From: Clemens Gruber <clemens.gruber@pqgruber.com>
To: Sven Van Asbroeck <thesven73@gmail.com>
Cc: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
"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: Fri, 29 Jan 2021 17:31:47 +0100 [thread overview]
Message-ID: <YBQ4c2cYYPDMjkeH@workstation.tuxnet> (raw)
In-Reply-To: <CAGngYiW=KhCOZX3tPMFykXzpWLpj3qusN2OXVPSfHLRcyts+wA@mail.gmail.com>
Hi Sven,
On Fri, Jan 29, 2021 at 08:42:13AM -0500, Sven Van Asbroeck wrote:
> On Mon, Jan 11, 2021 at 3:35 PM Uwe Kleine-König
> <u.kleine-koenig@pengutronix.de> wrote:
> >
> > 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 agree that this is the most user-friendly behaviour.
>
> The problem however with the pca9685 is that it has many degrees of
> freedom: there are many possible register values which produce the same
> physical chip outputs.
>
> This could lead to a situation where, if .probe() does not reset the register
> values, subsequent writes may lead to different outputs than expected.
>
> One possible solution is to write .get_state() so that it always reads the
> correct state, even if "unconventional" register settings are present, i.e.
> those written by an outside entity, e.g. a bootloader. Then write that
> state back using driver conventions.
>
> This may be trickier than it sounds - after all we've learnt that the pca9685
> looks simple on the surface, but is actually quite challenging to get right.
>
> Clemens, Uwe, what do you think?
Ok, so you suggest we extend our get_state logic to deal with cases
like the following:
If neither full OFF nor full ON is set && ON == OFF, we should probably
set the full OFF bit to disable the PWM and log a warning message?
(e.g. "invalid register setting detected: pwm disabled" ?)
If the ON registers are set and the nxp,staggered-outputs property is
not, I'd calculate (off - on) & 4095, set the OFF register to that value
and clear the ON register.
And then call our get_state in .probe, followed by a write of the
resulting / fixed-up state?
This would definitely solve the problem of invalid/unconventional values
set by the bootloader and avoid inconsistencies.
Sounds good to me!
If Thierry and Uwe have no objections, I can send out a new round of
patches in the upcoming weeks.
My current goal is to get the changes into 5.13.
Thanks,
Clemens
next prev parent reply other threads:[~2021-01-29 16:34 UTC|newest]
Thread overview: 36+ 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
2021-01-14 18:05 ` Uwe Kleine-König
2021-03-22 8:53 ` Thierry Reding
[not found] ` <CAGngYiW=KhCOZX3tPMFykXzpWLpj3qusN2OXVPSfHLRcyts+wA@mail.gmail.com>
2021-01-29 16:31 ` Clemens Gruber [this message]
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
[not found] ` <CAHp75Ve2FFEMsAv8S18bUDFsH2UkiQ5UvgcRtZ=j30syQtEirw@mail.gmail.com>
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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