	return ret;
+}
+
+int dma_heap_coherent_register(struct reserved_mem *rmem)
+{
+	int ret;
+
+	ret = __coherent_heap_register(rmem);
+	if (ret == -ENOMEM)
+		return coherent_heap_add_deferred(rmem);
+	return ret;
+}
+
+static int __init coherent_heap_register_deferred(void)
+{
+	unsigned int i;
+	int ret;
+
+	for (i = 0; i < rmem_areas_deferred_num; i++) {
+		struct reserved_mem *rmem = rmem_areas_deferred[i];
+
+		ret = __coherent_heap_register(rmem);
+		if (ret) {
+			pr_warn("Failed to add coherent heap %s",
+				rmem->name ? rmem->name : "unknown");
+			continue;
+		}
+	}
+
+	return 0;
+}
+late_initcall(coherent_heap_register_deferred);
+MODULE_DESCRIPTION("DMA-BUF heap for coherent reserved-memory regions");
diff --git a/include/linux/dma-heap.h b/include/linux/dma-heap.h
index 648328a64b27e..e894cfa1ecf1a 100644
--- a/include/linux/dma-heap.h
+++ b/include/linux/dma-heap.h
@@ -9,9 +9,11 @@
 #ifndef _DMA_HEAPS_H
 #define _DMA_HEAPS_H
 
+#include <linux/errno.h>
 #include <linux/types.h>
 
 struct dma_heap;
+struct reserved_mem;
 
 /**
  * struct dma_heap_ops - ops to operate on a given heap
@@ -48,4 +50,13 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info);
 
 extern bool mem_accounting;
 
+#if IS_ENABLED(CONFIG_DMABUF_HEAPS_COHERENT)
+int dma_heap_coherent_register(struct reserved_mem *rmem);
+#else
+static inline int dma_heap_coherent_register(struct reserved_mem *rmem)
+{
+	return -EOPNOTSUPP;
+}
+#endif
+
 #endif /* _DMA_HEAPS_H */
diff --git a/kernel/dma/coherent.c b/kernel/dma/coherent.c
index 1147497bc512c..f49d13e460e4b 100644
--- a/kernel/dma/coherent.c
+++ b/kernel/dma/coherent.c
@@ -9,6 +9,7 @@
 #include <linux/module.h>
 #include <linux/dma-direct.h>
 #include <linux/dma-map-ops.h>
+#include <linux/dma-heap.h>
 
 struct dma_coherent_mem {
 	void		*virt_base;
@@ -393,6 +394,14 @@ static int __init rmem_dma_setup(struct reserved_mem *rmem)
 	rmem->ops = &rmem_dma_ops;
 	pr_info("Reserved memory: created DMA memory pool at %pa, size %ld MiB\n",
 		&rmem->base, (unsigned long)rmem->size / SZ_1M);
+
+	if (IS_ENABLED(CONFIG_DMABUF_HEAPS_COHERENT)) {
+		int ret = dma_heap_coherent_register(rmem);
+
+		if (ret)
+			pr_warn("Reserved memory: failed to register coherent heap for %s (%d)\n",
+				rmem->name ? rmem->name : "unknown", ret);
+	}
 	return 0;
 }
 

---
base-commit: 6de23f81a5e08be8fbf5e8d7e9febc72a5b5f27f
change-id: 20260223-b4-dmabuf-heap-coherent-rmem-91fd3926afe9

Best regards,
-- 
Albert Esteve <aesteve@redhat.com>

Re: [PATCH] dma-buf: heaps: Add Coherent heap to dmabuf heaps

[Maxime Ripard]

[-- Attachment #1: Type: text/plain, Size: 11450 bytes --]

Hi Albert,

Thanks for your patch!

On Tue, Feb 24, 2026 at 08:57:33AM +0100, Albert Esteve wrote:
> Add a dma-buf heap for DT coherent reserved-memory
> (i.e., 'shared-dma-pool' without 'reusable' property),
> exposing one heap per region for userspace buffers.
> 
> The heap binds a synthetic platform device to each region
> so coherent allocations use the correct dev->dma_mem,
> and it defers registration until late_initcall when
> normal allocator are available.
> 
> This patch includes charging of the coherent heap
> allocator to the dmem cgroup.
> 
> Signed-off-by: Albert Esteve <aesteve@redhat.com>
> ---
> This patch introduces a new driver to expose DT coherent reserved-memory
> regions as dma-buf heaps, allowing userspace buffers to be created.
> 
> Since these regions are device-dependent, we bind a synthetic platform
> device to each region so coherent allocations use the correct dev->dma_mem.
> 
> Following Eric’s [1] and Maxime’s [2] work on charging DMA buffers
> allocated from userspace to cgroups (dmem), this patch adds the same
> charging pattern used by CMA heaps patch. Charging is done only through
> the dma-buf heap interface so it can be attributed to a userspace allocator.
> 
> This allows each device-specific reserved-memory region to enforce its
> own limits.
> 
> [1] https://lore.kernel.org/all/20260218-dmabuf-heap-cma-dmem-v2-0-b249886fb7b2@redhat.com/
> [2] https://lore.kernel.org/all/20250310-dmem-cgroups-v1-0-2984c1bc9312@kernel.org/
> ---
>  drivers/dma-buf/heaps/Kconfig         |  17 ++
>  drivers/dma-buf/heaps/Makefile        |   1 +
>  drivers/dma-buf/heaps/coherent_heap.c | 485 ++++++++++++++++++++++++++++++++++
>  include/linux/dma-heap.h              |  11 +
>  kernel/dma/coherent.c                 |   9 +
>  5 files changed, 523 insertions(+)
> 
> diff --git a/drivers/dma-buf/heaps/Kconfig b/drivers/dma-buf/heaps/Kconfig
> index a5eef06c42264..93765dca164e3 100644
> --- a/drivers/dma-buf/heaps/Kconfig
> +++ b/drivers/dma-buf/heaps/Kconfig
> @@ -12,3 +12,20 @@ config DMABUF_HEAPS_CMA
>  	  Choose this option to enable dma-buf CMA heap. This heap is backed
>  	  by the Contiguous Memory Allocator (CMA). If your system has these
>  	  regions, you should say Y here.
> +
> +config DMABUF_HEAPS_COHERENT
> +	bool "DMA-BUF Coherent Reserved-Memory Heap"
> +	depends on DMABUF_HEAPS && OF_RESERVED_MEM && DMA_DECLARE_COHERENT
> +	help
> +	  Choose this option to enable coherent reserved-memory dma-buf heaps.
> +	  This heap is backed by non-reusable DT "shared-dma-pool" regions.
> +	  If your system defines coherent reserved-memory regions, you should
> +	  say Y here.
> +
> +config COHERENT_AREAS_DEFERRED
> +	int "Max deferred coherent reserved-memory regions"
> +	depends on DMABUF_HEAPS_COHERENT
> +	default 16
> +	help
> +	  Maximum number of coherent reserved-memory regions that can be
> +	  deferred for later registration during early boot.
> diff --git a/drivers/dma-buf/heaps/Makefile b/drivers/dma-buf/heaps/Makefile
> index 974467791032f..96bda7a65f041 100644
> --- a/drivers/dma-buf/heaps/Makefile
> +++ b/drivers/dma-buf/heaps/Makefile
> @@ -1,3 +1,4 @@
>  # SPDX-License-Identifier: GPL-2.0
>  obj-$(CONFIG_DMABUF_HEAPS_SYSTEM)	+= system_heap.o
>  obj-$(CONFIG_DMABUF_HEAPS_CMA)		+= cma_heap.o
> +obj-$(CONFIG_DMABUF_HEAPS_COHERENT)	+= coherent_heap.o
> diff --git a/drivers/dma-buf/heaps/coherent_heap.c b/drivers/dma-buf/heaps/coherent_heap.c
> new file mode 100644
> index 0000000000000..870b2b89aefcb
> --- /dev/null
> +++ b/drivers/dma-buf/heaps/coherent_heap.c
> @@ -0,0 +1,485 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * DMABUF heap for coherent reserved-memory regions
> + *
> + * Copyright (C) 2026 Red Hat, Inc.
> + * Author: Albert Esteve <aesteve@redhat.com>
> + *
> + */
> +
> +#include <linux/cgroup_dmem.h>
> +#include <linux/dma-heap.h>
> +#include <linux/dma-buf.h>
> +#include <linux/dma-mapping.h>
> +#include <linux/err.h>
> +#include <linux/highmem.h>
> +#include <linux/iosys-map.h>
> +#include <linux/of_reserved_mem.h>
> +#include <linux/platform_device.h>
> +#include <linux/scatterlist.h>
> +#include <linux/slab.h>
> +#include <linux/vmalloc.h>
> +
> +#define DEFERRED_AREAS_MAX CONFIG_COHERENT_AREAS_DEFERRED

