Re: [PATCH 2/2] backlight: pwm_bl: Don't disable the PWM to disable the backlight

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

On Mon, Jan 09, 2023 at 09:47:58PM +0100, Uwe Kleine-König wrote:
> Most but not all PWMs drive the PWM pin to its inactive state when
> disabled. Rely on the lowlevel PWM implementation to implement
> duty_cycle = 0 in an energy efficient way and don't disable the PWM.

I'm a little worried about this one.

I thought the PWM APIs allow the duty cycle to be rounded up or down
slightly during the apply.

So when you say "rely on the lowlevel to implement duty_cycle = 0 to..."
is it confirmed that this is true (and that all PWMs *can* implement
a duty_cycle of 0 without rounding up)?


Daniel.


> This fixes backlight disabling e.g. on i.MX6 when an inverted PWM is
> used.
>
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
> ---
>  drivers/video/backlight/pwm_bl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index 0509fecd5715..7bdc5d570a12 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -109,7 +109,7 @@ static int pwm_backlight_update_status(struct backlight_device *bl)
>  		pwm_backlight_power_off(pb);
>
>  		pwm_get_state(pb->pwm, &state);
> -		state.enabled = false;
> +		state.enabled = true;
>  		state.duty_cycle = 0;
>  		pwm_apply_state(pb->pwm, &state);
>  	}
> --
> 2.39.0
>