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Date:   Thu, 11 Aug 2022 18:11:40 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Daniel Vetter <daniel@ffwll.ch>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@linux.ie>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        Linux/m68k <linux-m68k@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Javier Martinez Canillas <javierm@redhat.com>
Subject: Re: [PATCH v3 01/10] drm/fourcc: Add drm_format_info_bpp() helper
Message-ID: <YvUqPL5l8/+XbvaQ@phenom.ffwll.local>
Mail-Followup-To: Geert Uytterhoeven <geert@linux-m68k.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@linux.ie>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        Linux/m68k <linux-m68k@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Javier Martinez Canillas <javierm@redhat.com>
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On Thu, Aug 11, 2022 at 09:59:39AM +0200, Geert Uytterhoeven wrote:
> Hi Daniel,
> 
> On Wed, Aug 10, 2022 at 5:59 PM Daniel Vetter <daniel@ffwll.ch> wrote:
> > On Fri, Jul 08, 2022 at 08:20:46PM +0200, Geert Uytterhoeven wrote:
> > > Add a helper to retrieve the actual number of bits per pixel for a
> > > plane, taking into account the number of characters and pixels per
> > > block for tiled formats.
> > >
> > > Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
> > > Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
> 
> > > --- a/drivers/gpu/drm/drm_fourcc.c
> > > +++ b/drivers/gpu/drm/drm_fourcc.c
> > > @@ -370,6 +370,25 @@ unsigned int drm_format_info_block_height(const struct drm_format_info *info,
> > >  }
> > >  EXPORT_SYMBOL(drm_format_info_block_height);
> > >
> > > +/**
> > > + * drm_format_info_bpp - number of bits per pixel
> > > + * @info: pixel format info
> > > + * @plane: plane index
> > > + *
> > > + * Returns:
> > > + * The actual number of bits per pixel, depending on the plane index.
> > > + */
> > > +unsigned int drm_format_info_bpp(const struct drm_format_info *info, int plane)
> > > +{
> > > +     if (!info || plane < 0 || plane >= info->num_planes)
> > > +             return 0;
> > > +
> > > +     return info->char_per_block[plane] * 8 /
> > > +            (drm_format_info_block_width(info, plane) *
> > > +             drm_format_info_block_height(info, plane));
> >
> > Do we really needs this for blocky formats where this is potentially
> > ill-defined? I think if there's no need then this should also return 0
> > when block_width/height != 1, it doesn't make much sense to compute bpp
> > when it's not really bits per _pixel_.
> 
> Yes, we do need this.  For low-color formats, the number of bits
> per pixel is less than eight, and block_width is larger than one.
> That is actually the point of this patch.

Hm right, I didn't realize that this is how we have to describe the
formats with less than 8 bpp.

I think we can include them easily with a check for char_per_block == 1
and then making sure that the division does not have a reminder (just in
case someone does something really funny, it could e.g. be a 332 layout or
something like that for 3 pixels).

> > Minimally this needs to check whether the division actually makes sense or
> > whether there's a reminder, and if there's  reminder, then fail. But that
> > feels like a bad hack and I think we should avoid it if it's not
> > absolutely necessary.
> 
> Looking at drivers/gpu/drm/drm_fourcc.c, the only supported format
> where there can be a remainder is P030, which has 2 spare bits per
> 32-bit word, and thus is special anyway.
> Still, 4 * 8 / 3 = 10, so you get the correct numbers of bits for
> the first plane.  For the second plane, you get 8 * 8 / 3 = 21,
> but as .is_yuv = true, you have to divide that result by two again,
> so you get 10 again.

Yeah I don't think we should describe these with bpp or cpp or anything
like that. bpp < 8 makes sense since that's how this has been done since
decades, but trying to extend these to funny new formats is a bad idea.
This is also why cpp and depth refuse to compute these (or at least
should).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch