[RFC] arm: DMA-API contiguous cacheable memory

[Arnd Bergmann]

On Wednesday 20 May 2015 00:05:54 Lorenzo Nava wrote:
> 
> On Tue, May 19, 2015 at 6:34 PM, Catalin Marinas
> <catalin.marinas at arm.com> wrote:
> > 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.

That is not what Lorenzo was asking about though.

> > 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.
>
> Thanks for the answer. I do agree with you on that: I'll take a look
> at arm64 code and I'll be glad to contribute with patches as soon as
> possible.
> 
> Anyway I'd like to focus on a different aspect: I think that this
> solution can manage cache coherent DMA, so devices which guarantees
> the coherency using cache snooping mechanism. However how can I manage
> devices which needs contiguous memory and don't guarantee cache
> coherency? If the device doesn't implement sg functionality, I can't
> allocate buffers which is greater than 4MB because I can't use neither
> dma_alloc_coherent() nor accessing directly to CMA (well, actually I
> can use dma_alloc_coherent(), but it sounds a little bit confusing).

So you have a device that is not cache-coherent, and you want to
allocate cacheable memory and manage coherency manually.

This is normally done using alloc_pages() and dma_map_single(),
but as you have realized, that does not use the CMA area.

> Do you think that dma_alloc_coherent() can be used as well with this
> type of devices? Do you think that a new dma_alloc_contiguous()
> function would help in this case?
> Maybe my interpretation of dma_alloc_coherent() is not correct, and
> the coherency can be managed using the dma_sync_single_for_* functions
> and it doesn't require hardware mechanism.

I believe dma_alloc_attrs is the interface you want, with attributes
DMA_ATTR_FORCE_CONTIGUOUS and DMA_ATTR_NON_CONSISTENT. I don't
know if that is already implemented on arm64, but this is something
that can definitely be done.

With that memory, you should be able to use the normal streaming
API (dma_sync_single_for_*). There is an older interface called
dma_alloc_noncoherent(), but that cannot be easily implemented on
ARM.

	Arnd