I'm not sure we need to make it configurable. Distros are going to set
it to the user with the highest number of regions anyway. How about
using MAX_RESERVED_REGIONS for now?

>
> [...]
>
> +struct coherent_heap {
> +	struct dma_heap *heap;
> +	struct reserved_mem *rmem;
> +	char *name;
> +	struct device *dev;
> +	struct platform_device *pdev;
> +#if IS_ENABLED(CONFIG_CGROUP_DMEM)
> +	struct dmem_cgroup_region *cg;
> +#endif

We might want to leave the dmem accounting out for now so we can focus
on the heap itself. That being said, it ended up being pretty trivial
for CMA, so maybe it's not too much of a concern.

>
> [...]
>
> +static int __coherent_heap_register(struct reserved_mem *rmem)
> +{
> +	struct dma_heap_export_info exp_info;
> +	struct coherent_heap *coh_heap;
> +#if IS_ENABLED(CONFIG_CGROUP_DMEM)
> +	struct dmem_cgroup_region *region;
> +#endif
> +	const char *rmem_name;
> +	int ret;
> +
> +	if (!rmem)
> +		return -EINVAL;
> +
> +	rmem_name = rmem->name ? rmem->name : "unknown";

If the reserved region has no name, we probably shouldn't expose it to
userspace at all. Using unknown will probably create some bugs if you
have several, but also it's pretty like to have a name at some point and
thus we wouldn't have a stable name for the uAPI.

> +	coh_heap = kzalloc_obj(*coh_heap);
> +	if (!coh_heap)
> +		return -ENOMEM;
> +
> +	coh_heap->name = kasprintf(GFP_KERNEL, "coherent_%s", rmem_name);
> +	if (!coh_heap->name) {
> +		ret = -ENOMEM;
> +		goto free_coherent_heap;
> +	}

Similarly, we shouldn't use the coherent_ prefix for the heap name. If
the backing allocator ever changes (and between contiguous and coherent,
the difference is just a single property value in the DT), then the name
would change and userspace would break. We should directly use the name
of the region here.

> +	coh_heap->rmem = rmem;
> +
> +	/* create a platform device per rmem and bind it */
> +	coh_heap->pdev = platform_device_register_simple("coherent-heap",
> +							 PLATFORM_DEVID_AUTO,
> +							 NULL, 0);
> +	if (IS_ERR(coh_heap->pdev)) {
> +		ret = PTR_ERR(coh_heap->pdev);
> +		goto free_name;
> +	}

We probably should use a faux_device here instead of a platform_device,
but more importantly, the heap itself already has a device allocated for
it (dev_ret in dma_heap_add).

It's not in struct dma_heap yet, but there's a patch that moves it there
that we should probably carry:
https://lore.kernel.org/r/20210120210937.15069-2-john.stultz@linaro.org/

> +	if (rmem->ops && rmem->ops->device_init) {
> +		ret = rmem->ops->device_init(rmem, &coh_heap->pdev->dev);
> +		if (ret)
> +			goto pdev_unregister;
> +	}

I'm not really a fan of calling ops directly. Maybe we should create an
of_reserved_mem_device_init_with_mem function that would do it for us
(and would be called by of_reserved_mem_device_init_by_idx and the
likes).

> +	coh_heap->dev = &coh_heap->pdev->dev;
> +#if IS_ENABLED(CONFIG_CGROUP_DMEM)
> +	region = dmem_cgroup_register_region(rmem->size, "coh/%s", rmem_name);
> +	if (IS_ERR(region)) {
> +		ret = PTR_ERR(region);
> +		goto pdev_unregister;
> +	}
> +	coh_heap->cg = region;
> +#endif

Same comment than for CMA here: it should really be created by the
coherent memory region itself.

