[Mesa-dev] [PATCH] st/va: add mpeg4 startcode workaround

[Christian König]

On 06.11.2015 20:28, Ilia Mirkin wrote:
> On Fri, Nov 6, 2015 at 2:15 PM, Christian König <deathsimple at vodafone.de> wrote:
>> On 06.11.2015 20:10, Ilia Mirkin wrote:
>>> On Fri, Nov 6, 2015 at 1:48 PM, Zhang, Boyuan <Boyuan.Zhang at amd.com>
>>> wrote:
>>>> Hi Emil,
>>>>
>>>> Please see the following information about this patch.
>>>>
>>>> - Issue: For Mpeg4, the VOP and GOV headers were truncated. With the
>>>> existing workaround in st/va, playback shows massive corruptions.
>>>> - This Patch: Provide another way to get the truncated headers back.
>>>> Massive corruptions are gone with this patch. At the same time, add an
>>>> environmental variable to allow user to decide whether to use this patch.
>>> Why would the user not want to use this? Sounds like a correctness
>>> fix, no? Or is it some thing that a hypothetical gallium driver might
>>> not need but the radeon uvd-based ones do? In that case it should be
>>> behind a PIPE_VIDEO_CAP_bla (sorry, I'm still not too clear on what
>>> "bla" is here...)
>>
>> The problem is that this is a rather extreme hack.
>>
>> As you probably knew VA-API didn't correctly specify which start code should
>> be included and which shouldn't for MPEG-4. This is an issue for AMD as well
>> as NVidia hardware and pretty much everybody which sticks close to an
>> elementary stream.
>>
>> What we do in this hack is just searching the bytes *before* the pointer and
>> size we got from the application for the stuff that's missing. E.g. we
>> access memory the application didn't told us to access.
>>
>> This is rather speculative, but works surprisingly well with a lot of
>> applications.
> Hm, that is a little dodgy indeed. But making user-selectable options
> (provided via env var) for correct decoding... doesn't seem ideal
> either. Is there some "correct" way to resolve this without changing
> the va api?

Unfortunately no. I wasn't involved in everything but we had a couple of 
people working on this which have more knowledge about MPEG-4 part 2 
than me.

A couple of month back somebody from a different team at AMD even tried 
to convince Intel to fix this, but as far as I know without success.

The over all conclusion is that the interface definition of VA-API for 
MPEG-4 part 2 is just a bloody mess.

Regards,
Christian.

>
>    -ilia