[v2] drm/fb-helper: Scale back depth to supported maximum

The following happened when migrating an old fbdev driver to DRM:

The Integrator/CP PL111 supports 16BPP but only ARGB1555/ABGR1555
or XRGB1555/XBGR1555 i.e. the maximum depth is 15.

This makes the initialization of the framebuffer fail since
the code in drm_fb_helper_single_fb_probe() assigns the same value
to sizes.surface_bpp and sizes.surface_depth. I.e. it simply assumes
a 1-to-1 mapping between BPP and depth, which is true in most cases
but not for this hardware that only support odd formats.

To support the odd case of a driver supporting 16BPP with only 15
bits of depth, this patch will make the code loop over the formats
supported on the primary plane on each CRTC managed by the FB
helper and cap the depth to the maximum supported on any primary
plane.

On the PL110 Integrator, this makes drm_mode_legacy_fb_format()
select DRM_FORMAT_XRGB1555 which is acceptable for this driver, and
thus we get framebuffer, penguin and console on the Integrator/CP.

Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
ChangeLog v1->v2:
- Loop over the CRTCs managed by the helper and check the
  crtc->primary on each CRTC for the applicable formats and
  thus depths.
- Skip over YUV formats. The framebuffer emulation cannot
  handle these formats.

The v1 was sent some while back in february and I only recently
got back to fixing this up to support the last CLCD displays.
It was agreed that it is probably best to augment the framebuffer
initializer to pass a desired pixel format instead of just
BPP as today, but that is a bit daunting, and Daniel said that
we would probably anyways need a fallback like this.
---
 drivers/gpu/drm/drm_fb_helper.c | 47 ++++++++++++++++++++++++++++++++-
 1 file changed, 46 insertions(+), 1 deletion(-)

On Fri, Dec 28, 2018 at 4:40 PM Linus Walleij <linus.walleij@linaro.org> wrote:
>
> The following happened when migrating an old fbdev driver to DRM:
>
> The Integrator/CP PL111 supports 16BPP but only ARGB1555/ABGR1555
> or XRGB1555/XBGR1555 i.e. the maximum depth is 15.
>
> This makes the initialization of the framebuffer fail since
> the code in drm_fb_helper_single_fb_probe() assigns the same value
> to sizes.surface_bpp and sizes.surface_depth. I.e. it simply assumes
> a 1-to-1 mapping between BPP and depth, which is true in most cases
> but not for this hardware that only support odd formats.
>
> To support the odd case of a driver supporting 16BPP with only 15
> bits of depth, this patch will make the code loop over the formats
> supported on the primary plane on each CRTC managed by the FB
> helper and cap the depth to the maximum supported on any primary
> plane.
>
> On the PL110 Integrator, this makes drm_mode_legacy_fb_format()
> select DRM_FORMAT_XRGB1555 which is acceptable for this driver, and
> thus we get framebuffer, penguin and console on the Integrator/CP.
>
> Cc: Noralf Trønnes <noralf@tronnes.org>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
> ---
> ChangeLog v1->v2:
> - Loop over the CRTCs managed by the helper and check the
>   crtc->primary on each CRTC for the applicable formats and
>   thus depths.
> - Skip over YUV formats. The framebuffer emulation cannot
>   handle these formats.
>
> The v1 was sent some while back in february and I only recently
> got back to fixing this up to support the last CLCD displays.
> It was agreed that it is probably best to augment the framebuffer
> initializer to pass a desired pixel format instead of just
> BPP as today, but that is a bit daunting, and Daniel said that
> we would probably anyways need a fallback like this.
> ---
>  drivers/gpu/drm/drm_fb_helper.c | 47 ++++++++++++++++++++++++++++++++-
>  1 file changed, 46 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index d3af098b0922..57be06d932e4 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -1797,6 +1797,7 @@ static int drm_fb_helper_single_fb_probe(struct drm_fb_helper *fb_helper,
>         int i;
>         struct drm_fb_helper_surface_size sizes;
>         int gamma_size = 0;
> +       int best_depth = 0;
>
>         memset(&sizes, 0, sizeof(struct drm_fb_helper_surface_size));
>         sizes.surface_depth = 24;
> @@ -1804,7 +1805,10 @@ static int drm_fb_helper_single_fb_probe(struct drm_fb_helper *fb_helper,
>         sizes.fb_width = (u32)-1;
>         sizes.fb_height = (u32)-1;
>
> -       /* if driver picks 8 or 16 by default use that for both depth/bpp */
> +       /*
> +        * If driver picks 8 or 16 by default use that for both depth/bpp
> +        * to begin with
> +        */
>         if (preferred_bpp != sizes.surface_bpp)
>                 sizes.surface_depth = sizes.surface_bpp = preferred_bpp;
>
> @@ -1839,6 +1843,47 @@ static int drm_fb_helper_single_fb_probe(struct drm_fb_helper *fb_helper,
>                 }
>         }
>
> +       /*
> +        * If we run into a situation where, for example, the primary plane
> +        * supports RGBA5551 (16 bpp, depth 15) but not RGB565 (16 bpp, depth
> +        * 16) we need to scale down the depth of the sizes we request.
> +        */
> +       for (i = 0; i < fb_helper->crtc_count; i++) {
> +               struct drm_mode_set *mode_set = &fb_helper->crtc_info[i].mode_set;
> +               struct drm_crtc *crtc = mode_set->crtc;
> +               struct drm_plane *plane = crtc->primary;
> +               int j;
> +
> +               DRM_DEBUG("test CRTC %d primary plane\n", i);
> +
> +               for (j = 0; j < plane->format_count; j++) {
> +                       const struct drm_format_info *fmt;
> +
> +                       fmt = drm_format_info(plane->format_types[j]);
> +
> +                       /* Do not consider YUV formats for framebuffers */
> +                       if (fmt->is_yuv)