> +	exp_info.name = coh_heap->name;
> +	exp_info.ops = &coherent_heap_ops;
> +	exp_info.priv = coh_heap;
> +
> +	coh_heap->heap = dma_heap_add(&exp_info);
> +	if (IS_ERR(coh_heap->heap)) {
> +		ret = PTR_ERR(coh_heap->heap);
> +		goto cg_unregister;
> +	}
> +
> +	return 0;
> +
> +cg_unregister:
> +#if IS_ENABLED(CONFIG_CGROUP_DMEM)
> +	dmem_cgroup_unregister_region(coh_heap->cg);
> +#endif
> +pdev_unregister:
> +	platform_device_unregister(coh_heap->pdev);
> +	coh_heap->pdev = NULL;
> +free_name:
> +	kfree(coh_heap->name);
> +free_coherent_heap:
> +	kfree(coh_heap);
> +
> +	return ret;
> +}
> +
> +int dma_heap_coherent_register(struct reserved_mem *rmem)
> +{
> +	int ret;
> +
> +	ret = __coherent_heap_register(rmem);
> +	if (ret == -ENOMEM)
> +		return coherent_heap_add_deferred(rmem);
> +	return ret;
> +}

I appreciate you did it like we did for CMA, but if we ever want to make
that heap a module you'll end up with a dependency from the core kernel
on a module which doesn't work. The best here might be to wait a bit
until we have somewhat of an agreement on

https://lore.kernel.org/r/20260225-dma-buf-heaps-as-modules-v1-0-2109225a090d@kernel.org

> +static int __init coherent_heap_register_deferred(void)
> +{
> +	unsigned int i;
> +	int ret;
> +
> +	for (i = 0; i < rmem_areas_deferred_num; i++) {
> +		struct reserved_mem *rmem = rmem_areas_deferred[i];
> +
> +		ret = __coherent_heap_register(rmem);
> +		if (ret) {
> +			pr_warn("Failed to add coherent heap %s",
> +				rmem->name ? rmem->name : "unknown");
> +			continue;
> +		}
> +	}
> +
> +	return 0;
> +}
> +late_initcall(coherent_heap_register_deferred);

Why do you need a late_initcall here? Isn't module_init enough?

> +MODULE_DESCRIPTION("DMA-BUF heap for coherent reserved-memory regions");
> diff --git a/include/linux/dma-heap.h b/include/linux/dma-heap.h
> index 648328a64b27e..e894cfa1ecf1a 100644
> --- a/include/linux/dma-heap.h
> +++ b/include/linux/dma-heap.h
> @@ -9,9 +9,11 @@
>  #ifndef _DMA_HEAPS_H
>  #define _DMA_HEAPS_H
>  
> +#include <linux/errno.h>
>  #include <linux/types.h>
>  
>  struct dma_heap;
> +struct reserved_mem;
>  
>  /**
>   * struct dma_heap_ops - ops to operate on a given heap
> @@ -48,4 +50,13 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info);
>  
>  extern bool mem_accounting;
>  
> +#if IS_ENABLED(CONFIG_DMABUF_HEAPS_COHERENT)
> +int dma_heap_coherent_register(struct reserved_mem *rmem);
> +#else
> +static inline int dma_heap_coherent_register(struct reserved_mem *rmem)
> +{
> +	return -EOPNOTSUPP;
> +}
> +#endif
> +
>  #endif /* _DMA_HEAPS_H */
> diff --git a/kernel/dma/coherent.c b/kernel/dma/coherent.c
> index 1147497bc512c..f49d13e460e4b 100644
> --- a/kernel/dma/coherent.c
> +++ b/kernel/dma/coherent.c
> @@ -9,6 +9,7 @@
>  #include <linux/module.h>
>  #include <linux/dma-direct.h>
>  #include <linux/dma-map-ops.h>
> +#include <linux/dma-heap.h>
>  
>  struct dma_coherent_mem {
>  	void		*virt_base;
> @@ -393,6 +394,14 @@ static int __init rmem_dma_setup(struct reserved_mem *rmem)
>  	rmem->ops = &rmem_dma_ops;
>  	pr_info("Reserved memory: created DMA memory pool at %pa, size %ld MiB\n",
>  		&rmem->base, (unsigned long)rmem->size / SZ_1M);
> +
> +	if (IS_ENABLED(CONFIG_DMABUF_HEAPS_COHERENT)) {
> +		int ret = dma_heap_coherent_register(rmem);
> +
> +		if (ret)
> +			pr_warn("Reserved memory: failed to register coherent heap for %s (%d)\n",
> +				rmem->name ? rmem->name : "unknown", ret);
> +	}

I think this should be split into a patch of its own. It's going to be
reviewed (and possibly merged) by another maintainer, through another
tree.

Maxime

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Re: [PATCH] dma-buf: heaps: Add Coherent heap to dmabuf heaps

[Robin Murphy]

On 24/02/2026 7:57 am, Albert Esteve wrote:
> Add a dma-buf heap for DT coherent reserved-memory
> (i.e., 'shared-dma-pool' without 'reusable' property),
> exposing one heap per region for userspace buffers.

Despite the "shared" naming of the compatible, these kinds of 
reservations are often for highly device-specific purposes, and 
sometimes may not even be accessible to other devices at all, so it's 
far from clear that there's a generic use-case for connecting them to 
dma-buf. Certainly it doesn't seem like a good idea to unconditionally 
create heaps for *everyting*, and give userspace free reign to mess with 
things it doesn't necessarily understand (especially where 
usage-specific restrictions implied by "no-map" are involved) and which 
may break drivers.

Most drivers that accomodate a memory-region expect to manage it 
themselves, so I would think it should be up to the drivers to opt into 
delegating "their" pool to userspace by registering it as a heap. Or at 
very worst, at least some additional DT compatible or property to 
indicate that it really is safe and desirable to use a given pool in a 
truly shared manner.

Otherwise, If we just present some heaps named "memory@xyz" to userspace 
(arch/arm64/boot/dts/ti/k3-j784s4-j742s2-ti-ipc-firmware-common.dtsi is 
a fun example), do we really expect it to maintain exhaustive 
platform-specific knowledge of which actual device(s) they belong to and 
what they're for? And if it does try to just mess around and allocate 
and map stuff, how does the dma-buf layer also have all of that 
usage-specific detail to know what memory attributes are safe to map 
with etc.?

Thanks,
Robin.

