linux-kernel - Re: [PATCH] drm: add drm_format_alpha_bits

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Date:   Fri, 12 Jan 2018 16:11:38 +0000
From:   Ayan Halder <ayan.halder@....com>
To:     Ville Syrj?l? <ville.syrjala@...ux.intel.com>
Cc:     liviu.dudau@....com, brian.starkey@....com,
        daniel.vetter@...el.com, jani.nikula@...ux.intel.com,
        seanpaul@...omium.org, airlied@...ux.ie,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        maxime.ripard@...e-electrons.com, nd@....com
Subject: Re: [PATCH] drm: add drm_format_alpha_bits

On Fri, Jan 12, 2018 at 05:53:33PM +0200, Ville Syrj?l? wrote:
> 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@....com>
> > > > Reviewed-by: Liviu Dudau <liviu.dudau@....com>
> > > > ---
> > > >  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.
> 
> No point in adding the function then IMO.
We have helper functions for the other fields so I followed the same
and added the helper function for the new field ('alpha') too.
> 
> > 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. 
> 
> I think that's OK. Not sure if it really helps much though since you
> generally have to configure other things based on the format as well, so
> I would expect almost everyone will end up with some kind of
> swicth/table to configure the hardware based on the format. Based on a
> quick look this doesn't really help i915 at least.

My understanding was based on the fact that alpha blending could be a
common feature for all the drivers (I might be wrong). So, if we keep
the number of 'alpha' bits in 'drm_format_info', it would be useful to
fetch this info similar to the existing fields.
> > > 
> > > > diff --git a/include/drm/drm_fourcc.h b/include/drm/drm_fourcc.h
> > > > index 6942e84..5513510 100644
> > > > --- a/include/drm/drm_fourcc.h
> > > > +++ b/include/drm/drm_fourcc.h
> > > > @@ -38,6 +38,7 @@ struct drm_mode_fb_cmd2;
> > > >   * @cpp: Number of bytes per pixel (per plane)
> > > >   * @hsub: Horizontal chroma subsampling factor
> > > >   * @vsub: Vertical chroma subsampling factor
> > > > + * @alpha: Number of bits per pixel representing alpha
> > > >   */
> > > >  struct drm_format_info {
> > > >  	u32 format;
> > > > @@ -46,6 +47,7 @@ struct drm_format_info {
> > > >  	u8 cpp[3];
> > > >  	u8 hsub;
> > > >  	u8 vsub;
> > > > +	u8 alpha;
> > > >  };
> > > >  
> > > >  /**
> > > > @@ -57,6 +59,7 @@ struct drm_format_name_buf {
> > > >  };
> > > >  
> > > >  const struct drm_format_info *__drm_format_info(u32 format);
> > > > +int drm_format_alpha_bits(u32 format);
> > > >  const struct drm_format_info *drm_format_info(u32 format);
> > > >  const struct drm_format_info *
> > > >  drm_get_format_info(struct drm_device *dev,
> > > > -- 
> > > > 2.7.4
> > > > 
> > > > _______________________________________________
> > > > dri-devel mailing list
> > > > dri-devel@...ts.freedesktop.org
> > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> > > 
> > > -- 
> > > Ville Syrj??l??
> > > Intel OTC
> > > _______________________________________________
> > > dri-devel mailing list
> > > dri-devel@...ts.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> 
> -- 
> Ville Syrj?l?
> Intel OTC

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