Re: [PATCH] backlight: pwm_bl: Set pin to sleep state when powered down

[Daniel Thompson <daniel.thompson@linaro.org>]

On 22/05/2019 17:34, Paul Cercueil wrote:
> When the driver probes, the PWM pin is automatically configured to its
> default state, which should be the "pwm" function.

At which point in the probe... and by who?

> However, at this
> point we don't know the actual level of the pin, which may be active or
> inactive. As a result, if the driver probes without enabling the
> backlight, the PWM pin might be active, and the backlight would be
> lit way before being officially enabled.
> 
> To work around this, if the probe function doesn't enable the backlight,
> the pin is set to its sleep state instead of the default one, until the
> backlight is enabled. Whenk the backlight is disabled, the pin is reset
> to its sleep state.
Doesn't this workaround result in a backlight flash between whatever 
enables it and the new code turning it off again?

> 
> Signed-off-by: Paul Cercueil <paul@crapouillou.net> > ---
>   drivers/video/backlight/pwm_bl.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index fb45f866b923..422f7903b382 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -16,6 +16,7 @@
>   #include <linux/module.h>
>   #include <linux/kernel.h>
>   #include <linux/init.h>
> +#include <linux/pinctrl/consumer.h>
>   #include <linux/platform_device.h>
>   #include <linux/fb.h>
>   #include <linux/backlight.h>
> @@ -50,6 +51,8 @@ static void pwm_backlight_power_on(struct pwm_bl_data *pb)
>   	struct pwm_state state;
>   	int err;
>   
> +	pinctrl_pm_select_default_state(pb->dev);
> +
>   	pwm_get_state(pb->pwm, &state);
>   	if (pb->enabled)
>   		return;
> @@ -90,6 +93,8 @@ static void pwm_backlight_power_off(struct pwm_bl_data *pb)
>   
>   	regulator_disable(pb->power_supply);
>   	pb->enabled = false;
> +
> +	pinctrl_pm_select_sleep_state(pb->dev);
>   }
>   
>   static int compute_duty_cycle(struct pwm_bl_data *pb, int brightness)
> @@ -626,6 +631,10 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>   	backlight_update_status(bl);
>   
>   	platform_set_drvdata(pdev, bl);
> +
> +	if (bl->props.power == FB_BLANK_POWERDOWN)
> +		pinctrl_pm_select_sleep_state(&pdev->dev);

Didn't backlight_update_status(bl) already do this?


Daniel.


> +
>   	return 0;
>   
>   err_alloc:
>