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

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Message-ID: <20170706080132.GL16144@ulmo.fritz.box>
Date:   Thu, 6 Jul 2017 10:01:32 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Enric Balletbo i Serra <enric.balletbo@...labora.com>
Cc:     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>, Pavel Machek <pavel@....cz>,
        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 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.

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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