Re: [PATCH 1/3] drm/vkms: Set preferred depth correctly

From: Melissa Wen <melissa.srw@gmail.com>
To: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Daniel Vetter <daniel.vetter@intel.com>,
	Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>,
	DRI Development <dri-devel@lists.freedesktop.org>,
	Haneen Mohammed <hamohammed.sa@gmail.com>
Subject: Re: [PATCH 1/3] drm/vkms: Set preferred depth correctly
Date: Fri, 16 Oct 2020 14:02:03 -0300	[thread overview]
Message-ID: <20201016170203.53wjzysqh2p7rfry@smtp.gmail.com> (raw)
In-Reply-To: <CAKMK7uFtesxom12TL4MX2rAb2eW2nEmh5gfeCG-wVAyfNvkGRw@mail.gmail.com>

On 10/16, Daniel Vetter wrote:
> On Fri, Oct 16, 2020 at 12:38 PM Simon Ser <contact@emersion.fr> wrote:
> >
> > > The only thing we support is xrgb8888.
> > >
> > > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > > Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
> > > Cc: Melissa Wen <melissa.srw@gmail.com>
> > > Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
> > > Cc: Daniel Vetter <daniel@ffwll.ch>
> > > ---
> > >  drivers/gpu/drm/vkms/vkms_drv.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/gpu/drm/vkms/vkms_drv.c b/drivers/gpu/drm/vkms/vkms_drv.c
> > > index 726801ab44d4..eb4007443706 100644
> > > --- a/drivers/gpu/drm/vkms/vkms_drv.c
> > > +++ b/drivers/gpu/drm/vkms/vkms_drv.c
> > > @@ -124,7 +124,7 @@ static int vkms_modeset_init(struct vkms_device *vkmsdev)
> > >       dev->mode_config.max_height = YRES_MAX;
> > >       dev->mode_config.cursor_width = 512;
> > >       dev->mode_config.cursor_height = 512;
> > > -     dev->mode_config.preferred_depth = 24;
> > > +     dev->mode_config.preferred_depth = 32;
> >
> > Are you sure xrgb8888's depth is 32 and not 24? Looking at drmdb [1], *all*
> > drivers set it to 24.
> 
> Uh there's a problem I think. Depth should indeed stay at 24, the
> problem is that fb helpers directly take that to be the bpp parameter,
> which is a different thing. And if you look at how most drivers set up
> that, they pick 32.
> 
> I guess I need to revert this here, and unconfuse the fb helpers about
> depth vs bpp.

Hi guys,

Perhaps it deserves to be pointed out: the documentation says
"preferred_depth: preferred RBG(sic) pixel depth, used by fb helpers",
and looking to fb helpers, preferred_depth is only used by
generic_setup, as bits by pixel (if I didn't miss something there).

In fact, the alpha channel is not used for final display (perhaps in the
future); however, I saw another driver with a change similar to this
here and, possibly like me, following the same misunderstanding.

Melissa
> 
> Maybe best would be to just switch over to preferred drm_fourcc format
> code, or maybe just pick this up from the first format the primary
> plane supports.
> 
> This is all getting slightly tricky and a lot more work :-/
> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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