On 09.07.2026 17:59, Thierry Reding wrote:

On Thu, Jul 09, 2026 at 07:56:45AM +0200, Marek Szyprowski wrote:

quoted

On 08.07.2026 10:35, David Hildenbrand (Arm) wrote:

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On 7/7/26 12:02, Marek Szyprowski wrote:

quoted

On 01.07.2026 18:08, Thierry Reding wrote:

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From: Thierry Reding <redacted>

There is no technical reason why there should be a limited number of CMA
regions, so extract some code into helpers and use them to create extra
functions (cma_create() and cma_free()) that allow creating and freeing,
respectively, CMA regions dynamically at runtime.

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).



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

Are you saying you're not convinced that CMA is the right API to use for
this? Or something else?

I read this again and indeed CMA seems to be right solution. I only wonder
why do You want to create the CMA areas dynamically? Imho it would work if
You just create large enough CMA area on boot, what would automatically
share the memory with buddy allocator and then allocate dynamic VPR regions
with cma_alloc(), potentially unmapping or marking the allocated region as
reserved in linear kernel mapping to avoid any potential speculative access
to the protected memory.

In both cases You will probably won't need the DMA-mapping API on top of
it, although it might be even possible to partially use with by
registering custom dma_ops for the devices using the protected region
(assuming that it would support only DMA_ATTR_NO_KERNEL_MAPPING
allocations).

I certainly don't think we want to get the DMA-mapping API involved for
this because that always implies that we perform cache operations, which
we specifically don't want for this memory.

Thierry

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