linux-kernel - Re: [PATCH 1/1 v2] pwm_bl: Add support for backlight enable GPIO

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Message-ID: <20130305071043.GB7212@avionic-0098.mockup.avionic-design.de>
Date:	Tue, 5 Mar 2013 08:10:43 +0100
From:	Thierry Reding <thierry.reding@...onic-design.de>
To:	Andrew Chew <achew@...dia.com>
Cc:	acourbot@...dia.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1 v2] pwm_bl: Add support for backlight enable GPIO

On Mon, Mar 04, 2013 at 08:43:50PM -0800, Andrew Chew wrote:
> The backlight enable GPIO is specified in the device tree node for
> backlight.
> 
> Signed-off-by: Andrew Chew <achew@...dia.com>
> ---
> I decided to go ahead with disabling/enabling the backlight via GPIO as
> needed.  Note that I named the new functions pwm_backlight_enable() and
> pwm_backlight_disable() (instead of something more gpio-specific) because
> I thought it would be convenient to have a generic hook for when someone
> wants to add yet more stuff to be done on enable/disable.
> 
> I tested this by going into /sys/class/backlight/backlight.n and manually
> adjusting the brightness, and checked the gpio state to see that it had
> the appropriate value.
> 
>  .../bindings/video/backlight/pwm-backlight.txt     |    2 +
>  drivers/video/backlight/pwm_bl.c                   |   50 ++++++++++++++++++--
>  include/linux/pwm_backlight.h                      |    2 +
>  3 files changed, 50 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt
> index 1e4fc72..1ed4f0f 100644
> --- a/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt
> +++ b/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt
> @@ -14,6 +14,8 @@ Required properties:
>  Optional properties:
>    - pwm-names: a list of names for the PWM devices specified in the
>                 "pwms" property (see PWM binding[0])
> +  - enable-gpio: a GPIO that needs to be used to enable the backlight

According to Documentation/devicetree/bindings/gpio/gpio.txt this should
be called "enable-gpios".

> +  - enable-gpio-active-high: polarity of GPIO is active high (default is low)

There is OF_GPIO_ACTIVE_LOW, which is automatically parsed from the
second cell of a GPIO specifier. It will only work if you request the
GPIO using of_get_named_gpio_flags(), though, so it will only work in
the non-DT case.

I think using a regulator would be more appropriate, since it gives you
more flexibility than a plain GPIO. Does anybody see a problem with
using a regulator instead?

Thierry

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