linux-kernel - Re: [PATCH] ARM: multi_v7_defconfig: Enable BACKLIGHT_CLASS

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Date: Fri, 2 Feb 2024 12:07:43 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Marek Szyprowski <m.szyprowski@...sung.com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, Russell King <linux@...linux.org.uk>, 
	Arnd Bergmann <arnd@...db.de>, Geert Uytterhoeven <geert+renesas@...der.be>, 
	Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>, 
	Alexandre Torgue <alexandre.torgue@...s.st.com>, Andrew Davis <afd@...com>, 
	Mark Brown <broonie@...nel.org>, Lee Jones <lee@...nel.org>, 
	Daniel Thompson <daniel.thompson@...aro.org>, Jingoo Han <jingoohan1@...il.com>, 
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH] ARM: multi_v7_defconfig: Enable BACKLIGHT_CLASS_DEVICE

Hi Marek,

On Fri, Feb 2, 2024 at 10:51 AM Marek Szyprowski
<m.szyprowski@...sung.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@...sung.com>

Sounds like a good interim solution.

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

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68korg

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