I think better to skip all funny formats with fmt->depth == 0. With that:

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> +                               continue;
> +
> +                       /* We found a perfect fit, great */
> +                       if (fmt->depth == sizes.surface_depth)
> +                               break;
> +
> +                       /* Skip depths above what we're looking for */
> +                       if (fmt->depth > sizes.surface_depth)
> +                               continue;
> +
> +                       /* Best depth found so far */
> +                       if (fmt->depth > best_depth)
> +                               best_depth = fmt->depth;
> +               }
> +       }
> +       if (sizes.surface_depth != best_depth) {
> +               DRM_INFO("requested bpp %d, scaled depth down to %d",
> +                         sizes.surface_bpp, best_depth);
> +               sizes.surface_depth = best_depth;
> +       }
> +
>         crtc_count = 0;
>         for (i = 0; i < fb_helper->crtc_count; i++) {
>                 struct drm_display_mode *desired_mode;
> --
> 2.19.2
>
> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

On Fri, Dec 28, 2018 at 5:05 PM Daniel Vetter <daniel@ffwll.ch> wrote:
> On Fri, Dec 28, 2018 at 4:40 PM Linus Walleij <linus.walleij@linaro.org> wrote:

> > - Skip over YUV formats. The framebuffer emulation cannot
> >   handle these formats.
(...)
> > +                       /* Do not consider YUV formats for framebuffers */
> > +                       if (fmt->is_yuv)
>
> I think better to skip all funny formats with fmt->depth == 0. With that:
>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

I can easily make that change but this comment in the drm_fourcc.h
header makes me insecure:

/**
 * @depth:
 *
 * Color depth (number of bits per pixel excluding padding bits),
 * valid for a subset of RGB formats only. This is a legacy field, do
 * not use in new code and set to 0 for new formats.
 */
u8 depth;

This would mean that we make the framebuffer only support
"legacy formats". It might be that "legacy formats" include
all reasonable ones such as any RGB/BGA variants.

I will make the change and put in some comments about this
so it's clear.

Yours,
Linus Walleij

On Thu, Jan 10, 2019 at 10:19 AM Linus Walleij <linus.walleij@linaro.org> wrote:
>
> On Fri, Dec 28, 2018 at 5:05 PM Daniel Vetter <daniel@ffwll.ch> wrote:
> > On Fri, Dec 28, 2018 at 4:40 PM Linus Walleij <linus.walleij@linaro.org> wrote:
>
> > > - Skip over YUV formats. The framebuffer emulation cannot
> > >   handle these formats.
> (...)
> > > +                       /* Do not consider YUV formats for framebuffers */
> > > +                       if (fmt->is_yuv)
> >
> > I think better to skip all funny formats with fmt->depth == 0. With that:
> >
> > Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>
> I can easily make that change but this comment in the drm_fourcc.h
> header makes me insecure:
>
> /**
>  * @depth:
>  *
>  * Color depth (number of bits per pixel excluding padding bits),
>  * valid for a subset of RGB formats only. This is a legacy field, do
>  * not use in new code and set to 0 for new formats.
>  */
> u8 depth;
>
> This would mean that we make the framebuffer only support
> "legacy formats". It might be that "legacy formats" include
> all reasonable ones such as any RGB/BGA variants.

Yeah it's not a perfect fit either, but there's a lot non-yuv format
that are very special, and checking for ->depth excludes them. And yes
all the usual rgb/bga formats have bpp/depth descriptions. That's also
all that the fbdev format stuff can describe. Long term we maybe want
to switch over to directly using the format fourcc (and -EINVAL the
ones we don't understand), because atm we're shrugging the bgr vs rgb
difference under the rug.

Cheers, Daniel

> I will make the change and put in some comments about this
> so it's clear.
>
> Yours,
> Linus Walleij