Re: [RFC PATCH 08/15] backlight: pwm_bl: remove useless call to pwm_set_period

From: Boris Brezillon <boris.brezillon@free-electrons.com>
To: Thierry Reding <thierry.reding@gmail.com>
Cc: linux-pwm@vger.kernel.org, Mark Brown <broonie@kernel.org>,
	Liam Girdwood <lgirdwood@gmail.com>,
	Bryan Wu <cooloney@gmail.com>, Richard Purdie <rpurdie@rpsys.net>,
	Jacek Anaszewski <j.anaszewski@samsung.com>,
	linux-leds@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>,
	Tomi Valkeinen <tomi.valkeinen@ti.com>,
	linux-fbdev@vger.kernel.org,
	Stephen Warren <swarren@wwwdotorg.org>,
	Alexandre Courbot <gnurou@gmail.com>,
	linux-tegra@vger.kernel.org,
	Maxime Ripard <maxime.ripard@free-electrons.com>,
	Jingoo Han <jingoohan1@gmail.com>,
	Lee Jones <lee.jones@linaro.org>,
	Doug Anderson <dianders@google.com>
Subject: Re: [RFC PATCH 08/15] backlight: pwm_bl: remove useless call to pwm_set_period
Date: Mon, 20 Jul 2015 10:50:03 +0200	[thread overview]
Message-ID: <20150720105003.29b5813a@bbrezillon> (raw)
In-Reply-To: <20150720083648.GK29614@ulmo>

On Mon, 20 Jul 2015 10:36:50 +0200
Thierry Reding <thierry.reding@gmail.com> wrote:

> On Mon, Jul 20, 2015 at 10:21:43AM +0200, Boris Brezillon wrote:
> > On Mon, 20 Jul 2015 10:16:00 +0200
> > Thierry Reding <thierry.reding@gmail.com> wrote:
> > 
> > > On Wed, Jul 01, 2015 at 10:21:54AM +0200, Boris Brezillon wrote:
> > > > The PWM period will be set when calling pwm_config. Remove this useless
> > > > call to pwm_set_period, which might mess up with the initial PWM state
> > > > once we have added proper support for PWM init state retrieval.
> > > > 
> > > > Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
> > > > ---
> > > >  drivers/video/backlight/pwm_bl.c | 4 +---
> > > >  1 file changed, 1 insertion(+), 3 deletions(-)
> > > > 
> > > > diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> > > > index ae498c1..fe5597c 100644
> > > > --- a/drivers/video/backlight/pwm_bl.c
> > > > +++ b/drivers/video/backlight/pwm_bl.c
> > > > @@ -295,10 +295,8 @@ static int pwm_backlight_probe(struct platform_device *pdev)
> > > >  	 * via the PWM lookup table.
> > > >  	 */
> > > >  	pb->period = pwm_get_default_period(pb->pwm);
> > > > -	if (!pb->period && (data->pwm_period_ns > 0)) {
> > > > +	if (!pb->period && (data->pwm_period_ns > 0))
> > > >  		pb->period = data->pwm_period_ns;
> > > > -		pwm_set_period(pb->pwm, data->pwm_period_ns);
> > > > -	}
> > > >  
> > > >  	pb->lth_brightness = data->lth_brightness * (pb->period / pb->scale);
> > > 
> > > As far as I remember this line is there in order to pass in a period if
> > > the backlight driver is initialized from board setup files. In such a
> > > case there won't be an period associated with the PWM channel in the
> > > first place.
> > > 
> > > I think even with the introduction of a default period, we'd be missing
> > > out on the board setup case because there is no standard place where it
> > > is being set, so it must come from the platform data.
> > 
> > AFAICT, we don't need to explicitly set the period when probing the
> > backlight device, because it will be set next time we call
> > pwm_config(), and since we're passing pb->period when calling
> > pwm_config() everything should be fine.
> 
> Calling pwm_set_period() is still good for consistency. Consider for
> example what happens if after the driver were to call pwm_get_period().
> It would return some more or less random value (likely 0 or whatever it
> had been set to by an earlier user).

Yes, that's true in general, but in this specific driver
pwm_get_period() is never called, and the driver only relies on the
pb->period value.

> 
> Technically I think the most proper equivalent here would be to set the
> default state's period to data->pwm_period_ns, but I don't think that's
> proper to do. Perhaps since this is only relevant to boards where the
> backlight device is created from board setup code we don't have to care
> so much about messing up the initial state because either the board
> setup code has been carefully written to match what the bootloader set
> up, or because they don't match at all, in which case we don't have to
> worry anyway.

IMHO, if we had to support default period values for non DT boards, the
proper way would be to pass something in the PWM platform data and let
the PWM driver (or PWM core) initialize the default PWM state.
This way the PWM user could rely on the pwm_get_default_period() helper
to extract the default period value.

-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com