[PATCH 3/4] drm: Add drm_format_{horz, vert}_chroma_subsampling() utility functions

[Roland Scheidegger]

Am 12.04.2012 14:23, schrieb Ville Syrjälä:
> On Wed, Apr 11, 2012 at 08:53:08PM +0200, Roland Scheidegger wrote:
>> Am 05.04.2012 20:35, schrieb ville.syrjala at linux.intel.com:
>>> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
>>>
>>> These functions return the chroma subsampling factors for the specified
>>> pixel format.
>> Hmm not really related but looking at it this reminds me these formats
>> always look a bit underspecified wrt chroma subsample positions. Are
>> these fixed, undefined, or what (even mpeg1 and mpeg2 differ there, and
>> let's not even talk about the big mess that interlaced mpeg2 is)?
> 
> My thinking is that the sample positions can be configured through
> some other means. It doesn't affect the in memory layout at all so
> tying it to the format isn't necessary.
> 
> This also depends on the hardware. On some hardware you can configure
> it quite freely, but on some you may be left with just the one option,
> which is often not even documented.
> 
> I sent another patch [1] some time ago which provides a helper function
> for drivers dealing with good hardware.
> 
> [1] http://lists.freedesktop.org/archives/dri-devel/2011-December/017585.html
> 

I guess that makes sense. As long as those "other means" are available.
Not like with xv where I think drivers just default to what is probably
the most likely subsample positions, so assuming mpeg2/4 progressive or
something (which gets mpeg1 slightly wrong), if they even bother to care
about such subtleties :-).

Roland