linux-kernel - Re: [PATCH v2 2/4] pwm-backlight: add support for pwm-delay-us property

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Message-ID: <20170706091231.GA6741@amd>
Date:   Thu, 6 Jul 2017 11:12:31 +0200
From:   Pavel Machek <pavel@....cz>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Lee Jones <lee.jones@...aro.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Rob Herring <robh+dt@...nel.org>,
        Richard Purdie <rpurdie@...ys.net>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Heiko Stuebner <heiko@...ech.de>, linux-pwm@...r.kernel.org,
        linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        groeck@...omium.org, linux-rockchip@...ts.infradead.org,
        huang lin <hl@...k-chips.com>
Subject: Re: [PATCH v2 2/4] pwm-backlight: add support for pwm-delay-us
 property

On Thu 2017-07-06 10:01:32, Thierry Reding wrote:
> On Fri, Jun 30, 2017 at 01:21:07PM +0200, Enric Balletbo i Serra wrote:
> > From: huang lin <hl@...k-chips.com>
> > 
> > Some panels (i.e. N116BGE-L41), in their power sequence specifications,
> > request a delay between set the PWM signal and enable the backlight and
> > between clear the PWM signal and disable the backlight. Add support for
> > the new pwm-delay-us property to meet the timing.
> > 
> > Note that this patch inverts current sequence. Before this patch the
> > enable signal was set before the PWM signal and vice-versa on power off.
> > 
> > I assumed that this sequence was wrong, at least it is on different panel
> > datasheets that I checked, so I inverted the sequence to follow:
> > 
> >   On power on, set the PWM signal, wait, and set the LED_EN signal.
> >   On power off, clear the LED_EN signal, wait, and stop the PWM signal.
> 
> I think this should be two separate patches to make it easier to revert
> the inverted sequence should it prove to regress on other panels.

Don't make this overly complex. This is trivial. No need to split it
into more patches.


> Two more comments below.
> 
> > Signed-off-by: huang lin <hl@...k-chips.com>
> > Signed-off-by: Enric Balletbo i Serra <enric.balletbo@...labora.com>
> > ---
> > Changes since v1:
> >  - As suggested by Daniel Thompson
> >    - Do not assume power-on delay and power-off delay will be the same
> >  - Move the check of dt property to the parse dt function.
> > 
> > v1: https://lkml.org/lkml/2017/6/28/219
> > 
> >  drivers/video/backlight/pwm_bl.c | 24 ++++++++++++++++++++----
> >  include/linux/pwm_backlight.h    |  1 +
> >  2 files changed, 21 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> > index 002f1ce..0f5470e 100644
> > --- a/drivers/video/backlight/pwm_bl.c
> > +++ b/drivers/video/backlight/pwm_bl.c
> > @@ -10,6 +10,7 @@
> >   * published by the Free Software Foundation.
> >   */
> >  
> > +#include <linux/delay.h>
> >  #include <linux/gpio/consumer.h>
> >  #include <linux/gpio.h>
> >  #include <linux/module.h>
> > @@ -35,6 +36,7 @@ struct pwm_bl_data {
> >  	struct gpio_desc	*enable_gpio;
> >  	unsigned int		scale;
> >  	bool			legacy;
> > +	unsigned int		pwm_delay[2];
> >  	int			(*notify)(struct device *,
> >  					  int brightness);
> >  	void			(*notify_after)(struct device *,
> > @@ -54,10 +56,14 @@ static void pwm_backlight_power_on(struct pwm_bl_data *pb, int brightness)
> >  	if (err < 0)
> >  		dev_err(pb->dev, "failed to enable power supply\n");
> >  
> > +	pwm_enable(pb->pwm);
> > +
> > +	if (pb->pwm_delay[0])
> > +		usleep_range(pb->pwm_delay[0], pb->pwm_delay[0] + 2000);
> 
> 2000 us is kind of arbitrary. What if pwm_delay[0] is on the order of 20
> us? Making the delay 2 ms longer (in the worst case) seems somewhat
> excessive. Why not something like:
> 
> 	usleep_range(pb->pwm_delay[0], pb->pwm_delay[0] * 2);
> 
> ?
> 
> > +
> >  	if (pb->enable_gpio)
> >  		gpiod_set_value_cansleep(pb->enable_gpio, 1);
> >  
> > -	pwm_enable(pb->pwm);
> >  	pb->enabled = true;
> >  }
> >  
> > @@ -66,12 +72,15 @@ static void pwm_backlight_power_off(struct pwm_bl_data *pb)
> >  	if (!pb->enabled)
> >  		return;
> >  
> > -	pwm_config(pb->pwm, 0, pb->period);
> > -	pwm_disable(pb->pwm);
> > -
> >  	if (pb->enable_gpio)
> >  		gpiod_set_value_cansleep(pb->enable_gpio, 0);
> >  
> > +	if (pb->pwm_delay[1])
> > +		usleep_range(pb->pwm_delay[1], pb->pwm_delay[1] + 2000);
> > +
> > +	pwm_config(pb->pwm, 0, pb->period);
> > +	pwm_disable(pb->pwm);
> > +
> >  	regulator_disable(pb->power_supply);
> >  	pb->enabled = false;
> >  }
> > @@ -174,6 +183,12 @@ static int pwm_backlight_parse_dt(struct device *dev,
> >  		data->max_brightness--;
> >  	}
> >  
> > +	/* read pwm to enable pre/post delays from DT property */
> 
> This comment is confusing. This isn't reading anything from the PWM.
> 
> Thierry



-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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