Commit 72fee6b0a3a4 ("fbdev: Restrict FB_SH_MOBILE_LCDC to SuperH")
disabled availablity of the SH_MOBILE_LCDC driver on the RENESAS arch.
This innocent change has a significant side-effect on the ARM's
multi_v7_defconfig, because FB_BACKLIGHT symbol is no longer selected,
what in turn leaves BACKLIGHT_CLASS_DEVICE symbol selected only as
a module. The latter disables some backlight related code in the DRM
core, because the DRM core is set to be compiled-in in this defconfig.
This leaves all DRM display panels without integrated backlight control,
even if the needed modules have been properly loaded and probed.

Fix this by selecting BACKLIGHT_CLASS_DEVICE to be compiled-in in
multi_v7_defconfig.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
---
 arch/arm/configs/multi_v7_defconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm/configs/multi_v7_defconfig b/arch/arm/configs/multi_v7_defconfig
index 8f7445729cd0..b2955dcb5a53 100644
--- a/arch/arm/configs/multi_v7_defconfig
+++ b/arch/arm/configs/multi_v7_defconfig
@@ -777,6 +777,7 @@ CONFIG_FB_EFI=y
 CONFIG_FB_WM8505=y
 CONFIG_FB_SH_MOBILE_LCDC=y
 CONFIG_FB_SIMPLE=y
+CONFIG_BACKLIGHT_CLASS_DEVICE=y
 CONFIG_BACKLIGHT_PWM=y
 CONFIG_BACKLIGHT_AS3711=y
 CONFIG_BACKLIGHT_GPIO=y
-- 
2.34.1

Hi Marek,

On Fri, Feb 2, 2024 at 10:51 AM Marek Szyprowski
<m.szyprowski@samsung.com> wrote:
> Commit 72fee6b0a3a4 ("fbdev: Restrict FB_SH_MOBILE_LCDC to SuperH")
> disabled availablity of the SH_MOBILE_LCDC driver on the RENESAS arch.
> This innocent change has a significant side-effect on the ARM's
> multi_v7_defconfig, because FB_BACKLIGHT symbol is no longer selected,
> what in turn leaves BACKLIGHT_CLASS_DEVICE symbol selected only as
> a module. The latter disables some backlight related code in the DRM

Oops, sorry for that.

> core, because the DRM core is set to be compiled-in in this defconfig.
> This leaves all DRM display panels without integrated backlight control,
> even if the needed modules have been properly loaded and probed.

Hmm, that's bad.

Is there any way to fix this in DRM?
A quick grep shows that DRM is using the full monty of
IS_{BUILTIN,ENABLED,MODULE,REACHABLE}(CONFIG_BACKLIGHT_CLASS_DEVICE).
Probably not all of them are in perfect sync?

Several DRM drivers do select BACKLIGHT_CLASS_DEVICE, but if that
does not work in the modular case, it should be fixed.

> Fix this by selecting BACKLIGHT_CLASS_DEVICE to be compiled-in in
> multi_v7_defconfig.
>
> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>

Sounds like a good interim solution.

Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>

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