Re: [PATCH] dma/contiguous: Fix broken build

[Marek Szyprowski <m.szyprowski@samsung.com>]

On 30.03.2026 13:41, Maxime Ripard wrote:
> On Mon, Mar 30, 2026 at 10:59:48AM +0200, Marek Szyprowski wrote:
>> On 30.03.2026 10:40, Maxime Ripard wrote:
>>> Commit 3a236f6a5cf2 ("dma: contiguous: Turn heap registration logic
>>> around") didn't remove one last call to dma_heap_cma_register_heap()
>>> that it removed, thus breaking the build.
>>>
>>> That last call is in dma_contiguous_reserve(), to handle the
>>> registration of the default CMA region heap instance if it's declared in
>>> the device tree.
>>>
>>> However, the default CMA region instance is already handled by
>>> retrieving it through dev_get_cma_area() in the CMA heap driver, so the
>>> call to dma_heap_cma_register_heap() wasn't actually needed.
>>>
>>> Let's remove this call, the now unused function definition, its now
>>> empty header, and all includes of this header.
>>>
>>> Fixes: 3a236f6a5cf2 ("dma: contiguous: Turn heap registration logic around")
>>> Reported-by: Mark Brown <broonie@kernel.org>
>>> Closes: https://lore.kernel.org/linux-next/acbjaDJ1a-YQC64d@sirena.co.uk/
>>> Signed-off-by: Maxime Ripard <mripard@kernel.org>
>>> ---
>>>  drivers/dma-buf/heaps/cma_heap.c  |  1 -
>>>  include/linux/dma-buf/heaps/cma.h | 16 ----------------
>>>  kernel/dma/contiguous.c           |  5 -----
>>>  3 files changed, 22 deletions(-)
>>>
>>> diff --git a/drivers/dma-buf/heaps/cma_heap.c b/drivers/dma-buf/heaps/cma_heap.c
>>> index f8a3d87f3ccee9630383ba28502eb40b10671cc2..cc517ac68a0bec0788abcb338c03f530d169013b 100644
>>> --- a/drivers/dma-buf/heaps/cma_heap.c
>>> +++ b/drivers/dma-buf/heaps/cma_heap.c
>>> @@ -12,11 +12,10 @@
>>>  
>>>  #define pr_fmt(fmt) "cma_heap: " fmt
>>>  
>>>  #include <linux/cma.h>
>>>  #include <linux/dma-buf.h>
>>> -#include <linux/dma-buf/heaps/cma.h>
>>>  #include <linux/dma-heap.h>
>>>  #include <linux/dma-map-ops.h>
>>>  #include <linux/err.h>
>>>  #include <linux/highmem.h>
>>>  #include <linux/io.h>
>>> diff --git a/include/linux/dma-buf/heaps/cma.h b/include/linux/dma-buf/heaps/cma.h
>>> deleted file mode 100644
>>> index e751479e21e703e24a5f799b4a7fc8bd0df3c1c4..0000000000000000000000000000000000000000
>>> --- a/include/linux/dma-buf/heaps/cma.h
>>> +++ /dev/null
>>> @@ -1,16 +0,0 @@
>>> -/* SPDX-License-Identifier: GPL-2.0 */
>>> -#ifndef DMA_BUF_HEAP_CMA_H_
>>> -#define DMA_BUF_HEAP_CMA_H_
>>> -
>>> -struct cma;
>>> -
>>> -#ifdef CONFIG_DMABUF_HEAPS_CMA
>>> -int dma_heap_cma_register_heap(struct cma *cma);
>>> -#else
>>> -static inline int dma_heap_cma_register_heap(struct cma *cma)
>>> -{
>>> -	return 0;
>>> -}
>>> -#endif // CONFIG_DMABUF_HEAPS_CMA
>>> -
>>> -#endif // DMA_BUF_HEAP_CMA_H_
>>> diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
>>> index ad50512d71d3088a73e4b1ac02d6e6122374888e..9fe001c712339f8388d3f40cca3dfff3f707fcbf 100644
>>> --- a/kernel/dma/contiguous.c
>>> +++ b/kernel/dma/contiguous.c
>>> @@ -40,11 +40,10 @@
>>>  #include <asm/page.h>
>>>  
>>>  #include <linux/memblock.h>
>>>  #include <linux/err.h>
>>>  #include <linux/sizes.h>
>>> -#include <linux/dma-buf/heaps/cma.h>
>>>  #include <linux/dma-map-ops.h>
>>>  #include <linux/cma.h>
>>>  #include <linux/nospec.h>
>>>  
>>>  #ifdef CONFIG_CMA_SIZE_MBYTES
>>> @@ -270,14 +269,10 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
>>>  						  selected_limit,
>>>  						  &dma_contiguous_default_area,
>>>  						  fixed);
>>>  		if (ret)
>>>  			return;
>>> -
>>> -		ret = dma_heap_cma_register_heap(dma_contiguous_default_area);
>>> -		if (ret)
>>> -			pr_warn("Couldn't register default CMA heap.");
>> After this change no dma-buf heap for the default CMA area is created if it has 
>> not been specified in device-tree. This might be especially a problem for the
>> non-dt systems.
> I don't think that's the case? My understanding is that
> dma_contiguous_reserve() is called by the arch code, and will create
> that region only if it hasn't be set by the DT (that excerpt above is
> run only if !dma_contiguous_default_area).
>
> However, for the DT case, dma_contiguous_default_area will be set by
> rmem_cma_setup() a bit below if linux,cma-default is set in the DT.
>
> dma_contiguous_reserved() is called (on arm64 at least) through
> bootmem_init(), called as part of setup_arch().
>
> rmem_cma_setup() is called through RESERVEDMEM_OF_DECLARE, so through
> __reserved_mem_init_node(), so, if we consider only the public
> functions, through:
>
>   * fdt_scan_reserved_mem_reg_nodes(), called by
>     unflatten_device_tree(), called right before bootmem_init() in
>     setup_arch();
>
>   * or fdt_scan_reserved_mem() and then
>     early_init_fdt_scan_reserved_mem(), called in arm64_memblock_init(),
>     itself called in setup_arch() earlier than both
>     unflatten_device_tree(), and bootmem_init().
>
> Thus, the DT case will run first and set dma_contiguous_default_area if
> relevant on that platform, and if it's not set, the non-DT case will set
> it up. Either way, dma_contiguous_default_area will be set.
>
> The CMA heap runs much later using a regular module_init. It will
> retrieve dma_contiguous_default_area through dev_get_cma_area(), and
> will create a heap instance for the default area.
>
> Am I misunderstanding something?

The default CMA area is also optional for DT platforms. 

If DT doesn't provide such, it will be created based on the kernel cmdline cma= parameter or the default configuration defined in the kernel .config (this is also true for the non-dt platforms). I know that for most Android GKI systems the default CMA area will be defined in DT, but I see no reason to disable dma-buf heap support for the default CMA area if it has been instantiated from kernel cmdline or .config.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland