On Sat, Jan 28, 2012 at 9:41 AM, Jean Delvare <khali@xxxxxxxxxxxx> wrote: > On Sat, 28 Jan 2012 00:52:17 +0100, CC wrote: >> We have that >> >> * fan1 is linked to pwm1 >> * fan2 is linked to pwm2 >> * fan4 can only be monitored >> * fan5 is linked to pwm3 >> >> Only fan2 has a 4-pin connector and could thus be controlled by PWM, >> the others can only be controlled by voltage. > > This assumption is wrong. 3-pin fans can be controlled by PWM too. It > is frequent that CPU fan == 4 pins controlled with PWM and case fan == > 3 pins controlled with DC but other combinations are possible too. > >> Let's assume all fans to be in manual mode. >> >> * regardless of pwm1, fan1 runs at full speed when pwm1_mode returns 0 >> * the fan's speed can be controlled by pwm1 when pwm1_mode returns 1 >> >> So DC mode is only (correctly) activated when pwm1_mode returns 1. >> >> * fan2 stops when pwm2 returns 0 and pwm2_mode also returns 0 >> * fan2 runs slowly when pwm2 returns 0 and pwm2_mode returns 1 >> >> I interpret this as DC mode being used when pwm2_mode returns 0. >> >> * regardless of pwm3, fan5 runs at full speed whenever pwm3_mode returns 0 >> * the fan's speed can be controlled when pwm1_mode returns 1 >> >> So DC mode is only (correctly) activated when pwm3_mode returns 1. > > You are not supposed to change pwm*_mode attribute values in the first > place. Your board should come up with the right settings from the BIOS. > In most cases the output pins of the chip aren't connected directly to > the fan headers, there is some electronics involved on the path which > only works as designed if you use the right output mode, so changing > the mode simply breaks fan control. > > -- > Jean Delvare It's bad to provide a writable attribute and documentation if one isn't supposed to ever change it... let me rewrite this after a cold boot, where I didn't change any settings, where I use the exact same fan, and where I employ pwmcontrol: pwm1_mode returns 1. Between "PWM 255" and "PWM 75" the fan speed correlation is very jagged, very non-linear. pwm2_mode returns 1. Between "PWM 210" and "PWM 60" the fan speed correlation is surprisingly close to linear. pwm3_mode returns 1. Between "PWM 255" and "PWM 75" the fan speed correlation is very jagged, very non-linear. So the behavior is different, although pwm*_mode returns the same value. Moreover: * Three pin connectors have ground, +12V and a tachometer signal. * Four pin connectors have ground, +12V, a tachometer signal and a PWM signal. Therefore, only unusual systems can control an unsual 3-pin fan by PWM. As per the standard, http://www.formfactors.org/developer/specs/4_Wire_PWM_Spec.pdf, the correlation between the fan speed and the PWM signal is supposed to be linear between its min and max. I conclude that PWM mode is working correctly for pwm2, and DC mode (the name is from the docs -- most likely it's linear voltage regulation) is working correctly for pwm1 and pwm3. So: I didn't change any settings, but it seems like the meaning of pwm*_mode does not agree with the documentation. Best, CC _______________________________________________ lm-sensors mailing list lm-sensors@xxxxxxxxxxxxxx http://lists.lm-sensors.org/mailman/listinfo/lm-sensors
