linux-kernel - Re: [PATCH] pwm: cros-ec: Let cros_ec_pwm_get

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Date:   Wed, 9 Oct 2019 11:42:36 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        linux-kernel@...r.kernel.org, thierry.reding@...il.com,
        heiko@...ech.de, dianders@...omium.org, mka@...omium.org,
        groeck@...omium.org, kernel@...labora.com, bleung@...omium.org,
        linux-pwm@...r.kernel.org, Lee Jones <lee.jones@...aro.org>
Subject: Re: [PATCH] pwm: cros-ec: Let cros_ec_pwm_get_state() return the
 last applied state

On Wed, Oct 09, 2019 at 12:16:37PM +0200, Uwe Kleine-König wrote:
> On Wed, Oct 09, 2019 at 10:56:35AM +0100, Daniel Thompson wrote:
> > On Wed, Oct 09, 2019 at 11:27:13AM +0200, Enric Balletbo i Serra wrote:
> > > Hi Uwe,
> > > 
> > > Adding Daniel and Lee to the discussion ...
> > 
> > Thanks!
> > 
> > > On 8/10/19 22:31, Uwe Kleine-König wrote:
> > > > On Tue, Oct 08, 2019 at 06:33:15PM +0200, Enric Balletbo i Serra wrote:
> > > >>> A few thoughts to your approach here ...:
> > > >>>
> > > >>>  - Would it make sense to only store duty_cycle and enabled in the
> > > >>>    driver struct?
> > > >>>
> > > >>
> > > >> Yes, in fact, my first approach (that I didn't send) was only storing enabled
> > > >> and duty cycle. For some reason I ended storing the full pwm_state struct, but I
> > > >> guess is not really needed.
> > > >>
> > > >>
> > > >>>  - Which driver is the consumer of your pwm? If I understand correctly
> > > >>>    the following sequence is the bad one:
> > > >>>
> > > >>
> > > >> The consumer is the pwm_bl driver. Actually I'n trying to identify
> > > >> other consumers.
> > > > 
> > > 
> > > So far, the pwm_bl driver is the only consumer of cros-ec-pwm.
> > > 
> > > > Ah, I see why I missed to identify the problem back when I checked this
> > > > driver. The problem is not that .duty_cycle isn't set but there .enabled
> > > > isn't set. So maybe we just want:
> > > > 
> > > > diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> > > > index 2201b8c78641..0468c6ee4448 100644
> > > > --- a/drivers/video/backlight/pwm_bl.c
> > > > +++ b/drivers/video/backlight/pwm_bl.c
> > > > @@ -123,6 +123,7 @@ static int pwm_backlight_update_status(struct backlight_device *bl)
> > > >         if (brightness > 0) {
> > > >                 pwm_get_state(pb->pwm, &state);
> > > >                 state.duty_cycle = compute_duty_cycle(pb, brightness);
> > > > +               state.enabled = true;
> > > >                 pwm_apply_state(pb->pwm, &state);
> > > >                 pwm_backlight_power_on(pb);
> > > >         } else
> > > > 
> > > > ? On a side note: It's IMHO strange that pwm_backlight_power_on
> > > > reconfigures the PWM once more.
> > > > 
> > > 
> > > Looking again to the pwm_bl code, now, I am not sure this is correct (although
> > > it probably solves the problem for me).
> > 
> > Looking at the pwm_bl code I wouldn't accept the above as it is but I'd
> > almost certainly accept a patch to pwm_bl to move the PWM enable/disable
> > out of both the power on/off functions so the duty-cycle/enable or
> > disable can happen in one go within the update_status function. I don't
> > think such a change would interfere with the power and enable sequencing
> > needed by panels and it would therefore be a nice continuation of the
> > work to convert over to the pwm_apply_state() API.
> 
> OK for me. Enric, do you care enough to come up with a patch for pwm_bl?
> I'd expect that this alone should already fix your issue.
>  
> > None of the above has anything to do with what is right or wrong for
> > the PWM API evolution. Of course, if this thread does conclude that it
> > is OK the duty cycle of a disabled PWM to be retained for some drivers
> > and not others then I'd hope to see some WARN_ON()s added to the PWM
> > framework to help bring problems to the surface with all drivers.
> 
> I think it's not possible to add a reliable WARN_ON for that issue. It
> is quite expected that .get_state returns something that doesn't
> completely match the requested configuration. So if a consumer requests
> 
> 	.duty_cycle = 1
> 	.period = 100000000
> 	.enabled = false
> 
> pwm_get_state possibly returns .duty_cycle = 0 even for drivers/hardware
> that has a concept of duty_cycle for disabled hardware.
> 
> A bit this is addressed in https://patchwork.ozlabs.org/patch/1147517/.

Isn't that intended to help identify "odd" PWM drivers rather than "odd"
clients?

Initially I was thinking that a WARN_ON() could be emitted when:

1. .duty_cycle is non-zero
2. .enabled is false
3. the PWM is not already enabled

(#3 included to avoid too many false positives when disabling a PWM)

A poisoning approach might be equally valid. If some drivers are
permitted to "round" .duty_cycle to 0 when .enabled is false then the
framework could get *all* drivers to behave in the same way by
zeroing it out before calling the drivers apply method. It is not that
big a deal but minimising the difference between driver behaviour should
automatically reduce the difference in API usage by clients.


Daniel.