[Mesa-dev] [PATCH] Revert "st/vdpau: use linear layout for output surfaces"

[Alex Deucher]

On Fri, Sep 16, 2016 at 4:03 AM, Christian König
<deathsimple at vodafone.de> wrote:
> Am 16.09.2016 um 09:50 schrieb Michel Dänzer:
>>
>> On 16/09/16 04:33 PM, Christian König wrote:
>>>
>>> Am 15.09.2016 um 21:43 schrieb Dave Airlie:
>>>>
>>>> On 15 September 2016 at 17:43, Christian König
>>>> <deathsimple at vodafone.de> wrote:
>>>>>
>>>>> Am 15.09.2016 um 06:00 schrieb Ilia Mirkin:
>>>>>>
>>>>>> On Wed, Sep 14, 2016 at 11:58 PM, Dave Airlie <airlied at gmail.com>
>>>>>> wrote:
>>>>>>>
>>>>>>> From: Dave Airlie <airlied at redhat.com>
>>>>>>>
>>>>>>> This reverts commit d180de35320eafa3df3d76f0e82b332656530126.
>>>>>>>
>>>>>>> This is a radeon specific hack that causes problems on nouveau
>>>>>>> when combined with the SHARED flag later. If radeonsi needs a fix
>>>>>>> for this, please fix it in the driver.
>>>>>
>>>>> Actually it isn't radeon specific. Using linear surfaces for this makes
>>>>> sense because tilling isn't beneficial and the surfaces can
>>>>> potentially be
>>>>> shared with other GPUs using the VDPAU OpenGL interop.
>>>>
>>>> Who says tiling isn't beneficial though? Maybe on other GPUs tiling
>>>> might be, it
>>>> still seems like a radeon centric view to me.
>>>
>>> Tiling helps with the memory throughput because it makes pixels which
>>> are rendered together appear near to each other in the memory layout as
>>> well.
>>>
>>> Since multimedia as well as compute applications usually always render
>>> to the whole texture/array/matrix it usually makes no sense at all to
>>> enable it for those tasks.
>>
>> Are you sure about that? Tiling also affects the order of memory accesses,
>> which could affect performance even when all pixels of a surface are
>> written.
>
>
> I can't 100% rule that out, but the hardware I've encountered so far orders
> the execution by the memory layout of the output buffer which is written to
> maximize throughput.
>
> On the other hand I never double checked how the MC on AMD hardware really
> works in the documentation, just toke some measurements and it didn't seemed
> to be beneficial at all.
>
> tiling/shuffling can actually hurt performance quite a bit when the whole
> buffer is written and the execution order doesn't follow the memory pattern,
> so I think we would have noticed that.

Tiling also hurts with GTT buffers due to the way pcie transactions work.

Alex

>
> Where tilling could help quite a bit is with the video surfaces, cause the
> deinterlacing shaders need to read them quite extensively, but unfortunately
> our decoding hardware can't fill it in the way it is needed.
>
> Regards,
> Christian.
>
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