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

[Melissa Wen <melissa.srw@gmail.com>]

On 10/16, Daniel Vetter wrote:
> On Fri, Oct 16, 2020 at 7:02 PM Melissa Wen <melissa.srw@gmail.com> wrote:
> >
> > 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.
> 
> Yeah the problem is that preferred_depth is depth, and that means 24
> bit for XRGB8888. But bpp as used by fb helpers would be 32 bit for
> XRGB8888.
> 
> I think the real fix here is to switch this entire mess over to using
> drm_fourcc codes directly, at least for atomic drivers. Which nowadays
> are most. Interim I'm not sure whether we should revert my patch (it
> breaks fbdev) or switch preferred_depth to 0, which means we get the
> default every, and that means both fbdev helpers and userspace will
> pick XRGB8888.

hmm... but why not keep preferred_depth = 24 and pass 32 as the
preferred_bpp parameter of fbdev_generic_setup?

> -Daniel
> 
> > 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
> 
> 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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