> The heap binds a synthetic platform device to each region
> so coherent allocations use the correct dev->dma_mem,
> and it defers registration until late_initcall when
> normal allocator are available.
> 
> This patch includes charging of the coherent heap
> allocator to the dmem cgroup.
> 
> Signed-off-by: Albert Esteve <aesteve@redhat.com>
> ---
> This patch introduces a new driver to expose DT coherent reserved-memory
> regions as dma-buf heaps, allowing userspace buffers to be created.
> 
> Since these regions are device-dependent, we bind a synthetic platform
> device to each region so coherent allocations use the correct dev->dma_mem.
> 
> Following Eric’s [1] and Maxime’s [2] work on charging DMA buffers
> allocated from userspace to cgroups (dmem), this patch adds the same
> charging pattern used by CMA heaps patch. Charging is done only through
> the dma-buf heap interface so it can be attributed to a userspace allocator.
> 
> This allows each device-specific reserved-memory region to enforce its
> own limits.
> 
> [1] https://lore.kernel.org/all/20260218-dmabuf-heap-cma-dmem-v2-0-b249886fb7b2@redhat.com/
> [2] https://lore.kernel.org/all/20250310-dmem-cgroups-v1-0-2984c1bc9312@kernel.org/
> ---
>   drivers/dma-buf/heaps/Kconfig         |  17 ++
>   drivers/dma-buf/heaps/Makefile        |   1 +
>   drivers/dma-buf/heaps/coherent_heap.c | 485 ++++++++++++++++++++++++++++++++++
>   include/linux/dma-heap.h              |  11 +
>   kernel/dma/coherent.c                 |   9 +
>   5 files changed, 523 insertions(+)
> 
> diff --git a/drivers/dma-buf/heaps/Kconfig b/drivers/dma-buf/heaps/Kconfig
> index a5eef06c42264..93765dca164e3 100644
> --- a/drivers/dma-buf/heaps/Kconfig
> +++ b/drivers/dma-buf/heaps/Kconfig
> @@ -12,3 +12,20 @@ config DMABUF_HEAPS_CMA
>   	  Choose this option to enable dma-buf CMA heap. This heap is backed
>   	  by the Contiguous Memory Allocator (CMA). If your system has these
>   	  regions, you should say Y here.
> +
> +config DMABUF_HEAPS_COHERENT
> +	bool "DMA-BUF Coherent Reserved-Memory Heap"
> +	depends on DMABUF_HEAPS && OF_RESERVED_MEM && DMA_DECLARE_COHERENT
> +	help
> +	  Choose this option to enable coherent reserved-memory dma-buf heaps.
> +	  This heap is backed by non-reusable DT "shared-dma-pool" regions.
> +	  If your system defines coherent reserved-memory regions, you should
> +	  say Y here.
> +
> +config COHERENT_AREAS_DEFERRED
> +	int "Max deferred coherent reserved-memory regions"
> +	depends on DMABUF_HEAPS_COHERENT
> +	default 16
> +	help
> +	  Maximum number of coherent reserved-memory regions that can be
> +	  deferred for later registration during early boot.
> diff --git a/drivers/dma-buf/heaps/Makefile b/drivers/dma-buf/heaps/Makefile
> index 974467791032f..96bda7a65f041 100644
> --- a/drivers/dma-buf/heaps/Makefile
> +++ b/drivers/dma-buf/heaps/Makefile
> @@ -1,3 +1,4 @@
>   # SPDX-License-Identifier: GPL-2.0
>   obj-$(CONFIG_DMABUF_HEAPS_SYSTEM)	+= system_heap.o
>   obj-$(CONFIG_DMABUF_HEAPS_CMA)		+= cma_heap.o
> +obj-$(CONFIG_DMABUF_HEAPS_COHERENT)	+= coherent_heap.o
> diff --git a/drivers/dma-buf/heaps/coherent_heap.c b/drivers/dma-buf/heaps/coherent_heap.c
> new file mode 100644
> index 0000000000000..870b2b89aefcb
> --- /dev/null
> +++ b/drivers/dma-buf/heaps/coherent_heap.c
> @@ -0,0 +1,485 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * DMABUF heap for coherent reserved-memory regions
> + *
> + * Copyright (C) 2026 Red Hat, Inc.
> + * Author: Albert Esteve <aesteve@redhat.com>
> + *
> + */
> +
> +#include <linux/cgroup_dmem.h>
> +#include <linux/dma-heap.h>
> +#include <linux/dma-buf.h>
> +#include <linux/dma-mapping.h>
> +#include <linux/err.h>
> +#include <linux/highmem.h>
> +#include <linux/iosys-map.h>
> +#include <linux/of_reserved_mem.h>
> +#include <linux/platform_device.h>
> +#include <linux/scatterlist.h>
> +#include <linux/slab.h>
> +#include <linux/vmalloc.h>
> +
> +#define DEFERRED_AREAS_MAX CONFIG_COHERENT_AREAS_DEFERRED
> +
> +/*
> + * Early init can't use normal memory management yet (memblock is used
> + * instead), so keep a small deferred list and retry at late_initcall.
> + */
> +static struct reserved_mem *rmem_areas_deferred[DEFERRED_AREAS_MAX];
> +static unsigned int rmem_areas_deferred_num;
> +
> +static int coherent_heap_add_deferred(struct reserved_mem *rmem)
> +{
> +	if (rmem_areas_deferred_num >= DEFERRED_AREAS_MAX) {
> +		pr_warn("Deferred heap areas list full, dropping %s\n",
> +			rmem->name ? rmem->name : "unknown");
> +		return -EINVAL;
> +	}
> +	rmem_areas_deferred[rmem_areas_deferred_num++] = rmem;
> +	return 0;
> +}
> +
> +struct coherent_heap {
> +	struct dma_heap *heap;
> +	struct reserved_mem *rmem;
> +	char *name;
> +	struct device *dev;
> +	struct platform_device *pdev;
> +#if IS_ENABLED(CONFIG_CGROUP_DMEM)
> +	struct dmem_cgroup_region *cg;
> +#endif
> +};
> +
> +struct coherent_heap_buffer {
> +	struct coherent_heap *heap;
> +	struct list_head attachments;
> +	struct mutex lock;
> +	unsigned long len;
> +	dma_addr_t dma_addr;
> +	void *alloc_vaddr;
> +	struct page **pages;
> +	pgoff_t pagecount;
> +	int vmap_cnt;
> +	void *vaddr;
> +#if IS_ENABLED(CONFIG_CGROUP_DMEM)
> +	struct dmem_cgroup_pool_state *pool;
> +#endif
> +};
> +
> +struct dma_heap_attachment {
> +	struct device *dev;
> +	struct sg_table table;
> +	struct list_head list;
> +	bool mapped;
> +};
> +
> +static int coherent_heap_attach(struct dma_buf *dmabuf,
> +				struct dma_buf_attachment *attachment)
> +{
> +	struct coherent_heap_buffer *buffer = dmabuf->priv;
> +	struct dma_heap_attachment *a;
> +	int ret;
> +
> +	a = kzalloc_obj(*a);
> +	if (!a)
> +		return -ENOMEM;
> +
> +	ret = sg_alloc_table_from_pages(&a->table, buffer->pages,
> +					buffer->pagecount, 0,
> +					buffer->pagecount << PAGE_SHIFT,
> +					GFP_KERNEL);
> +	if (ret) {
> +		kfree(a);
> +		return ret;
> +	}
> +
> +	a->dev = attachment->dev;
> +	INIT_LIST_HEAD(&a->list);
> +	a->mapped = false;
> +
> +	attachment->priv = a;
> +
> +	mutex_lock(&buffer->lock);
> +	list_add(&a->list, &buffer->attachments);
