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

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Date:   Tue, 8 Dec 2020 09:44:42 -0500
From:   Sven Van Asbroeck <thesven73@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Clemens Gruber <clemens.gruber@...ruber.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, linux-pwm@...r.kernel.org,
        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

Uwe, Thierry,

On Tue, Dec 8, 2020 at 4:10 AM Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> wrote:
>
> 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.)
>

I think you are 100% correct, this would be a severe bug. I have only used
this chip to drive LEDs, where the actual period is not that important. But
for motor control, it's a different story.

Basically you are suggesting: the period (prescaler) can only be changed iff
its use-count is 1.

This however brings up a whole load of additional questions: consider the case
where the chip outputs are also used in gpio mode. the gpio functionality
only sets "full on" and "full off" bits. On a scope, a gpio output will look
identical, no matter the value of the period. So when a gpio output is in use,
does it increment the prescaler use-count ?

Example:
1. output 1: set pwm mode (enabled=true, duty_cycle=50%, period=1/200Hz)
2. output 2: set led mode (full-on bit set)
3. output 1: change period(enabled=true, duty_cycle=50%, period=1/100Hz)

Do we have to make (3) fail? I would say no: although output 2 is in use,
it's not actually using the prescaler. Changing prescale won't modify
output 2 in any way.

Which brings us to an even trickier question: what happens if a pwm output
is set to 0% or 100% duty cycle? In that case, it'll behave like a gpio output.
So when it's enabled, it does not use the prescaler.
But! what happens if we now set that output to a different duty cycle?

Example:
1. output 1: set pwm mode (enabled=true, duty_cycle=50%,  period=1/200Hz)
2. output 2: set pwm mode (enabled=true, duty_cycle=100%, period=1/400Hz)
  fail? no, because it's not actually using the period (it's full on)
3. output 2: set pwm mode (enabled=true, duty_cycle=100%, period=1/200Hz)
  fail? no, because it's not actually using the period (it's full on)
4. output 1: set pwm mode (enabled=true, duty_cycle=50%,  period=1/400Hz)
  fail? no, because only output 1 is using the prescaler
5. output 2: set pwm mode (enabled=true, duty_cycle=50%, period=1/400Hz)
  fail? no, because output 2 is not changing the prescaler
6. output 2: set pwm mode (enabled=true, duty_cycle=50%, period=1/200Hz)
  fail? yes, because output 2 is changing prescaler and it's already in use

IMHO all this can get very complicated and tricky.

We can of course make this much simpler by assumung that gpio or on/off pwms
are actually using the prescaler. But then we'd be limiting this chip's
functionality.