Re: [PATCH v2 09/15] drm/fbconv: Mode-setting pipeline enable / disable

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Thomas,

On Mon, Oct 14, 2019 at 4:05 PM Thomas Zimmermann <tzimmermann@suse.de> wrote:
> The display mode is set by converting the DRM display mode to an
> fb_info state and handling it to the fbdev driver's fb_setvar()
> function. This also requires a color depth, which we take from the
> value of struct drm_mode_config.preferred_depth
>
> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>

> --- a/drivers/gpu/drm/drm_fbconv_helper.c
> +++ b/drivers/gpu/drm/drm_fbconv_helper.c
> @@ -919,6 +919,24 @@ static int drm_fbconv_update_fb_var_screeninfo_from_framebuffer(
>         return 0;
>  }
>
> +static int drm_fbconv_update_fb_var_screeninfo_from_simple_display_pipe(
> +       struct fb_var_screeninfo *fb_var, struct drm_simple_display_pipe *pipe)
> +{
> +       struct drm_plane *primary = pipe->crtc.primary;
> +       struct drm_fbconv_modeset *modeset = drm_fbconv_modeset_of_pipe(pipe);
> +
> +       if (primary && primary->state && primary->state->fb)
> +               return drm_fbconv_update_fb_var_screeninfo_from_framebuffer(
> +                       fb_var, primary->state->fb,
> +                       modeset->fb_info->fix.smem_len);
> +
> +       fb_var->xres_virtual = fb_var->xres;
> +       fb_var->yres_virtual = fb_var->yres;
> +       fb_var->bits_per_pixel = modeset->dev->mode_config.preferred_depth;

This looks wrong to me: IMHO bits_per_pixel should be derived from
the fourcc format of the _new_ mode to be set...

> +
> +       return 0;
> +}
> +
>  /**
>   * drm_fbconv_simple_display_pipe_mode_valid - default implementation for
>   *     struct drm_simple_display_pipe_funcs.mode_valid
> @@ -950,6 +968,28 @@ bool drm_fbconv_simple_display_pipe_mode_fixup(
>         struct drm_crtc *crtc, const struct drm_display_mode *mode,
>         struct drm_display_mode *adjusted_mode)
>  {
> +       struct drm_simple_display_pipe *pipe =
> +               container_of(crtc, struct drm_simple_display_pipe, crtc);
> +       struct drm_fbconv_modeset *modeset = drm_fbconv_modeset_of_pipe(pipe);
> +       struct fb_var_screeninfo fb_var;
> +       int ret;
> +
> +       if (!modeset->fb_info->fbops->fb_check_var)
> +               return true;
> +
> +       drm_fbconv_init_fb_var_screeninfo_from_mode(&fb_var, mode);
> +
> +       ret = drm_fbconv_update_fb_var_screeninfo_from_simple_display_pipe(
> +               &fb_var, &modeset->pipe);
> +       if (ret)
> +               return true;
> +
> +       ret = modeset->fb_info->fbops->fb_check_var(&fb_var, modeset->fb_info);

... hence this fails if the requested mode is valid with the new
fourcc format, but invalid with the old (but preferred) depth.
E.g. due to bandwidth limitations, a high-resolution mode is valid
with a low color depth, while a high color depth is limited to lower
resolutions.

Unfortunately we do not know the new fourcc format here, as both
drm_simple_display_pipe_funcs.mode_{valid,fixup}() are passed
the mode (from drm_mode_set.mode), but not the new format (from
drm_mode_set.fb->format).

Am I missing something? Is the new format available in some other way?
Thanks!

> +       if (ret < 0)
> +               return false;
> +
> +       drm_mode_update_from_fb_var_screeninfo(adjusted_mode, &fb_var);
> +
>         return true;
>  }
>  EXPORT_SYMBOL(drm_fbconv_simple_display_pipe_mode_fixup);

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds