Ticket #2380 (closed defect: fixed)
pwmconfig doesn't detect correlations properly
|Reported by:||khali||Owned by:||khali|
Original report from Charles Pillar on the lm-sensors list:
Unfortunately the post is incomplete in the archive, so I duplicate it below.
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 board has 2 fans, the other 2 have 3 fans each)
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...