From: Clemens Gruber <clemens.gruber@pqgruber.com>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: Sven Van Asbroeck <thesven73@gmail.com>,
Thierry Reding <thierry.reding@gmail.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-pwm@vger.kernel.org
Subject: Re: [PATCH v5 2/7] pwm: pca9685: Support hardware readout
Date: Thu, 14 Jan 2021 18:16:22 +0100 [thread overview]
Message-ID: <YAB8ZmtOxRV1QN4l@workstation.tuxnet> (raw)
In-Reply-To: <20210111203532.m3yvq6e5bcpjs7mc@pengutronix.de>
Hi,
On Mon, Jan 11, 2021 at 09:35:32PM +0100, Uwe Kleine-König wrote:
> Hello,
>
> On Thu, Dec 17, 2020 at 06:43:04PM +0100, Clemens Gruber wrote:
> > On Wed, Dec 16, 2020 at 11:00:59PM -0500, Sven Van Asbroeck wrote:
> > > On Wed, Dec 16, 2020 at 7:53 AM Clemens Gruber
> > > <clemens.gruber@pqgruber.com> wrote:
> > > >
> > > > Implements .get_state to read-out the current hardware state.
> > > >
> > >
> > > I am not convinced that we actually need this.
> > >
> > > Looking at the pwm core, .get_state() is only called right after .request(),
> > > to initialize the cached value of the state. The core then uses the cached
> > > value throughout, it'll never read out the h/w again, until the next .request().
> > >
> > > In our case, we know that the state right after request is always disabled,
> > > because:
> > > - we disable all pwm channels on probe (in PATCH v5 4/7)
> > > - .free() disables the pwm channel
> > >
> > > Conclusion: .get_state() will always return "pwm disabled", so why do we
> > > bother reading out the h/w?
> >
> > If there are no plans for the PWM core to call .get_state more often in
> > the future, we could just read out the period and return 0 duty and
> > disabled.
> >
> > Thierry, Uwe, what's your take on this?
>
> I have some plans here. In the past I tried to implement them (see
> commit 01ccf903edd65f6421612321648fa5a7f4b7cb10), but this failed
> (commit 40a6b9a00930fd6b59aa2eb6135abc2efe5440c3).
>
> > > Of course, if we choose to leave the pwm enabled after .free(), then
> > > .get_state() can even be left out! Do we want that? Genuine question, I do
> > > not know the answer.
> >
> > I do not think we should leave it enabled after free. It is less
> > complicated if we know that unrequested channels are not in use.
>
> My position here is: A consumer should disable a PWM before calling
> pwm_put. The driver should however not enforce this and so should not
> modify the hardware state in .free().
>
> Also .probe should not change the PWM configuration.
I see. This would also allow PWMs initialized in the bootloader (e.g.
backlights) to stay on between the bootloader and Linux and avoid
flickering.
If no one objects, I would then no longer reset period and duty cycles
in the driver (and for our projects, reset them in the bootloader code
to avoid leaving PWMs on after a kernel panic and watchdog reset, etc.)
And if there is no pre-known state of the registers, we actually need
the .get_state function fully implemented.
Thanks,
Clemens
next prev parent reply other threads:[~2021-01-14 17:17 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20201216125320.5277-1-clemens.gruber@pqgruber.com>
[not found] ` <20201216125320.5277-2-clemens.gruber@pqgruber.com>
2020-12-17 4:00 ` [PATCH v5 2/7] pwm: pca9685: Support hardware readout Sven Van Asbroeck
2020-12-17 17:43 ` Clemens Gruber
2020-12-17 17:52 ` Sven Van Asbroeck
2021-01-03 17:04 ` Clemens Gruber
2021-01-07 14:18 ` Sven Van Asbroeck
2021-01-11 20:43 ` Uwe Kleine-König
2021-03-22 8:34 ` Thierry Reding
2021-03-31 10:25 ` Uwe Kleine-König
2021-03-31 15:52 ` Thierry Reding
2021-04-06 6:33 ` Uwe Kleine-König
2021-04-06 13:47 ` Thierry Reding
2021-04-06 20:44 ` Uwe Kleine-König
2021-03-22 8:15 ` Thierry Reding
2021-01-11 20:35 ` Uwe Kleine-König
2021-01-14 17:16 ` Clemens Gruber [this message]
2021-01-14 18:05 ` Uwe Kleine-König
2021-03-22 8:53 ` Thierry Reding
2021-01-29 13:42 ` Sven Van Asbroeck
2021-01-29 16:31 ` Clemens Gruber
2021-01-29 18:05 ` Sven Van Asbroeck
2021-01-29 20:37 ` Clemens Gruber
2021-01-29 21:24 ` Sven Van Asbroeck
2021-01-29 22:16 ` Sven Van Asbroeck
2021-02-01 17:24 ` Clemens Gruber
2021-03-01 21:52 ` Uwe Kleine-König
2021-03-04 13:22 ` Clemens Gruber
2021-02-14 14:46 ` Clemens Gruber
2021-03-22 9:19 ` Thierry Reding
2021-03-22 9:38 ` Andy Shevchenko
2021-03-22 11:22 ` Uwe Kleine-König
2021-03-22 11:40 ` Andy Shevchenko
2021-03-22 11:48 ` Uwe Kleine-König
2021-03-22 12:15 ` Andy Shevchenko
2021-03-22 13:25 ` Uwe Kleine-König
2021-03-27 16:05 ` Clemens Gruber
2021-03-22 9:14 ` Thierry Reding
2021-03-22 8:47 ` Thierry Reding
2020-12-15 21:22 [PATCH v5 1/7] pwm: pca9685: Switch to atomic API Clemens Gruber
2020-12-15 21:22 ` [PATCH v5 2/7] pwm: pca9685: Support hardware readout Clemens Gruber
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