[Mesa-dev] [PATCH] vl/dri3: handle the case of different GPU

[Michel Dänzer]

On 08/09/16 05:05 PM, Christian König wrote:
> Am 08.09.2016 um 08:23 schrieb Michel Dänzer:
>> On 08/09/16 01:13 PM, Nayan Deshmukh wrote:
>>> On Thu, Sep 8, 2016 at 9:03 AM, Michel Dänzer <michel at daenzer.net
>>> <mailto:michel at daenzer.net>> wrote:
>>>      On 08/09/16 02:48 AM, Nayan Deshmukh wrote:
>>>      > use a linear buffer in case of back buffer
>>>      >
>>>      > Signed-off-by: Nayan Deshmukh <nayan26deshmukh at gmail.com
>>> <mailto:nayan26deshmukh at gmail.com>>
>>>
>>>      [...]
>>>
>>>      > @@ -226,8 +227,13 @@ dri3_alloc_back_buffer(struct
>>> vl_dri3_screen *scrn)
>>>      >        goto close_fd;
>>>      >
>>>      >     memset(&templ, 0, sizeof(templ));
>>>      > +   if (scrn->is_different_gpu)
>>>      > +   templ.bind = PIPE_BIND_RENDER_TARGET |
>>> PIPE_BIND_SAMPLER_VIEW |
>>>      > +                PIPE_BIND_SCANOUT | PIPE_BIND_SHARED |
>>> PIPE_BIND_LINEAR;
>>>      > +   else
>>>      >     templ.bind = PIPE_BIND_RENDER_TARGET |
>>> PIPE_BIND_SAMPLER_VIEW |
>>>      >                  PIPE_BIND_SCANOUT | PIPE_BIND_SHARED;
>>>
>>>      The indentation is wrong. Also, it would be better to make it
>>> something
>>>      like this:
>>>
>>>         templ.bind = PIPE_BIND_RENDER_TARGET | PIPE_BIND_SAMPLER_VIEW |
>>>                      PIPE_BIND_SCANOUT | PIPE_BIND_SHARED;
>>>         if (scrn->is_different_gpu)
>>>            templ.bind |= PIPE_BIND_LINEAR;
>>>
>>>
>>>      However, as we discussed before, for various reasons it would
>>> probably
>>>      be better to create separate linear buffers instead of making all
>>>      buffers linear.
>>>
>>> So should I maintain a single linear buffer and copy the back buffer to
>>> it before sending it via the present extension?
>> It's better to create one linear buffer corresponding to each non-linear
>> buffer with contents to be presented. Otherwise the rendering GPU may
>> overwrite the linear buffer contents while the presentation GPU is still
>> reading from it, resulting in tearing-like artifacts.
> 
> That approach isn't necessary. VDPAU has functions to query if an output
> surface is still displayed or not.
> 
> If the application starts to render into a buffer while it is still
> being displayed tearing-like artifacts are the expected result.

You're talking about the buffers exposed to applications via VDAPU. I
was talking about using a single separate linear buffer which would be
used for presentation of all VDPAU buffers. There's no way for the
application to know when that's idle.


> Additional to that I've made the VDPAU output surfaces linear a while
> ago anyway, because it showed that tiling actually wasn't beneficial in
> this use case (a single quad rendered over the whole texture).

That's fine as long as the buffers are in VRAM, but when they're pinned
to GTT for sharing between GPUs, rendering to them with the 3D engine
results in bad PCIe bandwidth utilization, as Marek explained recently.
So even if the original buffers are already linear, it's better to keep
those in VRAM and use separate buffers for sharing between GPUs.


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