On Thu, May 23, 2019 at 1:30 PM Daniel Vetter daniel@ffwll.ch wrote:
On Thu, May 23, 2019 at 1:21 PM Koenig, Christian Christian.Koenig@amd.com wrote:
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.
The dma-api doesn't flush device caches, ever. It might flush some iommu caches or some other bus cache somewhere in-between. So it also won't ever make sure that multiple devices don't trample on each another. For that you need something else (like reservation object, but I think that's not really followed outside of drm much).
The other bit is the coherent vs. non-coherent thing, which in the dma-api land just talks about whether cpu/device access need extra flushing or not. Now in practice that extra flushing is always only cpu side, i.e. will cpu writes/reads go through the cpu cache, and will device reads/writes snoop the cpu caches. That's (afaik at least, an in practice, not the abstract spec) the _only_ thing dma-api's cache maintenance does. For 0 copy that's all completely irrelevant, because as soon as you pick a mode where you need to do manual cache management you've screwed up, it's not 0-copy anymore really.
The other hilarious stuff is that on x86 we let userspace (at least with i915) do that cache management, so the kernel doesn't even have a clue. I think what we need in dma-buf (and dma-api people will scream about the "abstraction leak") is some notition about whether an importer should snoop or not (or if that device always uses non-snoop or snooped transactions). But that would shred the illusion the dma-api tries to keep up that all that matters is whether a mapping is coherent from the cpu's pov or not, and you can achieve coherence both with a cache cpu mapping + snooped transactions, or with wc cpu side and non-snooped transactions. Trying to add cache managment (which some dma-buf exporter do indeed attempt to) will be even worse.
Again, none of this is about preventing concurrent writes, or making sure device caches are flushed correctly around batches.
btw I just grepped for reservation_object, no one outside of drivers/gpu is using that. So for device access synchronization everyone else is relying on userspace ordering requests correctly on its own. Iirc v4l/media is pondering adding dma-fence support, but that's not going anywhere.
Also, for correctness reservations aren't needed, we allow explicit syncing userspace to managed dma-fences/drm_syncobj on their own, and they are allowed to get this wrong. -Daniel