Re: [PATCH v3 06/11] mm/cma: Allow dynamically creating CMA areas

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/9/26 07:56, Marek Szyprowski wrote:
> On 08.07.2026 10:35, David Hildenbrand (Arm) wrote:
>> On 7/7/26 12:02, Marek Szyprowski wrote:
>>>
>>> Well, the technical reason for not creating cma regions dynamically at
>>> runtime is that on some architectures (like 32bit ARM) the early fixup
>>> for the region is needed to make it functional for DMA.
>> Can you point me at the code that does that? Thanks!
> Check dma_contiguous_early_fixup() and dma_contiguous_remap() in 
> arch/arm/mm/dma-mapping.c. Those functions ensures that the CPU mappings for
> the CMA reserved region in linear map are remapped with 4k pages instead
> of the 1M sections, so later, it will be possible to alter the mappings and
> change them to coherent when needed (altering 1M sections is not possible,
> because each process has it's own level-1 array even for the kernel linear
> mapping).
> 

Thanks!

> 
> 
> However, in the use case in this patchset the reserved region is only shared
> with buddy allocator by using the CMA infrastructure, not registered to the
> regular DMA-mapping API, so it would work fine. 

Yes, exactly.

> I'm not convinced that this
> is the right API to use for this though.

If it's supposed to be special DMA memory, then indeed, it would be bypassing
the DMA layer.

-- 
Cheers,

David