linux-kernel - Re: (EXT) Re: [PATCH 1/4] pwm: pca9685: remove unused duty

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Message-ID: <20200330151231.GA1650@workstation.tuxnet>
Date:   Mon, 30 Mar 2020 17:12:31 +0200
From:   Clemens Gruber <clemens.gruber@...ruber.com>
To:     Matthias Schiffer <matthias.schiffer@...tq-group.com>
Cc:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, thierry.reding@...il.com,
        linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
        andy.shevchenko@...il.com
Subject: Re: (EXT) Re: [PATCH 1/4] pwm: pca9685: remove unused duty_cycle
 struct element

Hi,

On Wed, Feb 26, 2020 at 06:03:02PM +0100, Matthias Schiffer wrote:
> As it turns out, this driver is broken in yet another way I didn't find
> before: For changing the global prescaler the chip needs to be put into
> sleep mode, but the driver doesn't follow the restart sequence
> described in the datasheet when waking it back up. In consequence,
> changing the period of one PWM does not only modify the period of all
> PWMs (which is bad enough, but can't be avoided with this hardware),
> but it also leaves all PWMs disabled...

I am unable to reproduce this: If I set a specific duty cycle on a
channel and then change the period, the channel stays active.
I can see the brightness of an LED decrease if I increase the period.

This is expected, because after the SLEEP bit is set, we wait for
500usecs and then write to the LED ON/OFF registers.
This leaves the channel enabled with the new period (but with old
duty_ns value => different ratio)

A few years ago, I played around with the idea of remembering the
duty_ns to period_ns ratio and setting it accordingly after a period
change, possibly also with a shortcut of setting the RESTART bit if the
ratio did not change. Maybe after the switch to the atomic API, this
would be a nice improvement.

Best regards,
Clemens