Re: [PATCH] backlight: pwm_bl: configure pwm only once per backlight toggle

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

On Thu, Oct 17, 2019 at 10:10:59AM +0200, Uwe Kleine-König wrote:
> A previous change in the pwm core (namely 01ccf903edd6 ("pwm: Let
> pwm_get_state() return the last implemented state")) changed the
> semantic of pwm_get_state() and disclosed an (as it seems) common
> problem in lowlevel PWM drivers. By not relying on the period and duty
> cycle being retrievable from a disabled PWM this type of problem is
> worked around.
> 
> Apart from this issue only calling the pwm_get_state/pwm_apply_state
> combo once is also more effective.

I'm only interested in the second paragraph here.

There seems to be a reasonable consensus that the i.MX27 and cros-ec
PWM drivers should be fixed for the benefit of other PWM clients.
So we make this change because it makes the pwm-bl better... not to
work around bugs ;-).

> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index 746eebc411df..ddebd62b3978 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -67,40 +62,27 @@ static void pwm_backlight_power_on(struct pwm_bl_data *pb)
>  
>  static void pwm_backlight_power_off(struct pwm_bl_data *pb)
>  {
> -	struct pwm_state state;
> -
> -	pwm_get_state(pb->pwm, &state);
> -	if (!pb->enabled)
> -		return;
> -

Why remove the pb->enabled check? I thought that was there to ensure we
don't mess up the regular reference counts.


Daniel.