> +	mutex_unlock(&buffer->lock);
> +
> +	return 0;
> +}
> +
> +static void coherent_heap_detach(struct dma_buf *dmabuf,
> +				 struct dma_buf_attachment *attachment)
> +{
> +	struct coherent_heap_buffer *buffer = dmabuf->priv;
> +	struct dma_heap_attachment *a = attachment->priv;
> +
> +	mutex_lock(&buffer->lock);
> +	list_del(&a->list);
> +	mutex_unlock(&buffer->lock);
> +
> +	sg_free_table(&a->table);
> +	kfree(a);
> +}
> +
> +static struct sg_table *coherent_heap_map_dma_buf(struct dma_buf_attachment *attachment,
> +						  enum dma_data_direction direction)
> +{
> +	struct dma_heap_attachment *a = attachment->priv;
> +	struct sg_table *table = &a->table;
> +	int ret;
> +
> +	ret = dma_map_sgtable(attachment->dev, table, direction, 0);
> +	if (ret)
> +		return ERR_PTR(-ENOMEM);
> +	a->mapped = true;
> +
> +	return table;
> +}
> +
> +static void coherent_heap_unmap_dma_buf(struct dma_buf_attachment *attachment,
> +					struct sg_table *table,
> +					enum dma_data_direction direction)
> +{
> +	struct dma_heap_attachment *a = attachment->priv;
> +
> +	a->mapped = false;
> +	dma_unmap_sgtable(attachment->dev, table, direction, 0);
> +}
> +
> +static int coherent_heap_dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
> +						  enum dma_data_direction direction)
> +{
> +	struct coherent_heap_buffer *buffer = dmabuf->priv;
> +	struct dma_heap_attachment *a;
> +
> +	mutex_lock(&buffer->lock);
> +	if (buffer->vmap_cnt)
> +		invalidate_kernel_vmap_range(buffer->vaddr, buffer->len);
> +
> +	list_for_each_entry(a, &buffer->attachments, list) {
> +		if (!a->mapped)
> +			continue;
> +		dma_sync_sgtable_for_cpu(a->dev, &a->table, direction);
> +	}
> +	mutex_unlock(&buffer->lock);
> +
> +	return 0;
> +}
> +
> +static int coherent_heap_dma_buf_end_cpu_access(struct dma_buf *dmabuf,
> +						enum dma_data_direction direction)
> +{
> +	struct coherent_heap_buffer *buffer = dmabuf->priv;
> +	struct dma_heap_attachment *a;
> +
> +	mutex_lock(&buffer->lock);
> +	if (buffer->vmap_cnt)
> +		flush_kernel_vmap_range(buffer->vaddr, buffer->len);
> +
> +	list_for_each_entry(a, &buffer->attachments, list) {
> +		if (!a->mapped)
> +			continue;
> +		dma_sync_sgtable_for_device(a->dev, &a->table, direction);
> +	}
> +	mutex_unlock(&buffer->lock);
> +
> +	return 0;
> +}
> +
> +static int coherent_heap_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma)
> +{
> +	struct coherent_heap_buffer *buffer = dmabuf->priv;
> +	struct coherent_heap *coh_heap = buffer->heap;
> +
> +	return dma_mmap_coherent(coh_heap->dev, vma, buffer->alloc_vaddr,
> +				 buffer->dma_addr, buffer->len);
> +}
> +
> +static void *coherent_heap_do_vmap(struct coherent_heap_buffer *buffer)
> +{
> +	void *vaddr;
> +
> +	vaddr = vmap(buffer->pages, buffer->pagecount, VM_MAP, PAGE_KERNEL);
> +	if (!vaddr)
> +		return ERR_PTR(-ENOMEM);
> +
> +	return vaddr;
> +}
> +
> +static int coherent_heap_vmap(struct dma_buf *dmabuf, struct iosys_map *map)
> +{
> +	struct coherent_heap_buffer *buffer = dmabuf->priv;
> +	void *vaddr;
> +	int ret = 0;
> +
> +	mutex_lock(&buffer->lock);
> +	if (buffer->vmap_cnt) {
> +		buffer->vmap_cnt++;
> +		iosys_map_set_vaddr(map, buffer->vaddr);
> +		goto out;
> +	}
> +
> +	vaddr = coherent_heap_do_vmap(buffer);
> +	if (IS_ERR(vaddr)) {
> +		ret = PTR_ERR(vaddr);
> +		goto out;
> +	}
> +
> +	buffer->vaddr = vaddr;
> +	buffer->vmap_cnt++;
> +	iosys_map_set_vaddr(map, buffer->vaddr);
> +out:
> +	mutex_unlock(&buffer->lock);
> +
> +	return ret;
> +}
> +
> +static void coherent_heap_vunmap(struct dma_buf *dmabuf, struct iosys_map *map)
> +{
> +	struct coherent_heap_buffer *buffer = dmabuf->priv;
> +
> +	mutex_lock(&buffer->lock);
> +	if (!--buffer->vmap_cnt) {
> +		vunmap(buffer->vaddr);
> +		buffer->vaddr = NULL;
> +	}
> +	mutex_unlock(&buffer->lock);
> +	iosys_map_clear(map);
> +}
> +
> +static void coherent_heap_dma_buf_release(struct dma_buf *dmabuf)
> +{
> +	struct coherent_heap_buffer *buffer = dmabuf->priv;
> +	struct coherent_heap *coh_heap = buffer->heap;
> +
> +	if (buffer->vmap_cnt > 0) {
> +		WARN(1, "%s: buffer still mapped in the kernel\n", __func__);
> +		vunmap(buffer->vaddr);
> +		buffer->vaddr = NULL;
> +		buffer->vmap_cnt = 0;
> +	}
> +
> +	if (buffer->alloc_vaddr)
> +		dma_free_coherent(coh_heap->dev, buffer->len, buffer->alloc_vaddr,
> +			       buffer->dma_addr);
> +	kfree(buffer->pages);
> +#if IS_ENABLED(CONFIG_CGROUP_DMEM)
> +	dmem_cgroup_uncharge(buffer->pool, buffer->len);
> +#endif
> +	kfree(buffer);
> +}
> +
> +static const struct dma_buf_ops coherent_heap_buf_ops = {
> +	.attach = coherent_heap_attach,
> +	.detach = coherent_heap_detach,
> +	.map_dma_buf = coherent_heap_map_dma_buf,
> +	.unmap_dma_buf = coherent_heap_unmap_dma_buf,
> +	.begin_cpu_access = coherent_heap_dma_buf_begin_cpu_access,
> +	.end_cpu_access = coherent_heap_dma_buf_end_cpu_access,
> +	.mmap = coherent_heap_mmap,
> +	.vmap = coherent_heap_vmap,
> +	.vunmap = coherent_heap_vunmap,
> +	.release = coherent_heap_dma_buf_release,
> +};
> +
> +static struct dma_buf *coherent_heap_allocate(struct dma_heap *heap,
> +					      unsigned long len,
> +					      u32 fd_flags,
> +					      u64 heap_flags)
> +{
> +	struct coherent_heap *coh_heap;
> +	struct coherent_heap_buffer *buffer;
> +	DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
> +	size_t size = PAGE_ALIGN(len);
> +	pgoff_t pagecount = size >> PAGE_SHIFT;
> +	struct dma_buf *dmabuf;
> +	int ret = -ENOMEM;
> +	pgoff_t pg;
> +
> +	coh_heap = dma_heap_get_drvdata(heap);
> +	if (!coh_heap)
> +		return ERR_PTR(-EINVAL);
> +	if (!coh_heap->dev)
> +		return ERR_PTR(-ENODEV);
> +
> +	buffer = kzalloc_obj(*buffer);
> +	if (!buffer)
> +		return ERR_PTR(-ENOMEM);
> +
> +	INIT_LIST_HEAD(&buffer->attachments);
> +	mutex_init(&buffer->lock);
> +	buffer->len = size;
> +	buffer->heap = coh_heap;
> +	buffer->pagecount = pagecount;
> +
> +#if IS_ENABLED(CONFIG_CGROUP_DMEM)
> +	if (mem_accounting) {
> +		ret = dmem_cgroup_try_charge(coh_heap->cg, size,
> +					     &buffer->pool, NULL);
> +		if (ret)
> +			goto free_buffer;
> +	}
> +#endif
> +
> +	buffer->alloc_vaddr = dma_alloc_coherent(coh_heap->dev, buffer->len,
> +						 &buffer->dma_addr, GFP_KERNEL);
> +	if (!buffer->alloc_vaddr) {
> +		ret = -ENOMEM;
> +#if IS_ENABLED(CONFIG_CGROUP_DMEM)
> +		goto uncharge_cgroup;
> +#else
> +		goto free_buffer;
> +#endif
> +	}
> +
> +	buffer->pages = kmalloc_array(pagecount, sizeof(*buffer->pages),
> +				      GFP_KERNEL);
> +	if (!buffer->pages) {
> +		ret = -ENOMEM;
> +		goto free_dma;
> +	}
> +
> +	for (pg = 0; pg < pagecount; pg++)
> +		buffer->pages[pg] = virt_to_page((char *)buffer->alloc_vaddr +
> +						 (pg * PAGE_SIZE));
> +
> +	/* create the dmabuf */
> +	exp_info.exp_name = dma_heap_get_name(heap);
> +	exp_info.ops = &coherent_heap_buf_ops;
> +	exp_info.size = buffer->len;
> +	exp_info.flags = fd_flags;
> +	exp_info.priv = buffer;
> +	dmabuf = dma_buf_export(&exp_info);
> +	if (IS_ERR(dmabuf)) {
> +		ret = PTR_ERR(dmabuf);
> +		goto free_pages;
> +	}
> +	return dmabuf;
> +
> +free_pages:
> +	kfree(buffer->pages);
> +free_dma:
> +	dma_free_coherent(coh_heap->dev, buffer->len, buffer->alloc_vaddr,
> +			  buffer->dma_addr);
> +#if IS_ENABLED(CONFIG_CGROUP_DMEM)
> +uncharge_cgroup:
> +	dmem_cgroup_uncharge(buffer->pool, size);
> +#endif
> +free_buffer:
> +	kfree(buffer);
> +	return ERR_PTR(ret);
> +}
> +
> +static const struct dma_heap_ops coherent_heap_ops = {
> +	.allocate = coherent_heap_allocate,
> +};
> +
> +static int __coherent_heap_register(struct reserved_mem *rmem)
> +{
> +	struct dma_heap_export_info exp_info;
> +	struct coherent_heap *coh_heap;
> +#if IS_ENABLED(CONFIG_CGROUP_DMEM)
> +	struct dmem_cgroup_region *region;
> +#endif
> +	const char *rmem_name;
> +	int ret;
> +
> +	if (!rmem)
> +		return -EINVAL;
> +
> +	rmem_name = rmem->name ? rmem->name : "unknown";
> +
> +	coh_heap = kzalloc_obj(*coh_heap);
> +	if (!coh_heap)
> +		return -ENOMEM;
> +
> +	coh_heap->name = kasprintf(GFP_KERNEL, "coherent_%s", rmem_name);
> +	if (!coh_heap->name) {
> +		ret = -ENOMEM;
> +		goto free_coherent_heap;
> +	}
> +
> +	coh_heap->rmem = rmem;
> +
> +	/* create a platform device per rmem and bind it */
> +	coh_heap->pdev = platform_device_register_simple("coherent-heap",
> +							 PLATFORM_DEVID_AUTO,
> +							 NULL, 0);
> +	if (IS_ERR(coh_heap->pdev)) {
> +		ret = PTR_ERR(coh_heap->pdev);
> +		goto free_name;
> +	}
> +
> +	if (rmem->ops && rmem->ops->device_init) {
> +		ret = rmem->ops->device_init(rmem, &coh_heap->pdev->dev);
> +		if (ret)
> +			goto pdev_unregister;
> +	}
> +
> +	coh_heap->dev = &coh_heap->pdev->dev;
> +#if IS_ENABLED(CONFIG_CGROUP_DMEM)
> +	region = dmem_cgroup_register_region(rmem->size, "coh/%s", rmem_name);
> +	if (IS_ERR(region)) {
> +		ret = PTR_ERR(region);
> +		goto pdev_unregister;
> +	}
> +	coh_heap->cg = region;
> +#endif
> +
> +	exp_info.name = coh_heap->name;
> +	exp_info.ops = &coherent_heap_ops;
> +	exp_info.priv = coh_heap;
> +
> +	coh_heap->heap = dma_heap_add(&exp_info);
> +	if (IS_ERR(coh_heap->heap)) {
> +		ret = PTR_ERR(coh_heap->heap);
> +		goto cg_unregister;
> +	}
> +
> +	return 0;
> +
> +cg_unregister:
> +#if IS_ENABLED(CONFIG_CGROUP_DMEM)
> +	dmem_cgroup_unregister_region(coh_heap->cg);
> +#endif
> +pdev_unregister:
> +	platform_device_unregister(coh_heap->pdev);
> +	coh_heap->pdev = NULL;
> +free_name:
> +	kfree(coh_heap->name);
> +free_coherent_heap:
> +	kfree(coh_heap);
> +
> +	return ret;
> +}
> +
> +int dma_heap_coherent_register(struct reserved_mem *rmem)
> +{
> +	int ret;
> +
> +	ret = __coherent_heap_register(rmem);
> +	if (ret == -ENOMEM)
> +		return coherent_heap_add_deferred(rmem);
> +	return ret;
> +}
> +
> +static int __init coherent_heap_register_deferred(void)
> +{
> +	unsigned int i;
> +	int ret;
> +
> +	for (i = 0; i < rmem_areas_deferred_num; i++) {
> +		struct reserved_mem *rmem = rmem_areas_deferred[i];
> +
> +		ret = __coherent_heap_register(rmem);
> +		if (ret) {
> +			pr_warn("Failed to add coherent heap %s",
> +				rmem->name ? rmem->name : "unknown");
> +			continue;
> +		}
> +	}
> +
> +	return 0;
> +}
> +late_initcall(coherent_heap_register_deferred);
> +MODULE_DESCRIPTION("DMA-BUF heap for coherent reserved-memory regions");
> diff --git a/include/linux/dma-heap.h b/include/linux/dma-heap.h
> index 648328a64b27e..e894cfa1ecf1a 100644
> --- a/include/linux/dma-heap.h
> +++ b/include/linux/dma-heap.h
> @@ -9,9 +9,11 @@
>   #ifndef _DMA_HEAPS_H
>   #define _DMA_HEAPS_H
>   
> +#include <linux/errno.h>
>   #include <linux/types.h>
>   
>   struct dma_heap;
> +struct reserved_mem;
>   
>   /**
>    * struct dma_heap_ops - ops to operate on a given heap
> @@ -48,4 +50,13 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info);
>   
>   extern bool mem_accounting;
>   
> +#if IS_ENABLED(CONFIG_DMABUF_HEAPS_COHERENT)
> +int dma_heap_coherent_register(struct reserved_mem *rmem);
> +#else
> +static inline int dma_heap_coherent_register(struct reserved_mem *rmem)
> +{
> +	return -EOPNOTSUPP;
> +}
> +#endif
> +
>   #endif /* _DMA_HEAPS_H */
> diff --git a/kernel/dma/coherent.c b/kernel/dma/coherent.c
> index 1147497bc512c..f49d13e460e4b 100644
> --- a/kernel/dma/coherent.c
> +++ b/kernel/dma/coherent.c
> @@ -9,6 +9,7 @@
>   #include <linux/module.h>
>   #include <linux/dma-direct.h>
>   #include <linux/dma-map-ops.h>
> +#include <linux/dma-heap.h>
>   
>   struct dma_coherent_mem {
>   	void		*virt_base;
> @@ -393,6 +394,14 @@ static int __init rmem_dma_setup(struct reserved_mem *rmem)
>   	rmem->ops = &rmem_dma_ops;
>   	pr_info("Reserved memory: created DMA memory pool at %pa, size %ld MiB\n",
>   		&rmem->base, (unsigned long)rmem->size / SZ_1M);
> +
> +	if (IS_ENABLED(CONFIG_DMABUF_HEAPS_COHERENT)) {
> +		int ret = dma_heap_coherent_register(rmem);
> +
> +		if (ret)
> +			pr_warn("Reserved memory: failed to register coherent heap for %s (%d)\n",
> +				rmem->name ? rmem->name : "unknown", ret);
> +	}
>   	return 0;
>   }
>   
> 
> ---
> base-commit: 6de23f81a5e08be8fbf5e8d7e9febc72a5b5f27f
> change-id: 20260223-b4-dmabuf-heap-coherent-rmem-91fd3926afe9
> 
> Best regards,

