Re: [PATCH v2 4/6] dma-buf: heaps: Add Coherent heap to dmabuf heaps

[Albert Esteve <aesteve@redhat.com>]

On Tue, Mar 3, 2026 at 2:20 PM Maxime Ripard <mripard@redhat.com> wrote:
>
> Hi,
>
> On Tue, Mar 03, 2026 at 01:33:47PM +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 the heap device to each memory region so
> > coherent allocations use the correct dev->dma_mem, and
> > it defers registration until module_init when normal
> > allocators are available.
> >
> > Signed-off-by: Albert Esteve <aesteve@redhat.com>
> > ---
> >  drivers/dma-buf/dma-heap.c            |   4 +-
> >  drivers/dma-buf/heaps/Kconfig         |   9 +
> >  drivers/dma-buf/heaps/Makefile        |   1 +
> >  drivers/dma-buf/heaps/coherent_heap.c | 426 ++++++++++++++++++++++++++++++++++
> >  include/linux/dma-heap.h              |  11 +
> >  include/linux/dma-map-ops.h           |   7 +
> >  6 files changed, 456 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/dma-buf/dma-heap.c b/drivers/dma-buf/dma-heap.c
> > index 88189d4e48561..ba87e5ac16ae2 100644
> > --- a/drivers/dma-buf/dma-heap.c
> > +++ b/drivers/dma-buf/dma-heap.c
> > @@ -390,8 +390,8 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
> >
> >       heap = dma_heap_create(exp_info);
> >       if (IS_ERR(heap)) {
> > -             pr_err("dma_heap: failed to create heap (%d)\n", PTR_ERR(heap));
> > -             return PTR_ERR(heap);
> > +             pr_err("dma_heap: failed to create heap (%ld)\n", PTR_ERR(heap));
> > +             return ERR_CAST(heap);
>
> This looks unrelated and should possibly be squashed into the previous
> patch that introduces dma_heap_create()?
>
> > +static int coherent_heap_init_dma_mask(struct device *dev)
> > +{
> > +     int ret;
> > +
> > +     ret = dma_coerce_mask_and_coherent(dev, DMA_BIT_MASK(64));
> > +     if (!ret)
> > +             return 0;
> > +
> > +     /* Fallback to 32-bit DMA mask */
> > +     return dma_coerce_mask_and_coherent(dev, DMA_BIT_MASK(32));
> > +}
>
> Why do you need to mess with the DMA mask? I'd expect that device to be
> able to access everything.

When I tested I was getting: "reserved memory is beyond device's set
DMA address range", so I tested if it was fixed with
dma_coerce_mask_and_coherent() and/or dma_set_mask_coherent(). I did
not debug the value of coherent_dma_mask, but given the error I assume
it was not set properly? Ultimately, using the 64 bit mask fixed it,
and I added a 32-bit fallback to ensure support for 32-bit systems.

>
> > +static int __coherent_heap_register(struct reserved_mem *rmem)
> > +{
> > +     struct dma_heap_export_info exp_info;
> > +     struct coherent_heap *coh_heap;
> > +     struct device *heap_dev;
> > +     int ret;
> > +
> > +     if (!rmem || !rmem->name)
> > +             return -EINVAL;
> > +
> > +     coh_heap = kzalloc_obj(*coh_heap);
> > +     if (!coh_heap)
> > +             return -ENOMEM;
> > +
> > +     coh_heap->rmem = rmem;
> > +     coh_heap->name = kstrdup(rmem->name, GFP_KERNEL);
> > +     if (!coh_heap->name) {
> > +             ret = -ENOMEM;
> > +             goto free_coherent_heap;
> > +     }
> > +
> > +     exp_info.name = coh_heap->name;
> > +     exp_info.ops = &coherent_heap_ops;
> > +     exp_info.priv = coh_heap;
> > +
> > +     coh_heap->heap = dma_heap_create(&exp_info);
> > +     if (IS_ERR(coh_heap->heap)) {
> > +             ret = PTR_ERR(coh_heap->heap);
> > +             goto free_name;
> > +     }
> > +
> > +     heap_dev = dma_heap_get_dev(coh_heap->heap);
> > +     ret = coherent_heap_init_dma_mask(heap_dev);
> > +     if (ret) {
> > +             pr_err("coherent_heap: failed to set DMA mask (%d)\n", ret);
> > +             goto destroy_heap;
> > +     }
> > +
> > +     ret = of_reserved_mem_device_init_with_mem(heap_dev, rmem);
> > +     if (ret) {
> > +             pr_err("coherent_heap: failed to initialize memory (%d)\n", ret);
> > +             goto destroy_heap;
> > +     }
> > +
> > +     ret = dma_heap_register(coh_heap->heap);
> > +     if (ret) {
> > +             pr_err("coherent_heap: failed to register heap (%d)\n", ret);
> > +             goto destroy_heap;
> > +     }
>
> I guess it's more of a comment about your previous patch, but it's not
> clear to me why you needed to split dma_heap_add into dma_heap_create /
> _register. Can you expand a bit?

