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

[Michel Dänzer]

On 23/09/16 09:09 PM, Daniel Vetter wrote:
> On Fri, Sep 23, 2016 at 07:00:25PM +0900, Michel Dänzer wrote:
>> On 22/09/16 10:22 PM, Christian König wrote:
>>> Am 22.09.2016 um 15:05 schrieb Daniel Vetter:
>>>> 
>>>> 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 ;-)
>>>
>>> Well as far as I can see there is no way I can fix amdgpu in this case.
>>>
>>> The handling clearly needs to be changed on the receiving side of the
>>> reservation objects if I don't completely want to disable concurrent
>>> access to BOs in amdgpu.
>>
>> Anyway, we need a solution for this between radeon and amdgpu, and I
>> don't think a solution which involves those drivers using reservation
>> object semantics between them which are different from all other drivers
>> is a good idea.
> 
> Afaik there's also amd+intel machines out there,

Sure, what I meant was that even if we didn't care about those (which we
do), we'd still need a solution between our own drivers.


> so really the only option is to either fix amdgpu to correctly set
> exclusive fences on shared buffers (with the help of userspace hints).
> Or change all the existing drivers.

I got some fresh perspective on the problem while taking a walk, and I'm
now fairly convinced that neither amdgpu userspace nor other drivers
need to be modified:

It occurred to me that all the information we need is already there in
the exclusive and shared fences set by amdgpu. We just need to use it
differently to match the expectations of other drivers.

We should be able to set some sort of "pseudo" fence as the exclusive
fence of the reservation object. When we are asked by another driver to
wait for this fence to signal, we take the current "real" exclusive
fence (which we can keep track of e.g. in our BO struct) and shared
fences, and wait for all of those to signal; then we mark the "pseudo"
exclusive fence as signalled.

Am I missing anything which would prevent this from working?


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer