linux-kernel - Re: [PATCH v4 1/4] pwm: pca9685: Switch to atomic API

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Message-ID: <20201208091033.bxzrlad7mjbe3dsp@pengutronix.de>
Date:   Tue, 8 Dec 2020 10:10:33 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Clemens Gruber <clemens.gruber@...ruber.com>
Cc:     linux-pwm@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Lee Jones <lee.jones@...aro.org>, linux-kernel@...r.kernel.org,
        Sven Van Asbroeck <TheSven73@...il.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        David Jander <david@...tonic.nl>
Subject: Re: [PATCH v4 1/4] pwm: pca9685: Switch to atomic API

Hello Clemens,

On Tue, Dec 08, 2020 at 12:13:44AM +0100, Clemens Gruber wrote:
> On Mon, Dec 07, 2020 at 11:00:25PM +0100, Uwe Kleine-König wrote:
> > On Mon, Dec 07, 2020 at 08:36:27PM +0100, Clemens Gruber wrote:
> > > The hardware readout may return slightly different values than those
> > > that were set in apply due to the limited range of possible prescale and
> > > counter register values. If one channel is reconfigured with new duty
> > > cycle and period, the others will keep the same relative duty cycle to
> > > period ratio as they had before, even though the per-chip / global
> > > frequency changed. (The PCA9685 has only one prescaler!)
> > 
> > This is not acceptable, if you have two PWM outputs and a consumer
> > modifies one of them the other must change. So if this chip only
> > supports a single period length of all channels, the first consumer
> > enabling a channel defines the period to be used. All later consumers
> > must live with that. (Also the first must be denied modifying the period
> > if a second consumer has enabled its PWM.)
> 
> Good idea, but is it OK to potentially break users relying on the old
> behavior ("the last one who changes the period wins") ?

If this is already in the old code, this probably warrants a separate
fix, and yes, I consider this a severe bug. (Consider one channel
driving a motor and reconfiguring an LED modifies the motor's speed.)

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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