[PATCH] drm/i915: Before pageflip, also wait for shared dmabuf fences.

[Daniel Vetter]

On Thu, Sep 22, 2016 at 2:44 PM, Christian König
<deathsimple at vodafone.de> wrote:
>> - explicit fencing: Userspace passes around distinct fence objects for
>> any work going on on the gpu. The kernel doesn't insert any stall of
>> it's own (except for moving buffer objects around ofc). This is what
>> Android. This also seems to be what amdgpu is doing within one
>> process/owner.
>
>
> No, that is clearly not my understanding of explicit fencing.
>
> Userspace doesn't necessarily need to pass around distinct fence objects
> with all of it's protocols and the kernel is still responsible for inserting
> stalls whenever an userspace protocol or application requires this
> semantics.
>
> Otherwise you will never be able to use explicit fencing on the Linux
> desktop with protocols like DRI2/DRI3.

This is about mixing them. Explicit fencing still means userspace has
an explicit piece, separate from buffers, (either sync_file fd, or a
driver-specific cookie, or similar).

> I would expect that every driver in the system waits for all fences of a
> reservation object as long as it isn't told otherwise by providing a
> distinct fence object with the IOCTL in question.

Yup agreed. This way if your explicitly-fencing driver reads a shared
buffer passed over a protocol that does implicit fencing (like
DRI2/3), then it will work.

The other interop direction is explicitly-fencing driver passes a
buffer to a consumer which expects implicit fencing. In that case you
must attach the right fence to the exclusive slot, but _only_ in that
case. Otherwise you end up stalling your explicitly-fencing userspace,
since implicit fencing doesn't allow more than 1 writer. For amdgpu
one possible way to implement this might be to count how many users a
dma-buf has, and if it's more than just the current context set the
exclusive fence. Or do an uabi revision and let userspace decide (or
at least overwrite it).

But the current approach in amdgpu_sync.c of declaring a fence as
exclusive after the fact (if owners don't match) just isn't how
reservation_object works. You can of course change that, but that
means you must change all drivers implementing support for implicit
fencing of dma-buf. Fixing amdgpu will be easier ;-)
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch