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

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Message-ID: <CACRpkdbB8LFM0Vu8VRo4niqxwxUdS-c5gmmWaxbUTbnLCLZbMA@mail.gmail.com>
Date:	Wed, 5 Feb 2014 09:57:32 +0100
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Jingoo Han <jg1.han@...sung.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Eric Miao <eric.y.miao@...il.com>,
	Thierry Reding <thierry.reding@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>
Subject: Re: [PATCH] backlight: add PWM dependencies

On Wed, Feb 5, 2014 at 6:01 AM, Jingoo Han <jg1.han@...sung.com> wrote:
> On Tuesday, February 04, 2014 9:57 PM, 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.
>
> However, even though, when CONFIG_PWM is not enabled, the problem
> should not happen. pwm_config(),pwm_disable(), and pwm_enable()
> are already defined for CONFIG_PWM=n case as below.

So you may think but it does happen :-)

I reproduced this with the defconfig for ARM pxa255-idp and enabling
all boards for that platform, then enabling all available backlight drivers
as compiled-in objects (y).

> ./include/linux/pwm.h
> #if IS_ENABLED(CONFIG_PWM) || IS_ENABLED(CONFIG_HAVE_PWM)
>         .....
> #else

Hm PXA that I am using defines CONFIG_HAVE_PWM, but doesn't
provide the required signatures (pwm_config/pwm_disable/pwm_enable).

One of two things is wrong:

- Either the PXA platform is breaking the CONFIG_HAVE_PWM
  contract by not providing pwm_config/pwm_disable/pwm_enable
  functions. Then HAVE_PWM should be removed from the PXA
  Kconfig selects.

Or:

- There is no such contract that these functions must exist if
  CONFIG_HAVE_PWM is defined, and the
  #if IS_ENABLED(CONFIG_HAVE_PWM)
  should be removed from <linux/pwm.h>

Does anyone know which one it is?

PWM subsystem maintainer? :-)

Yours,
Linus Walleij
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