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

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Date:   Mon, 7 Dec 2020 23:00:25 +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

On Mon, Dec 07, 2020 at 08:36:27PM +0100, Clemens Gruber wrote:
> The switch to the atomic API goes hand in hand with a few fixes to
> previously experienced issues:
> - The duty cycle is no longer lost after disable/enable (previously the
>   OFF registers were cleared in disable and the user was required to
>   call config to restore the duty cycle settings)
> - If one sets a period resulting in the same prescale register value,
>   the sleep and write to the register is now skipped
> - The prescale register is now set to the default value in probe. On
>   systems without CONFIG_PM, the chip is woken up at probe time.
> 
> 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.)

> Note that although the datasheet mentions 200 Hz as default frequency
> when using the internal 25 MHz oscillator, the calculated period from
> the default prescaler register setting of 30 is 5079040ns.

That means the datasheet is lax because 5079040ns corresponds to
196.88760080645162 Hz but it calls that 200 Hz, right?

I didn't look in the patch in detail, but get the impression it is more
complicated than necessary. For example adding improved PM behaviour
should probably go into a separate patch, also adding the .get_state
callback should be split out.

Best regards
Uwe

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

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