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

[Linus Walleij]

On Thu, Feb 1, 2018 at 2:19 PM, Ville Syrjälä
<ville.syrjala at linux.intel.com> wrote:
> [Me]
>> +     /*
>> +      * 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.
>> +      */
>> +     drm_for_each_plane(plane, fb_helper->dev) {
>> +             /* Only check the primary plane */
>> +             if (plane->type != DRM_PLANE_TYPE_PRIMARY)
>> +                     continue;
>
> I think this should look at crtc->primary for each of the crtcs managed
> by the fb_helper.
>
> Also this probably shouldn't look at YUV formats at all?

I guess I can look into doing it this way, sorry for not knowing how to
properly inspect DRM objects, I'm lost sometimes...

> I do wonder if instead we should just have the driver specify the
> pixel format explicitly instead of trying to guess based on bpp?

That makes a lot more sense to me actually. It would
give a better sense of control so the driver feel it knows
what is actually going on.

So I would just update
drm_fb_cma_fbdev_init() and drm_fb_helper_initial_config()
to pass a reasonable pixel format instead and refactor all the
way down?

It does hit a lot of code on the way, but if everyone thinks this
is a good idea I can very well take a stab at it.

Yours,
Linus Walleij