So first I tried to just use dma_heap_add() and then use the heap_dev
afterward to call of_reserved_mem_device_init_with_mem(), but if that
call failed, the error path required some kind dma_heap_remove()
function as the heap was already registered by then.

In the CMA heap for example, dma_heap_add() is invoked at the end of
the `init` function. Therefore, you do not have this issue, if it
failed it means the heap was not added and you just need to clean
everything else.

However, performing a remove() does not sound like something that can
be done safely. I've spent some time thinking on alternatives, but
splitting felt the best pattern.

This way I can:
1. Create the device
2. Call of_reserved_mem_device_init_with_mem
3. Register the heap

This places registration at the end, making every error path and
cleanup easy to handle.

Also, the `dma_heap_add()` code already seemed to handle these two
parts/phases implicitly with device_create(), so splitting felt
architecturally sound.

>
> > diff --git a/include/linux/dma-heap.h b/include/linux/dma-heap.h
> > index 1b0ea43ba66c3..77e6cb66ffce1 100644
> > --- a/include/linux/dma-heap.h
> > +++ b/include/linux/dma-heap.h
> > @@ -9,10 +9,12 @@
> >  #ifndef _DMA_HEAPS_H
> >  #define _DMA_HEAPS_H
> >
> > +#include <linux/errno.h>
> >  #include <linux/types.h>
> >
> >  struct dma_heap;
> >  struct device;
> > +struct reserved_mem;
> >
> >  /**
> >   * struct dma_heap_ops - ops to operate on a given heap
> > @@ -53,4 +55,13 @@ 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 */
>
> Do you still need that now that you switched to an iterator-like
> function?
>
> > diff --git a/include/linux/dma-map-ops.h b/include/linux/dma-map-ops.h
> > index 60b63756df821..c87e5e44e5383 100644
> > --- a/include/linux/dma-map-ops.h
> > +++ b/include/linux/dma-map-ops.h
> > @@ -12,6 +12,7 @@
> >
> >  struct cma;
> >  struct iommu_ops;
> > +struct reserved_mem;
> >
> >  struct dma_map_ops {
> >       void *(*alloc)(struct device *dev, size_t size,
> > @@ -161,6 +162,7 @@ int dma_alloc_from_dev_coherent(struct device *dev, ssize_t size,
> >  int dma_release_from_dev_coherent(struct device *dev, int order, void *vaddr);
> >  int dma_mmap_from_dev_coherent(struct device *dev, struct vm_area_struct *vma,
> >               void *cpu_addr, size_t size, int *ret);
> > +struct reserved_mem *dma_coherent_get_reserved_region(unsigned int idx);
> >  #else
> >  static inline int dma_declare_coherent_memory(struct device *dev,
> >               phys_addr_t phys_addr, dma_addr_t device_addr, size_t size)
> > @@ -172,6 +174,11 @@ static inline int dma_declare_coherent_memory(struct device *dev,
> >  #define dma_release_from_dev_coherent(dev, order, vaddr) (0)
> >  #define dma_mmap_from_dev_coherent(dev, vma, vaddr, order, ret) (0)
> >  static inline void dma_release_coherent_memory(struct device *dev) { }
> > +static inline
> > +struct reserved_mem *dma_coherent_get_reserved_region(unsigned int idx)
> > +{
> > +     return NULL;
> > +}
> >  #endif /* CONFIG_DMA_DECLARE_COHERENT */
> >
> >  #ifdef CONFIG_DMA_GLOBAL_POOL
>
> To preserve bisectability, you shouldn't do it that way. Introduce this
> function into a preliminary patch, and then use it in this one.
>
> Maxime