Messages in this thread | | | Date | Tue, 8 Dec 2020 21:54:22 +0100 | From | Clemens Gruber <> | Subject | Re: [PATCH v4 1/4] pwm: pca9685: Switch to atomic API |
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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
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