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, 4 Mar 2021 14:22:12 +0100 [thread overview]
Message-ID: <YEDfBPsbZ5uxj/PP@workstation.tuxnet> (raw)
In-Reply-To: <20210301215248.ekclgxc7dq6asdz5@pengutronix.de>
Hi Uwe,
On Mon, Mar 01, 2021 at 10:52:48PM +0100, Uwe Kleine-König wrote:
> Hello,
>
> On Mon, Feb 01, 2021 at 06:24:02PM +0100, Clemens Gruber wrote:
> > Hi Sven, Thierry, Uwe,
> >
> > On Fri, Jan 29, 2021 at 05:16:51PM -0500, Sven Van Asbroeck wrote:
> > > Hi Clemens,
> > >
> > > On Fri, Jan 29, 2021 at 4:24 PM Sven Van Asbroeck <thesven73@gmail.com> wrote:
> > > >
> > > > LEN_ON = 409, LED_OFF = 1228 and
> > > > LED_ON = 419, LED_OFF = 1238
> > > > produce the same result. you can't see the difference between the two
> > > > when scoping the channel. there are probably more ways to do this,
> > > > some might surprise us. It's a tricky chip.
> > >
> > > Please ignore this example, it's bogus. In my defence, it's a Friday
> > > afternoon here :)
> >
> > Happens to the best of us :)
> >
> > >
> > > But consider the following: imagine the bootloader has enabled a few
> > > pwm channels, and the driver's .probe() has left them on/unchanged.
> > > Then the user enables another pwm channel, and tries to change the
> > > period/prescaler. How would pca9685_may_change_prescaler() know
> > > if changing the prescaler is allowed?
> > >
> > > And the following: imagine the bootloader has enabled a few
> > > pwm channels, and the driver's .probe() has left them on/unchanged.
> > > After .probe(), the runtime_pm will immediately put the chip to sleep,
> > > because it's unaware that some channels are alive.
> >
> > (We could read out the state in .probe. If a pwm is already enabled by
> > the bootloader, then the user can't change the period. Also, the chip
> > would not be put to sleep.
> >
> > The user then can export channels and see if they are enabled. If he
> > wants to change the period, he needs to find the one enabled by the
> > bootloader and change the period there, before he requests more.
> > If the bootloader enabled more than one, then he has to disable all but
> > one to change the period.
> >
> > Or did I miss something?)
> >
> > >
> > > I'm sure I'm overlooking a few complications here. probe not changing
> > > the existing configuration, will add a lot of complexity to the driver.
> > > I'm not saying this is necessarily bad, just a tradeoff. Or, a management
> > > decision.
> >
> > But I agree that it is simpler if we keep the resets in probe. It would
> > also avoid a potentially breaking change for users that do not reset
> > their pca9685 chips in their bootloader code.
>
> I would prefer to drop the reset. If the bootloader left with an invalid
> state, this is active for sure until the PWM driver is loaded. If you
> don't reset, the time is extended (usually) until the consumer comes
> along and corrects the setting. So the downside of not resetting is
> quite limited, but if you disable the PWM in .probe() the effect can be
> worse. And consistency dictates to not reset.
>
> > Removing the resets could then be left as something to discuss further
> > in the future and something that belongs in a separate patch series?
>
> That would be fine for me, too.
Great, then I will prepare a new series next week.
Thanks,
Clemens
next prev parent reply other threads:[~2021-03-04 13:23 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
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 [this message]
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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