RE: [PATCH 1/1] pwm_bl: Add support for backlight enable GPIO

[Andrew Chew]

> From: Alex Courbot
> Sent: Monday, March 04, 2013 7:00 PM
> To: Thierry Reding
> Cc: Andrew Chew; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH 1/1] pwm_bl: Add support for backlight enable GPIO
> 
> On 03/05/2013 07:46 AM, Thierry Reding wrote:
> > * PGP Signed by an unknown key
> >
> > On Mon, Mar 04, 2013 at 01:49:49PM -0800, Andrew Chew wrote:
> >> The backlight enable GPIO is specified in the device tree node for
> >> backlight.
> >>
> >> Signed-off-by: Andrew Chew <achew@xxxxxxxxxx>
> >> ---
> >>   .../bindings/video/backlight/pwm-backlight.txt     |    2 ++
> >>   drivers/video/backlight/pwm_bl.c                   |   32 +++++++++++++++++--
> -
> >>   include/linux/pwm_backlight.h                      |    2 ++
> >>   3 files changed, 32 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
> >> +  - enable-gpio-active-high: polarity of GPIO is active high
> >> + (default is low)
> >>
> >>   [0]: Documentation/devicetree/bindings/pwm/pwm.txt
> >>
> >> diff --git a/drivers/video/backlight/pwm_bl.c
> >> b/drivers/video/backlight/pwm_bl.c
> >> index 069983c..f29f9c7 100644
> >> --- a/drivers/video/backlight/pwm_bl.c
> >> +++ b/drivers/video/backlight/pwm_bl.c
> >> @@ -20,10 +20,13 @@
> >>   #include <linux/pwm.h>
> >>   #include <linux/pwm_backlight.h>
> >>   #include <linux/slab.h>
> >> +#include <linux/of_gpio.h>
> >>
> >>   struct pwm_bl_data {
> >>   	struct pwm_device	*pwm;
> >>   	struct device		*dev;
> >> +	int			enable_gpio;
> >> +	unsigned int		enable_gpio_flags;
> >>   	unsigned int		period;
> >>   	unsigned int		lth_brightness;
> >>   	unsigned int		*levels;
> >> @@ -146,10 +149,15 @@ static int pwm_backlight_parse_dt(struct device
> *dev,
> >>   	}
> >>
> >>   	/*
> >> -	 * TODO: Most users of this driver use a number of GPIOs to control
> >> -	 *       backlight power. Support for specifying these needs to be
> >> -	 *       added.
> >> +	 * If "enable-gpio" is present, use that GPIO to enable the backlight.
> >> +	 * The presence (or not) of "enable-gpio-active-high" will determine
> >> +	 * the value of the GPIO.
> >>   	 */
> >> +	data->enable_gpio = of_get_named_gpio(node, "enable-gpio", 0);
> >> +	if (of_property_read_bool(node, "enable-gpio-active-high"))
> >> +		data->enable_gpio_flags = GPIOF_OUT_INIT_HIGH;
> >> +	else
> >> +		data->enable_gpio_flags = GPIOF_OUT_INIT_LOW;
> >>
> >>   	return 0;
> >>   }
> >> @@ -207,12 +215,23 @@ static int pwm_backlight_probe(struct
> platform_device *pdev)
> >>   	} else
> >>   		max = data->max_brightness;
> >>
> >> +	pb->enable_gpio = data->enable_gpio;
> >> +	pb->enable_gpio_flags = data->enable_gpio_flags;
> >>   	pb->notify = data->notify;
> >>   	pb->notify_after = data->notify_after;
> >>   	pb->check_fb = data->check_fb;
> >>   	pb->exit = data->exit;
> >>   	pb->dev = &pdev->dev;
> >>
> >> +	if (gpio_is_valid(pb->enable_gpio)) {
> >> +		ret = gpio_request_one(pb->enable_gpio,
> >> +			GPIOF_DIR_OUT | pb->enable_gpio_flags, "bl_en");
> >> +		if (ret) {
> >> +			dev_err(&pdev->dev, "failed to allocate bl_en gpio");
> >> +			goto err_alloc;
> >> +		}
> >> +	}
> >> +
> >>   	pb->pwm = devm_pwm_get(&pdev->dev, NULL);
> >>   	if (IS_ERR(pb->pwm)) {
> >>   		dev_err(&pdev->dev, "unable to request PWM, trying legacy
> >> API\n"); @@ -221,7 +240,7 @@ static int pwm_backlight_probe(struct
> platform_device *pdev)
> >>   		if (IS_ERR(pb->pwm)) {
> >>   			dev_err(&pdev->dev, "unable to request legacy
> PWM\n");
> >>   			ret = PTR_ERR(pb->pwm);
> >> -			goto err_alloc;
> >> +			goto err_gpio;
> >>   		}
> >>   	}
> >>
> >> @@ -255,6 +274,9 @@ static int pwm_backlight_probe(struct
> platform_device *pdev)
> >>   	platform_set_drvdata(pdev, bl);
> >>   	return 0;
> >>
> >> +err_gpio:
> >> +	if (gpio_is_valid(data->enable_gpio))
