linux-kernel - Re: [PATCH] backlight: add PWM dependencies

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Date:	Mon, 10 Feb 2014 11:40:34 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	Jingoo Han <jg1.han@...sung.com>, Arnd Bergmann <arnd@...db.de>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-pwm@...r.kernel.org
Subject: Re: [PATCH] backlight: add PWM dependencies

On Tue, Feb 04, 2014 at 01:57:14PM +0100, Linus Walleij wrote:
> In some compilations the LM3630A and LP855X backlight drivers
> fail like this:
> 
> drivers/built-in.o: In function `lm3630a_pwm_ctrl':
> drivers/video/backlight/lm3630a_bl.c:168: undefined reference to `pwm_config'
> drivers/video/backlight/lm3630a_bl.c:172: undefined reference to `pwm_disable'
> drivers/video/backlight/lm3630a_bl.c:170: undefined reference to `pwm_enable'
> drivers/built-in.o: In function `lp855x_pwm_ctrl':
> drivers/video/backlight/lp855x_bl.c:249: undefined reference to `pwm_config'
> drivers/video/backlight/lp855x_bl.c:253: undefined reference to `pwm_disable'
> drivers/video/backlight/lp855x_bl.c:251: undefined reference to `pwm_enable'
> 
> This is because both drivers depend on the PWM framework, so
> add this dependency to their Kconfig entries.
> 
> Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
> ---
>  drivers/video/backlight/Kconfig | 2 ++
>  1 file changed, 2 insertions(+)

Hi Linus,

it seems like at least BACKLIGHT_LP8788 is missing a corresponding
dependency as well.

I have applied Sascha's patch to remove the obsolete HAVE_PWM symbol,
and this will fix at least the build issues. However it will also cause
the driver to fail at runtime because the pwm_*() functions won't work.

So I wonder if we should still apply this patch to make it clear that
PWM support is necessary to make the driver work. I guess the point is
somewhat moot because even if we had PWM enabled it could still happen
that no PWM driver is enabled to provide a PWM device... I guess it's
equally justifiable to leave that up to the defconfig.

Should we just drop this patch? Cc'ing Arnd who's commented on Jingoo's
alternate proposal.

Thierry

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