Uwe, Thierry, On Tue, Dec 8, 2020 at 4:10 AM Uwe Kleine-König <u.kleine-koe...@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. 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.
