linux-kernel - Re: [PATCH] leds: rt8515: add V4L2_FLASH_LED

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Message-ID: <CACRpkdYpHAN5XYLxRj4Rktzj+LbGRFYwtUU72gqUwE=AEt0j6g@mail.gmail.com>
Date:   Fri, 5 Feb 2021 10:16:02 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Pavel Machek <pavel@....cz>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>, Dan Murphy <dmurphy@...com>,
        Linux LED Subsystem <linux-leds@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] leds: rt8515: add V4L2_FLASH_LED_CLASS dependency

On Thu, Feb 4, 2021 at 4:39 PM Arnd Bergmann <arnd@...nel.org> wrote:

> From: Arnd Bergmann <arnd@...db.de>
>
> The leds-rt8515 driver can optionall use the v4l2 flash led class,
> but it causes a link error when that class is in a loadable module
> and the rt8515 driver itself is built-in:
>
> ld.lld: error: undefined symbol: v4l2_flash_init
> >>> referenced by leds-rt8515.c
> >>>               leds/flash/leds-rt8515.o:(rt8515_probe) in archive
> drivers/built-in.a
>
> Adding 'depends on V4L2_FLASH_LED_CLASS' in Kconfig would avoid that,
> but it would make it impossible to use the driver without the
> v4l2 support.
>
> Add the same dependency that the other users of this class have
> instead, which just prevents the broken configuration.
>
> Fixes: e1c6edcbea13 ("leds: rt8515: Add Richtek RT8515 LED driver")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Reviewed-by: Linus Walleij <linus.walleij@...aro.org>

> +       depends on V4L2_FLASH_LED_CLASS || !V4L2_FLASH_LED_CLASS

This kind of thing that looks like a truism but isn't is one of the
most unintuitive
things about KConfig :/ no idea what to do about it though.

I remember making a program for the C64 in early 1990ies with joke input
such as y/n/m (yes/no/maybe), sadly in Kconfig this is no joke.

Yours,
Linus Walleij