Reggression caused by "drm/fb-helper: Don't use the preferred depth for the BPP default"

[Fabio Estevam]

Hi,

I have a custom board populated with a cfaf240320x panel connected via
SPI and driven by the drivers/gpu/drm/tiny/panel-mipi-dbi.c driver.

It works well on kernel 6.1.

After upgrading the kernel to 6.12 (also tested Linux-next), the panel
no longer works correctly: the colors are wrong, and the image appears
twice, one in each half of the screen.

Running git bisect pointed to the following bad commit:

559358282e5b43b1b01e7f6afac6e0beb33cb4a2 is the first bad commit
commit 559358282e5b43b1b01e7f6afac6e0beb33cb4a2
Author: Thomas Zimmermann <tzimmermann at suse.de>
Date:   Wed Nov 23 12:53:48 2022 +0100

    drm/fb-helper: Don't use the preferred depth for the BPP default

    If no preferred value for bits-per-pixel has been given, fall back
    to 32. Never use the preferred depth. The color depth is the number
    of color/alpha bits per pixel, while bpp is the overall number of
    bits in most cases.

    Most noteworthy, XRGB8888 has a depth of 24 and a bpp value of 32.
    Using depth for bpp would make the value 24 as well and format
    selection in fbdev helpers fails. Unfortunately XRGB8888 is the most
    common format and the old heuristic therefore fails for most of
    the drivers (unless they implement the 24-bit RGB888 format).

    Picking a bpp of 32 will later on result in a default depth of 24
    and the format XRGB8888. As XRGB8888 is the default format for most
    of the current and legacy graphics stack, all drivers must support
    it. So it is the safe choice.

    v2:
            * fix commit-message typo (Javier)

    Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
    Reviewed-by: Javier Martinez Canillas <javierm at redhat.com>
    Acked-by: Daniel Vetter <daniel.vetter at ffwll.ch>
    Link: https://patchwork.freedesktop.org/patch/msgid/20221123115348.2521-8-tzimmermann@suse.de

Then I did a quick hack like this:

--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -425,7 +425,7 @@ void drm_fb_helper_prepare(struct drm_device *dev,
struct drm_fb_helper *helper,
         *       format.
         */
        if (!preferred_bpp)
-               preferred_bpp = 32;
+               preferred_bpp = 16;

        INIT_LIST_HEAD(&helper->kernel_fb_list);
        spin_lock_init(&helper->damage_lock);

and the display correctly again.

What is the proper fix for this issue?

Thanks,

Fabio Estevam