Re: [PATCH] drm: add drm_format_alpha_bits

[Ayan Halder]

On Fri, Jan 12, 2018 at 04:51:16PM +0100, Maxime Ripard wrote:
> Hi,
> 
> On Fri, Jan 12, 2018 at 03:43:49PM +0000, Ayan Halder wrote:
> > On Fri, Jan 12, 2018 at 04:28:34PM +0200, Ville Syrj?l? wrote:
> > > On Fri, Jan 12, 2018 at 02:21:16PM +0000, Ayan Halder wrote:
> > > > drm_format_info does not describe the number of bits used for the alpha
> > > > channel. That information is useful in a central place like drm_fourcc.c
> > > > where it can be queried by the drivers that want to determine if 'alpha
> > > > blending' is to be enabled or not.
> > > > 
> > > > Signed-off-by: Ayan Kumar Halder <ayan.halder@xxxxxxx>
> > > > Reviewed-by: Liviu Dudau <liviu.dudau@xxxxxxx>
> > > > ---
> > > >  drivers/gpu/drm/drm_fourcc.c | 154 ++++++++++++++++++++++++-------------------
> > > >  include/drm/drm_fourcc.h     |   3 +
> > > >  2 files changed, 89 insertions(+), 68 deletions(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
> > > > index 9c0152d..073001b 100644
> > > > --- a/drivers/gpu/drm/drm_fourcc.c
> > > > +++ b/drivers/gpu/drm/drm_fourcc.c
> > > <snip>
> > > > @@ -348,3 +348,21 @@ int drm_format_plane_height(int height, uint32_t format, int plane)
> > > >  	return height / info->vsub;
> > > >  }
> > > >  EXPORT_SYMBOL(drm_format_plane_height);
> > > > +
> > > > +/**
> > > > + * drm_format_alpha_bits - get the number of bits per pixel
> > > > + * representing alpha for format
> > > > + * @format: pixel format (DRM_FORMAT_*)
> > > > + *
> > > > + * Returns:
> > > > + * The number of bits per pixel representing alpha used by the
> > > > + * specified pixel format.
> > > > + */
> > > > +int drm_format_alpha_bits(uint32_t format)
> > > > +{
> > > > +	const struct drm_format_info *info;
> > > > +
> > > > +	info = drm_format_info(format);
> > > > +	return info ? info->alpha : 0;
> > > > +}
> > > > +EXPORT_SYMBOL(drm_format_alpha_bits);
> > > 
> > > Do you have an actual use for this function somewhere?
> >
> > Currently, we do not have a usage for this function. We need 'alpha'
> > field for each entry in 'drm_format_info' so as to determine whether
> > to enable/disable alpha blending in Mali display processor for the
> > particular color format.
> 
> Do you need only the info that a particular channel has an alpha
> component, or do you also need the number of bits?

Currently, only the info whether alpha component is present in a
particular channel. I saw your patch and thought that instead of
returning true/false, it might be useful to capture the number of
alpha bits for each format.
> In the former case, then my serie sent earlier this week would help I
> guess.
> 
> Maxime
> 
> -- 
> Maxime Ripard, Free Electrons
> Embedded Linux and Kernel engineering
> http://free-electrons.com