[Mesa-dev] Update: UVD status on loongson 3a platform

[Jerome Glisse]

On Thu, Sep 05, 2013 at 03:29:52PM -0400, Jerome Glisse wrote:
> On Thu, Sep 05, 2013 at 10:14:32PM +0800, Chen Jie wrote:
> > Hi all,
> > 
> > This thread is about
> > http://lists.freedesktop.org/archives/dri-devel/2013-April/037598.html.
> > 
> > We recently find some interesting thing about UVD based playback on
> > loongson 3a plaform, and also find a way to fix the problem.
> > 
> > First, we find memcpy in [mesa]src/gallium/drivers/radeon/radeon_uvd.c
> > caused the problem:
> > * If memcpy is implemented though 16B or 8B load/store instructions,
> > it will normally caused video mosaic. When insert a memcmp after the
> > copying code in memcpy, it will report the src and dest are not equal.
> > * If memcpy use 1B load/store instructions only, the memcmp after the
> > copying code reports equal.
> > 
> > Then we find the following changeset fixs out problem:
> > 
> > diff --git a/src/gallium/drivers/radeon/radeon_uvd.c
> > b/src/gallium/drivers/radeon/radeon_uvd.c
> > index 2f98de2..f9599b6 100644
> > --- a/src/gallium/drivers/radeon/radeon_uvd.c
> > +++ b/src/gallium/drivers/radeon/radeon_uvd.c
> > @@ -162,7 +162,7 @@ static bool create_buffer(struct ruvd_decoder *dec,
> >    unsigned size)
> >  {
> >   buffer->buf = dec->ws->buffer_create(dec->ws, size, 4096, false,
> > -     RADEON_DOMAIN_GTT | RADEON_DOMAIN_VRAM);
> > +     RADEON_DOMAIN_GTT);
> >   if (!buffer->buf)
> >   return false;
> > 
> > The VRAM is mapped to an uncached area in out platform, so, my
> > question is what could go wrong while using  >4B load/store
> > instructions in UVD workflow? Any idea?
> > 
> 
> How do you map the VRAM into user process mapping ? ie do you have
> something like Intel PAT or something like MTRR or something else.
> 
> In other word, can you map into process address space a region of
> io memory (GPU VRAM in this case) and mark it as uncached so that
> none of the access to it goes through CPU cache.
> 
> Cheers,
> Jerome

Also it might be that you can't do write combining on your platform,
which would be a major drawback as it's assume by radeon userspace.
I would need to check the pcie specification, but write combining is
probably not mandatory meaning that your architecture might not have
it. This would explain why only memset with byte size copy works.

Don't think there is any easy way to work around that.

Cheers,
Jerome