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

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Message-ID: <X865s8SRLEn4EAIi@workstation.tuxnet>
Date:   Tue, 8 Dec 2020 00:24:35 +0100
From:   Clemens Gruber <clemens.gruber@...ruber.com>
To:     Sven Van Asbroeck <thesven73@...il.com>
Cc:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, linux-pwm@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        David Jander <david@...tonic.nl>
Subject: Re: [PATCH v4 1/4] pwm: pca9685: Switch to atomic API

On Mon, Dec 07, 2020 at 05:34:58PM -0500, Sven Van Asbroeck wrote:
> Hi Uwe,
> 
> On Mon, Dec 7, 2020 at 5:00 PM Uwe Kleine-König
> <u.kleine-koenig@...gutronix.de> wrote:
> >
> > 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.)
> 
> That makes sense. However, a possible wrinkle: when more than one pwm channel
> is requested, which one is able to change the period?
> 
> Example:
> 1. start with all pwms free
> 2. pwm_request(0), pwm_apply(period=200Hz)
> 3. pwm_request(1)
> 4. pwm_apply(1, period=400Hz) fails?
> 5. pwm_apply(0, period=400Hz) succeeds?
> 
> And if (5) succeeds, then pwm_get_state(1) will still return period=200Hz,
> because the pwm core doesn't realize anything has changed. Are you ok
> with this behaviour?

I think we'd have to deny the pwm_apply in step 5 as well. So, only the
first consumer is allowed to change the period and only as long as it is
the only one that is in use / was requested.

But that's definitely a breaking change.

Thanks,
Clemens