On Sat, Jan 28, 2012 at 1:53 PM, Jean Delvare <khali@xxxxxxxxxxxx> wrote: > On Sat, 28 Jan 2012 13:08:14 +0100, CC wrote: >> It's bad to provide a writable attribute and documentation if one >> isn't supposed to ever change it... > > With this reasoning we wouldn't make any attribute writable, as ideally > the BIOS would have set up everything properly in the first place. And > same would hold for a lot of other sysfs attributes. > > I agree the limitations of changing the value of the pwm[1-*]_mode > attribute could be better documented. > >> 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: > > Using the same fan for 4-pin headers and 3-pin headers makes little > sense. If you used a 3-pin fan, it wouldn't be controllable when > connected to a 4-pin header. As your results below contradict this, I > get that you used a 4-pin fan, which means it was controlled > differently between 3-pin headers (control on the +12V pin) and 4-pin > headers (control on the tachometer). Beware that some 3-wire fans are > happy to be controlled by PWM on +12V, but most 4-wire fans are not, as > they are really designed for a separate control signal. > >> 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: > > Different behavior is explained by 4-pin vs. 3-pin headers, see above. > >> >> * 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. > > This is plain wrong, sorry. A lot of motherboards have been using 3-pin > fan headers and offered fan speed control using PWM between, say 2000 > and 2008. Most monitoring chips did not even support DC mode before > 2007 (with the notable exception of the W83627THF.) There's nothing > "unusual" about this. > > Some 3-wire fans behave better under PWM, while others prefer DC. > Unfortunately you usually don't get to chose the method, the > motherboard maker decides for you, because the value at the output pin > generally needs to be transformed before it goes to the fan. For PWM, > the output is a logical 0/1 which is applied to +12V before reaching > the fan. For DC the output is an analog voltage but most controllers > can't output +12V, they'll output 2.5V max for example, and an external > amplifier is needed to actually drive the fan. That's why I said that > changing the mode generally doesn't work, unless the BIOS got it wrong > in the first place. Feeding an amplifier with a PWM signal won't get > you anywhere I'm afraid, just like modulating +12V based on another > voltage output may result in bad things. [1] > > 4-wire headers are a completely different story, as power and control > are on different pins and the control is really only a signal. I think > 4-pin fans are always controlled by PWM, but if DC is ever possible on > such fans, then changing the chip settings would work, because no extra > circuitry on the board is needed, I think the output pin of the control > chip is routed directly to the fan. > >> 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. > > That's my observation as well... for 4-wire fans and assuming proper > PWM frequency. For example: > http://khali.linux-fr.org/articles/sts100/STS100C_PWM_response__large.png > > For 3-wire fans this is a completely different story, the response is > generally very non-linear at high frequencies, with 75% top duty cycles > controlling about nothing and only 20% of the range allowing for > control. Lower frequencies give better results but are often unpleasant > to the ears. For example: > http://khali.linux-fr.org/devel/lm-sensors/pwm_response_curves.png > >> I conclude that PWM mode is working >> correctly for pwm2, > > As expected, for a 4-pin header with a 4-pin fan. > >> and DC mode (the name is from the docs -- most >> likely it's linear voltage regulation) is working correctly for pwm1 >> and pwm3. > > As far as I know DC stands for "direct current" and indeed means > voltage regulation. > > But I fail to see how you come to this conclusion from the experiment > above. Your reports about pwm1 and pwm3 do match what I would expect > from PWM sent to a 3-wire fan (or 4-wire fan on a 3-pin header - > similar behavior.) > >> So: I didn't change any settings, but it seems like the meaning of >> pwm*_mode does not agree with the documentation. > > The only way to demonstrate this would be to measure the values at the > output pin. At this point I have no reason to believe that the values > reported by the w83627ehf driver are incorrect. > > [1] Disclaimer: I am not an electronics guy, so take what I say with a > grain of salt. The electronics schematics I read over the years for > various boards and my experience make me believe I'm correct in what I > say here, but I could be wrong on specific details. > > -- > Jean Delvare I wasn't aware that mainboard chips can and do modulate +12V by PWM themself. So there's actually *three* methods/modes to drive a fan: 1) Voltage regulation ("DC") 2) PWM by modulating the +12V line 3) PWM by providing a control signal In my previous statements, I did assume that "PWM" always means (3). Maybe we could adapt the documentation to reflect the situation. I guess it's a valid distinction, and you surely wouldn't want another smart-ass guy complaining about it. Best, CC _______________________________________________ lm-sensors mailing list lm-sensors@xxxxxxxxxxxxxx http://lists.lm-sensors.org/mailman/listinfo/lm-sensors
