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

Thierry Reding
2015-07-20T08:36:50Z

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).

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.

Of course the right thing to do would be to replace all initialization
of the data->pwm_period_ns by proper PWM lookup tables, but that's
proven difficult in the past since very few people still have access to
hardware where that code gets executed.

Thierry
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 819 bytes
Desc: not available
URL: <http://lists.infradead.org/pipermail/linux-arm-kernel/attachments/20150720/fa5731d1/attachment.sig>