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

[Clemens Gruber <clemens.gruber@pqgruber.com>]

Hi everyone,

On Tue, Dec 08, 2020 at 07:26:37PM +0100, Uwe Kleine-König wrote:
> Hello Thierry, hello Sven,
> 
> On Tue, Dec 08, 2020 at 05:57:12PM +0100, Thierry Reding wrote:
> > On Tue, Dec 08, 2020 at 09:44:42AM -0500, Sven Van Asbroeck wrote:
> > > On Tue, Dec 8, 2020 at 4:10 AM Uwe Kleine-König
> > > <u.kleine-koenig@pengutronix.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.
> > 
> > Is this really that complicated?
> 
> I think it is.
> 
> > I sounds to me like the only thing that you need is to have some sort
> > of usage count for the prescaler. Whenever you want to use the
> > prescaler you check that usage count. If it is zero, then you can just
> > set it to whatever you need. If it isn't zero, that means somebody
> > else is already using it and you can't change it, which means you have
> > to check if you're trying to request the value that's already set. If
> > so, you can succeed, but otherwise you'll have to fail.
> 
> With this abstraction Sven's questions are changed to:
> 
> Does a PWM that runs at 0% or 100% use the prescaler?
> 
> If yes, you limit the possibilities of the brother channels. And if not,
> it will not be possible to go to a 50% relative duty cycle while
> retaining the period. Both sounds not optimal.

In my opinion, limiting the possibilities of brother channels is
preferrable to introducing another restriction: Not being able to
reconfigure a duty cycle from 0%/100% to something else while keeping
the previously set period.
Better deny the period change in the first place, even if the duty cycle
is 0% or 100%.

>  
> > > 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.
> > 
> > Yeah, this is obviously much simpler, but the cost is a bit high, in my
> > opinion. I'm fine with this alternative if there aren't any use-cases
> > where multiple outputs are actually used.
> 
> This metric is wishy-washy; of course you can construct a use-case. I'd
> still go for this simpler option and assume the prescaler used if the
> PWM runs at 0% or 100%, but not when it is a GPIO.

I'd also prefer this solution.

Thanks,
Clemens