Re: [PATCH] staging: ion: ion_cma_heap: Don't directly use dma_common_get_sgtable

[Robin Murphy <robin.murphy@arm.com>]

Hi Laura,

On 17/07/15 17:50, Laura Abbott wrote:
> On 07/17/2015 08:21 AM, Robin Murphy wrote:
>> Hi Tixy,
>>
>> On 17/07/15 12:01, Jon Medhurst (Tixy) wrote:
>>> Use dma_get_sgtable rather than dma_common_get_sgtable so a device's
>>> dma_ops aren't bypassed. This is essential in situations where a device
>>> uses an IOMMU and the physical memory is not contiguous (as the common
>>> function assumes).
>>>
>>> Signed-off-by: Jon Medhurst <tixy@linaro.org>
>>
>> The lack of obvious users of this code makes it hard to tell if "dev"
>>   hereis always the right, real, device pointer and never null or some
>>   dummy device with the wrong dma_ops, but the rest of the calls in this
>>   file are to the proper DMA API interface so at least this patch definitely
>>   makes things less wrong in that respect.
>>
>
>
> Ion currently lacks any standard way to set up heaps and associate a device
> with a heap. This means it's basically a free for all for what devices get
> associated (getting something mainlined might help...). I agree that using
> the proper DMA APIs is a step in the right direction.

I suspected as much, thanks for the confirmation.

>
>> Reviewed-by: Robin Murphy <robin.murphy@arm.com>
>>
>>> ---
>>>
>>> This also begs the question as to what happens if the memory region _is_
>>> contiguous but is in highmem or an ioremapped region. Should a device
>>> always provide dma_ops for that case? Because I believe the current
>>> implementation of dma_common_get_sgtable won't work for those as it uses
>>> virt_to_page.
>>>
>>> I see that this point has been raised before [1] by Zeng Tao, and I
>>> myself have been given a different fix to apply to a Linaro kernel tree.
>>> However, both solutions looked wrong to me as they treat a dma_addr_t as
>>> a physical address, so should at least be using dma_to_phys.
>>> So, should we fix dma_common_get_sgtable or mandate that the device
>>> has dma_ops? The latter seems to be implied by the commit message which
>>> introduced the function:
>>>
>>>           This patch provides a generic implementation based on
>>>           virt_to_page() call. Architectures which require more
>>>           sophisticated translation might provide their own get_sgtable()
>>>           methods.
>>
>> Given that we're largely here due to having poked this on arm64 systems,
>>   I'm inclined to think that implementing our own get_sgtable as per arch/arm
>>   is the right course of action. Since a lot of architectures using
>>   dma_common_get_sgtable don't even implement dma_to_phys, I don't think it
>>   would be right to try complicating the common code for a case that seems to
>>   be all but common. I can spin an arm64 patch if you like.
>>
>
> This would be hit on any system that has non-coherent DMA or highmem. I'm
> not sure I agree this isn't a common case. How many of the other
> architectures are actually using the dma_get_sgtable and would have the
> potential to find a problem?

This appears to be pretty much exclusively a graphics/video thing. 
Surveying in-tree callers (other than Ion) gives DRM, V4L, and a couple 
of specific ARM SoC drivers - my hunch is that none of those see much 
action on the likes of Blackfin and 68k.

That said, going through the git logs, the primary purpose of 
dma_common_get_sgtable would appear to be not breaking allmodconfig 
builds on architectures other than ARM. Thus I'm not really sure which 
is the least worst option - having "common" code which doesn't actually 
represent the common use case, or adding bogus dma_to_phys definitions 
to loads of architectures that don't even have proper DMA mapping 
implementations for the sake of some code they don't even use...

Robin.

>
> Thanks,
> Laura
>