> >> +		gpio_free(data->enable_gpio);
> >>   err_alloc:
> >>   	if (data->exit)
> >>   		data->exit(&pdev->dev);
> >> @@ -269,6 +291,8 @@ static int pwm_backlight_remove(struct
> platform_device *pdev)
> >>   	backlight_device_unregister(bl);
> >>   	pwm_config(pb->pwm, 0, pb->period);
> >>   	pwm_disable(pb->pwm);
> >> +	if (gpio_is_valid(pb->enable_gpio))
> >> +		gpio_free(pb->enable_gpio);
> >>   	if (pb->exit)
> >>   		pb->exit(&pdev->dev);
> >>   	return 0;
> >> diff --git a/include/linux/pwm_backlight.h
> >> b/include/linux/pwm_backlight.h index 56f4a86..2706805 100644
> >> --- a/include/linux/pwm_backlight.h
> >> +++ b/include/linux/pwm_backlight.h
> >> @@ -8,6 +8,8 @@
> >>
> >>   struct platform_pwm_backlight_data {
> >>   	int pwm_id;
> >> +	int enable_gpio;
> >> +	unsigned int enable_gpio_flags;
> >>   	unsigned int max_brightness;
> >>   	unsigned int dft_brightness;
> >>   	unsigned int lth_brightness;
> >
> > Hi Andrew,
> >
> > I'm Cc'ing Alexandre Courbot, who has been working on supporting this
> > in a more generic way using power sequences. Generally this kind of
> > support really belongs in the common display framework, but I guess we
> > could add this one GPIO since it really is related only to the
> > backlight. Usually more than just an enable for the backlight is
> > required so I'm not sure how useful this really is.
> >
> > Alex, any thought?
> 
> It is very common for a GPIO to be involved in powering the backlight on,
> indeed. However it seems that in this patch the GPIO is set once and for all
> during probe and never touched afterwards. This means the backlight is still
> enabled (and consuming power) even when its value is zero - I'd at least like
> to see the GPIO disabled when this is the case to save power. Otherwise you
> can achieve the same result with a gpio-regulator defined to be always on in
> the DT, without touching the pwm-backlight driver.
> 
> Another issue is that if the GPIO is not explicitly set to -1 in the platform data,
> probe will try to acquire GPIO 0 and will fail. This would break compatibility
> with all existing users of pwm-backlight that rely on platform data.
> 
> And there is also the fact that the powering of backlights is often slightly
> more complicated than just an enabling GPIO - for Ventana we have at least
> one more regulator/GPIO involved. Maybe this regulator could be turned on
> forever in the DT - then pwm-backlight could use the enable GPIO to save
> power, but I suspect we would save even more power if we could turn the
> regulator off as well.
> 
> But overall I'm not against having enable GPIO support in this driver if this
> helps getting the job done while I finish proper power-sequence support.
> Andrew, does this single patch allow you to enable the backlight on some
> boards?

I did come to the same conclusion regarding the platform data breakage.
I'm expecting that the use of platform data will go away, at least on ARM,
since we are all aggressively moving what used to be in platform data into
the device tree.  Do other platforms use this driver?

I remember hearing that there is some work in progress to encapsulate
gpios into a struct, rather than passing it around as a bare integer, so when
that happens, we can use NULL for no-gpio, which should take care of the
platform data issue as well.  It's kind of difficult to work around this problem
otherwise.

I agree that we should be turning on and off the backlight enable gpio as
needed to save power.  I just haven't gotten there yet.  I can try to modify
this patch if that's your preference, or I can follow up with a patch to add
this in the very near future.

To answer your last question, yes, this single patch does allow me to enable
the backlight on some boards (in particular, the one I'm working on).
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