Re: [PATCH 01/12] dma-buf: add dynamic caching of sg_table

[Koenig, Christian]

Am 22.05.19 um 20:30 schrieb Daniel Vetter:
> [SNIP]
>> Well, it seems you are making incorrect assumptions about the cache
>> maintenance of DMA-buf here.
>>
>> At least for all DRM devices I'm aware of mapping/unmapping an
>> attachment does *NOT* have any cache maintenance implications.
>>
>> E.g. the use case you describe above would certainly fail with amdgpu,
>> radeon, nouveau and i915 because mapping a DMA-buf doesn't stop the
>> exporter from reading/writing to that buffer (just the opposite actually).
>>
>> All of them assume perfectly coherent access to the underlying memory.
>> As far as I know there is no documented cache maintenance requirements
>> for DMA-buf.
> I think it is documented. It's just that on x86, we ignore that
> because the dma-api pretends there's never a need for cache flushing
> on x86, and that everything snoops the cpu caches. Which isn't true
> since over 20 ago when AGP happened. The actual rules for x86 dma-buf
> are very much ad-hoc (and we occasionally reapply some duct-tape when
> cacheline noise shows up somewhere).

Well I strongly disagree on this. Even on x86 at least AMD GPUs are also 
not fully coherent.

For example you have the texture cache and the HDP read/write cache. So 
when both amdgpu as well as i915 would write to the same buffer at the 
same time we would get a corrupted data as well.

The key point is that it is NOT DMA-buf in it's map/unmap call who is 
defining the coherency, but rather the reservation object and its 
attached dma_fence instances.

So for example as long as a exclusive reservation object fence is still 
not signaled I can't assume that all caches are flushed and so can't 
start with my own operation/access to the data in question.

Regards,
Christian.

>
> I've just filed this away as another instance of the dma-api not
> fitting gpus, and I think giving recent discussions that won't improve
> anytime soon. So we're stuck with essentially undefined dma-buf
> behaviour.
> -Daniel
>