Claude review: dma-buf: heaps: Add Coherent heap to dmabuf heaps

[Claude Code Review Bot]

Overall Series Review

Subject: dma-buf: heaps: Add Coherent heap to dmabuf heaps
Author: Albert Esteve <aesteve@redhat.com>
Patches: 3
Reviewed: 2026-02-27T15:38:23.597909

---

This is a single patch adding a new dma-buf heap driver for DT coherent reserved-memory (`shared-dma-pool` without `reusable`). The goal is to expose these regions to userspace via the dma-buf heaps interface, with dmem cgroup accounting.

**Overall assessment: Needs significant rework.** The patch has several critical issues including unsafe use of `virt_to_page()` on `dma_alloc_coherent()` return values, missing memory zeroing (information leak to userspace), and an architectural concern where core DMA code is modified to call into the dma-buf heap subsystem. The `#if IS_ENABLED(CONFIG_CGROUP_DMEM)` ifdefs throughout are unnecessary since the header provides stubs, and there is no mechanism to prevent unconditional exposure of all coherent reserved-memory to userspace.

---

---
Generated by Claude Code Patch Reviewer

Claude review: dma-buf: heaps: Add Coherent heap to dmabuf heaps

[Claude Code Review Bot]

Patch Review

#### Critical Issues

**1. `virt_to_page()` on `dma_alloc_coherent()` return value is unsafe**

```c
for (pg = 0; pg < pagecount; pg++)
    buffer->pages[pg] = virt_to_page((char *)buffer->alloc_vaddr +
                     (pg * PAGE_SIZE));
```

`dma_alloc_coherent()` on a device backed by reserved coherent memory goes through `dma_alloc_from_coherent()`, which returns a pointer into `mem->virt_base`. That virtual base comes from `memremap(phys_addr, size, MEMREMAP_WC)` (in `dma_init_coherent_memory()` at `kernel/dma/coherent.c`). On architectures where `memremap()` returns a vmalloc/ioremap-space address rather than a linear-map address, `virt_to_page()` will produce garbage or crash.

The existing `coherent.c` code avoids this — it stores `pfn_base = PFN_DOWN(phys_addr)` and uses PFN arithmetic directly in `remap_pfn_range()` calls. The correct approach here would be to derive PFNs from the DMA address:

```c
phys_addr_t phys = dma_to_phys(coh_heap->dev, buffer->dma_addr);
for (pg = 0; pg < pagecount; pg++)
    buffer->pages[pg] = pfn_to_page(PFN_DOWN(phys) + pg);
```

This is the single most critical issue in the patch.

**2. Missing memory zeroing — information leak to userspace**

The CMA heap explicitly zeroes allocated pages before exporting them:
```c
/* CMA heap does: */
memset(page_address(cma_pages), 0, size);
```

The coherent heap does not zero the memory returned by `dma_alloc_coherent()`. For reserved-memory coherent pools, `dma_alloc_from_coherent()` returns a pointer into pre-mapped memory without zeroing it. Between allocate/free cycles, stale data from prior users is exposed. Since these buffers are exported to userspace, this is a security concern. A `memset(buffer->alloc_vaddr, 0, buffer->len)` is needed after allocation.

