Robert: I haven't quite solved the problem, but I've made some progress.
Reporting back both FYI and to help anyone else with this problem.
I downloaded your dtb-rebuilder and spent some time looking at it. I
anticipated having to write those changes into the patched file, but as it
happened, I found that they're already included in
am57xx-beagle-x15-common.dtsi. I checked the more specific file
(am57xx-beagle-x15-revc.dts) to verify that it didn't have any overriding
settings.
I built the DTB:
make all
make install
...and verified that /usr/bin/dtb (aliased to /usr/bin/local/dtb) had been
updated. So far, so good.
I rebooted and found that device behavior hadn't changed. But at least I
had a lead as to where the behavior was controlled.
I tried tweaking the hystersis values in common.dtsi, making sure that the
most relevant hysteresis values are reasonably large:
cpu_alert1: cpu_alert1 {
hysteresis = <2000>; /* millicelsius */
};
board_alert0: board_alert {
hysteresis = <5000>; /* millicelsius */
};
board_crit: board_crit {
hysteresis = <1000>; /* millicelsius */
};
I rebuilt, reinstalled, and rebooted... no change in behavior - the fan
still turned on and off every couple of seconds. Watching the output of
tmp102 via sensors, I can see that the CPU temperature only fluctuates
about 0.1-0.2 C between fan cycles. I set them even higher - up to 5000 mC.
Rebuild, reinstall, reboot... no effect.
I realized that I could tackle the problem in a more crude way - by
increasing the temperature polling frequency... bordering on stupidly large:
&thermal_zones {
board_thermal: board_thermal {
polling-delay-passive = <30000>; /* milliseconds */
polling-delay = <30000>; /* milliseconds */
};
};
Rebuild, reinstall, reboot - that seems to have worked. Well, kind of. The
fan typically cycles in 30-second intervals now. Oddly, it's not 100% of
the time: sometimes it conducts three or four 30-second on/off cycles and
then exhibits a few short cycles (like: turning on just long enough to get
up to speed, turning off just long enough to stop, and then turning on for
30 seconds again).
Honestly, though, this solves the immediate problem: rapid fan cycling was
both getting on my nerves, and creating some concerns about wear and tear
from constantly turning on and off. I suppose I can live with a 30-second
cycle time.
However, I'm still really puzzled by the inconsistent behavior: why
hysteresis temperatures didn't work, and why the cycle time doesn't seem to
be fixed. Curious results that bear further exploration.
Regards,
David Stein
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