[PATCH v3 02/15] drm: Centralize format information

[Ville Syrjälä]

On Thu, Jun 09, 2016 at 02:40:28PM +0200, Daniel Vetter wrote:
> On Thu, Jun 09, 2016 at 03:23:17PM +0300, Ville Syrjälä wrote:
> > On Thu, Jun 09, 2016 at 10:52:23AM +0200, Daniel Vetter wrote:
> > > On Thu, Jun 09, 2016 at 02:32:06AM +0300, Laurent Pinchart wrote:
> > > > Various pieces of information about DRM formats (number of planes, color
> > > > depth, chroma subsampling, ...) are scattered across different helper
> > > > functions in the DRM core. Callers of those functions often need to
> > > > access more than a single parameter of the format, leading to
> > > > inefficiencies due to multiple lookups.
> > > > 
> > > > Centralize all format information in a data structure and create a
> > > > function to look up information based on the format 4CC.
> > > > 
> > > > Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > > > ---
> > > >  drivers/gpu/drm/drm_fourcc.c | 84 ++++++++++++++++++++++++++++++++++++++++++++
> > > >  include/drm/drm_fourcc.h     | 19 ++++++++++
> > > >  2 files changed, 103 insertions(+)
> > > > 
> > > > diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
> > > > index 0645c85d5f95..47b9abd743be 100644
> > > > --- a/drivers/gpu/drm/drm_fourcc.c
> > > > +++ b/drivers/gpu/drm/drm_fourcc.c
> > > > @@ -62,6 +62,90 @@ const char *drm_get_format_name(uint32_t format)
> > > >  EXPORT_SYMBOL(drm_get_format_name);
> > > >  
> > > >  /**
> > > > + * drm_format_info - query information for a given format
> > > > + * @format: pixel format (DRM_FORMAT_*)
> > > > + *
> > > > + * Returns:
> > > > + * The instance of struct drm_format_info that describes the pixel format, or
> > > > + * NULL if the format is unsupported.
> > > > + */
> > > > +const struct drm_format_info *drm_format_info(u32 format)
> > > 
> > > Bikeshed on your pixel format description table. I think the approach I've
> > > seen in gallium/mesa to describe pixel formats is a lot more generic, and
> > > we might as well adopt it when we change. Idea is to have a block size
> > > measure in pixels (both h and v), and then cpp is bytes_per_block. This is
> > > essentially what you have with hsub and vsub already, except confusing
> > > names, more ill-defined (since it only makes sense for yuv) and less
> > > generic. A few examples:
> > 
> > I think you have your confusion backwards. Calling something a block in
> > planar formats would be more confusing. The only thing that really
> > matters is the relative position of the samples between the planes.
> > So there really is no "block" in there.
> 
> Atm U/V planes have a cpp of 1, which is definitely not true. There's much
> less than 1 byte per visible pixel in those planes. And that's the part
> that annoys me.

That's exactly as it should be. The cpp value isn't some average thing
for the whole, it's per-plane.

> 
> block here is an entirely free-standing concept that just means "group of
> pixels over which the bytes-per-group is counted in each group". It's a
> concept stolen from gallium and makes a lot more sense when talking about
> compressed formats. But I think it also makes sense when talking about yuv
> formats.

For packed YUV formats the usual term I've heard is macropixel, and there
it does make sense. I could live with calling it a block. So I guess eg.
for 422 packed formats we'd have h_block_size=2 v_block_size=1, and
bytes_per_block=4.

For planar formats, each plane should be considered individually,
and trying to come up with some kind of cpp value etc. for the whole
thing is pointless. I think eg. with for all the NVxx formats the
chroma plane should have h_block_size=2 v_block_size=1 bytes_per_block=2
regardless the sub-sampling factor.

So if we start using the block size concept, I think that too should be
per-plane. Anything else will just get really confusing.

-- 
Ville Syrjälä
Intel OTC