#### Architectural Issues

**3. Reverse dependency: `kernel/dma/coherent.c` calling into dma-buf heaps**

```c
/* kernel/dma/coherent.c: */
+#include <linux/dma-heap.h>
...
+    if (IS_ENABLED(CONFIG_DMABUF_HEAPS_COHERENT)) {
+        int ret = dma_heap_coherent_register(rmem);
```

This creates a layering violation — core DMA infrastructure now calls up into the dma-buf heaps subsystem. The CMA heap does not do this; it uses `cma_for_each_area()` to iterate and register CMA areas autonomously from `drivers/dma-buf/heaps/cma_heap.c`, without touching any CMA core code.

The coherent heap should similarly find reserved-memory regions on its own at `late_initcall` time (e.g., by iterating the reserved-memory DT nodes), rather than hooking into `rmem_dma_setup()`. This would keep the modification entirely within `drivers/dma-buf/heaps/` and avoid touching `kernel/dma/coherent.c`.

**4. All coherent reserved-memory regions are unconditionally exposed to userspace**

Every `shared-dma-pool` (non-reusable) region automatically gets a dma-buf heap. Some of these regions may be intended exclusively for specific kernel drivers. A malicious or buggy userspace process could exhaust a device-specific reserved pool, starving the kernel driver. There should be a DT opt-in mechanism (e.g., a `dma-buf-heap` property) to control which regions are exposed, rather than exposing all of them.

#### Code Quality Issues

**5. Unnecessary `#if IS_ENABLED(CONFIG_CGROUP_DMEM)` guards throughout**

The header `linux/cgroup_dmem.h` already provides no-op stubs when `CONFIG_CGROUP_DMEM` is disabled (`dmem_cgroup_try_charge()` returns 0, `dmem_cgroup_uncharge()` is a no-op, etc.). All the `#if IS_ENABLED(CONFIG_CGROUP_DMEM)` blocks in the patch are unnecessary and clutter the code. The entire cgroup charging path can be written without any `#if` guards:

```c
/* In struct coherent_heap — no #if needed: */
struct dmem_cgroup_region *cg;

/* In allocate — no #if needed: */
if (mem_accounting) {
    ret = dmem_cgroup_try_charge(coh_heap->cg, size,
                     &buffer->pool, NULL);
    if (ret)
        goto free_buffer;
}
```

The stubs will compile to nothing when disabled. This would significantly clean up `coherent_heap_allocate()`, `coherent_heap_dma_buf_release()`, `__coherent_heap_register()`, and the error paths.

**6. `cg_unregister` error label structure is confusing**

```c
cg_unregister:
#if IS_ENABLED(CONFIG_CGROUP_DMEM)
    dmem_cgroup_unregister_region(coh_heap->cg);
#endif
pdev_unregister:
```

When `CONFIG_CGROUP_DMEM` is disabled, `cg_unregister` is an empty label that falls through to `pdev_unregister`. This compiles but is confusing. Removing the ifdefs (as per point 5) would fix this naturally.

**7. Missing `device_release()` call in error/cleanup paths**

In `__coherent_heap_register()`, after a successful `rmem->ops->device_init()` call, the error paths only call `platform_device_unregister()` but never call `rmem->ops->device_release()` to undo the init:

```c
if (rmem->ops && rmem->ops->device_init) {
    ret = rmem->ops->device_init(rmem, &coh_heap->pdev->dev);
    if (ret)
        goto pdev_unregister;
}
/* ... later errors goto cg_unregister or pdev_unregister ... */
/* but never call rmem->ops->device_release() */
```

If `dma_heap_add()` or `dmem_cgroup_register_region()` fails after a successful `device_init()`, the reserved memory binding is leaked. Need a `device_release` cleanup step.

**8. Deferred registration only on `-ENOMEM` is fragile**

```c
int dma_heap_coherent_register(struct reserved_mem *rmem)
{
    int ret;
    ret = __coherent_heap_register(rmem);
    if (ret == -ENOMEM)
        return coherent_heap_add_deferred(rmem);
    return ret;
}
```

`rmem_dma_setup()` runs from `RESERVEDMEM_OF_DECLARE`, which is during very early boot before slab is available. The assumption is that `kzalloc()` will fail with `-ENOMEM` and trigger deferral. But `platform_device_register_simple()` or other calls could fail with different error codes (e.g., `-ENODEV`, `-EPROBE_DEFER`) before the allocator is ready. If the intent is to unconditionally defer during early boot, just always defer — or better yet, don't hook into `rmem_dma_setup()` at all (see point 3).

**9. `COHERENT_AREAS_DEFERRED` Kconfig naming is inconsistent**

```
config COHERENT_AREAS_DEFERRED
    int "Max deferred coherent reserved-memory regions"
```

This name doesn't follow the `DMABUF_HEAPS_` prefix convention used by other options in this Kconfig. Should be something like `DMABUF_HEAPS_COHERENT_MAX_DEFERRED`. Also, making this a Kconfig integer for what is essentially an implementation detail of early boot deferral seems like unnecessary user-facing configuration.

**10. `begin_cpu_access`/`end_cpu_access` sync correctness for coherent memory**

```c
static int coherent_heap_dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
                          enum dma_data_direction direction)
{
    ...
    list_for_each_entry(a, &buffer->attachments, list) {
        if (!a->mapped)
            continue;
        dma_sync_sgtable_for_cpu(a->dev, &a->table, direction);
    }
```

This is copied from the CMA heap, but the semantics are subtly different. The underlying allocation is DMA-coherent (uncached/write-combining), so the sync on the allocation-side device is unnecessary. The sync on *attaching* devices' sg_tables may still be needed if those devices' DMA mappings are not coherent. The code works but a comment explaining this would be helpful, since the reader may wonder why coherent memory needs sync at all.

**11. Dependency on unmerged patches**

The `mem_accounting` variable and the dmem cgroup charging pattern are from Eric's series ([1] in the cover letter) which is not yet merged. This should be clearly stated as a dependency, and ideally this patch should be part of or follow that series.

#### Minor Issues

- `kmalloc_array()` is used for `buffer->pages` while `kzalloc_obj()` is used elsewhere — inconsistent with the CMA heap which uses `kmalloc_objs()` for the same purpose.
- The `pr_warn` in `coherent_heap_register_deferred()` is missing a newline: `pr_warn("Failed to add coherent heap %s",` → should have `\n`.
- No `MODULE_LICENSE()` (though since it's `bool` not `tristate`, this is cosmetic — but `MODULE_DESCRIPTION` without `MODULE_LICENSE` is inconsistent).

---
Generated by Claude Code Patch Reviewer