[Nouveau] [Bug 79820] [NVE6] Strange readout on fan1_input when pwm1_min < 33

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=79820

--- Comment #5 from Martin Peres <martin.peres at ensi-bourges.fr> ---
(In reply to comment #2)
> With nouveau loaded, can you grab vbios from
> /sys/kernel/debug/dri/0/vbios.rom? Or use -s prom with nvagetbios?

I just pushed a patch to nvagetbios to suggest to use -s prom when the
signature is invalid. Let's stop wasting time suggesting this.

> 
> It's moderately likely that the reported fan rpm is just 2x what it should
> be -- did the fan sound louder with nouveau than with the blob? See
> http://cgit.freedesktop.org/~darktama/nouveau/commit/
> ?id=e3601d0869a18acc19e58f2d1216b810632d75f0 (which will be in 3.16-rc1).

Yes, this is likely the problem here.

> 
> Martin -- any idea about the weird RPMs reported for the lower values?

It probably is a bouncing problem of the Hall effect sensor. This one may not
have a proper hysteresis cycle which leads to an unstable signal during the
transition phases. The longer the transition takes, the more un-wanted
oscillation is introduced. It is possible that the fan manufacturer added a
low-pass filter on the fan that only works down to the minimum speed (35%) and
fails to clean up the signal after that.

In any case, this is not our bug unless nvidia is able to cope with it (which
is impossible to test with the coolbits since they don't allow a fan speed
lower than the minimum speed). I could edit the vbios to change the minimum
speed limit and ask you to re-upload it and check if nvidia yields the same
result. Would you be interested in doing that?

(In reply to comment #3)
> The noise didn’t really noticeably increase. And I just checked the values
> reported by nvidia-settings. When idle, these always seem to jump in
> increments of 30 RPM (usually between 1050 and 1080). With nouveau, the
> output value jumps in increments of 60 RPM, so you are probably right and
> the difference is only nominal.

Perfect! Thank you :)

> 
> In any case, I’m attaching the BIOS file obtained from nvagetbios -s prom.
> No warnings were reported this time.

Thanks, but I don't think it will be necessary...

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