I'm new here. I believe that I have found a bug in pwmconfig. I first observed this behavior many many months ago and couldn't find anyone else with the problem so I just assumed it was just me. I've since stumbled on it again so I decided to look into it myself. I don't know if anyone is aware of the behavior I am seeing, but here it is...
Take for example a board with two or more PWM controllable fans both which of which the speed can be measured. Thus I have:
/sys/class/hwmon/hwmon0/pwm1
/sys/class/hwmon/hwmon0/pwm1_enable
/sys/class/hwmon/hwmon0/fan1_input
/sys/class/hwmon/hwmon0/pwm2
/sys/class/hwmon/hwmon0/pwm2_enable
/sys/class/hwmon/hwmon0/fan2_input
(etc...)
pwm1 & pwm1_enable = fan1_input
pwm2 & pwm2_enable = fan2_input
(etc...)
I think this would be a fairly common set up? Indeed I have three machines that are setup this way (1 has 2 fans, the other 2 have 3 fans)
From what I can see, pwmconfig does this:
pwm1_enable=0
pwm2_enable=0
wait...
for each PWM:
this pwm_enable=1
this pwm=0
for each fan
compare this fan before / after
this pwm_enable=0
check fan returns to normal
next fan
next pwm
The problem with this logic is that for each PWM, the pwm_enable is set to 1, then the first fan is tested, after the first fan is tested, the pwm is disabled and never re-enabled (until the next pwm)...
This means the pwm1=fan1 correlation is detected, but pwm2=fan2 is not - but only because the pwm_enable is still set to 0 when the second and subsequent fans are tested...
There are several ways to fix this, but after some pondering, I think that this logic might be most appropriate:
pwm1_enable=0
pwm2_enable=0
wait...
for each PWM:
for each fan
this pwm_enable=1
this pwm=0
compare this fan before / after
this pwm_enable=0
check fan returns to normal
next fan
next pwm
This causes the given pwm/fan to stop/start once for each fan input, which might seem bad, but then each fan is individually checked to make sure that it has begun spinning again after pwm_enable is set to 0... (as compared to stopping and starting once per pwm...)
I have a patch which I can supply. It fixes the problem for me, but I'm not 100% sure if it introduces any other problems. Let me know if I should email it or submit somewhere etc...
I would value hearing what others think about this problem...
Thanks
Charles
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