[RFC] arm: DMA-API contiguous cacheable memory

[Catalin Marinas]

On Mon, May 18, 2015 at 10:56:06PM +0200, Lorenzo Nava wrote:
> it's been a while since I've started working with DMA on ARM processor
> for a smart camera project. Typically the requirements is to have a
> large memory area which can be accessed by both DMA and user. I've
> already noticed that many people wonder about which would be the best
> way to have data received from DMA mapped in user space and, more
> important, mapped in a cacheable area of memory. Having a memory
> mapped region which is cacheable is very important if the user must
> access the data and make some sort of processing on that.
> My question is: why don't we introduce a function in the DMA-API
> interface for ARM processors which allows to allocate a contiguous and
> cacheable area of memory (> 4MB)?
> This new function can take advantage of the CMA mechanism as
> dma_alloc_coherent() function does, but using different PTE attribute
> for the allocated pages. Basically making a function similar to
> arm_dma_alloc() and set the attributes differently would do the trick:
> 
> pgprot_t prot = __pgprot_modify(prot, L_PTE_MT_MASK,
>         L_PTE_MT_WRITEALLOC | L_PTE_XN)

We already have a way to specify whether a device is coherent via the
"dma-coherent" DT property. This allows the correct dma_map_ops to be
set for a device. For cache coherent devices, the
arm_coherent_dma_alloc() and __dma_alloc() should return cacheable
memory.

However, looking at the code, it seems that __dma_alloc() does not use
the CMA when is_coherent == true, though you would hit a limit on the
number of pages that can be allocated.

As for mmap'ing to user space, there is arm_dma_mmap(). This one sets
the vm_page_prot to what __get_dma_pgprot() returns which is always
non-cacheable.

I haven't checked the history cache coherent DMA support on arm but I
think some of the above can be changed. As an example, on arm64
__dma_alloc() allocates from CMA independent of whether the device is
coherent or not. Also __get_dma_pgprot() returns cacheable attributes
for coherent devices, which in turn allows cacheable user mapping of
such buffers. You don't really need to implement additional functions,
just tweaks to the existing ones.

Patches welcome ;)

-- 
Catalin