Hi.
On 03/09/2024 17:47, Uwe Kleine-König wrote:
Hello Benjamin,
On Tue, Sep 03, 2024 at 01:58:30PM +0200, Benjamin Larsson wrote:
On 2024-09-03 12:46, Uwe Kleine-König wrote:
Would you please add a "Limitations" paragraph here covering the
following questions:
- How does the hardware behave on changes of configuration (does it
complete the currently running period? Are there any glitches?)
- How does the hardware behave on disabling?
Please stick to the format used in several other drivers such that
sed -rn '/Limitations:/,/\*\/?$/p' drivers/pwm/*.c
emits the informations.
The answer to your questions are currently unknown. Is this information
needed for a merge of the driver ?
It would be very welcome and typically isn't that hard to work out if
you have an LED connected to the output or a similar means to observe
the output. An oscilloscope makes it still easier.
For example to check if the current period is completed configure the
PWM with period = 1s and duty_cycle = 0 disabling the LED. (I leave it
as an exercise for the reader what to do if duty_cycle = 0 enables the
LED :-) Then do:
pwm_apply_might_sleep(mypwm, &(struct pwm_state){
.period = NSEC_PER_SEC,
.duty_cycle = NSEC_PER_SEC,
.enabled = true,
});
pwm_apply_might_sleep(mypwm, &(struct pwm_state){
.period = NSEC_PER_SEC,
.duty_cycle = 0,
.enabled = true,
});
Iff that enables the LED for a second, the period is completed. The
question about glitches is a bit harder to answer, but with a tool like
memtool should be possible to answer. Alternatively add delays and
printk output to .apply() in the critical places.
I connected a logic analyzer to a pin and configured the pwm for it.
I then configured the pwm with these parameters (setup for 2Hz).
echo 1000000000 > /sys/class/pwm/pwmchip0/pwm12/period
echo 0 > /sys/class/pwm/pwmchip0/pwm12/duty_cycle
If I then ran the following (in a script) no pulse was detected:
echo 500000000 > /sys/class/pwm/pwmchip0/pwm12/duty_cycle
echo 0 > /sys/class/pwm/pwmchip0/pwm12/duty_cycle
If I added a sleep 1 in between I always got 1 500ms pulse.
I then did the same but with direct register access with the same
result. Setting the duty cycle to 0 disables the pwm function on the
pin, it seems to take a while before it properly activates but before it
disables it the cycle completes.
I also tested with enabling the pwn signal and then setting a 0 duty
cycle. The last observed pulse was always 500ms long.
I am not sure what of your questions this answers and is there some
other tests I should perform ?
For the record while toggling the registers I noticed that it was
actually possible to generate 1 second long pulses. The documentation is
not clear on this part.
MvH
Benjamin Larsson
