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

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

On Thu, Oct 17, 2019 at 12:47:27PM +0100, Daniel Thompson wrote:
> 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 ;-).

That's fine, still I think it's fair to explain the motivation of
creating this patch.

> > 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.

I havn't looked yet, but I guess I have to respin. Expect a v2 later
today.

Best regards
Uwe

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