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Create a feature to allow fans to run at failsafe PWM when in failsafe mode #17

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brandonkimbk opened this issue Apr 1, 2021 · 1 comment
Open

Create a feature to allow fans to run at failsafe PWM when in failsafe mode #17

brandonkimbk opened this issue Apr 1, 2021 · 1 comment

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@brandonkimbk
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We are exploring ways to dynamically set the failsafe PWM at runtime. However, we're finding that fans tend to drive up to max RPM when some sensors go missing, overshooting the lower failsafe PWM that we set for them.

Create a feature flag to allow us to bypass the comparison here:

https://github.com/openbmc/phosphor-pid-control/blob/master/pid/fancontroller.cpp#L128-L139
bradbishop pushed a commit that referenced this issue Aug 24, 2022
@brandonkimbk
Add --strict-failsafe-pwm compile flag
bcdeb83 
This build flag is used to set the fans strictly at the failsafe
percent when in failsafe mode, even when the calculated PWM is higher
than failsafe PWM. Without this enabled, the PWM is calculated and set
to the calculated PWM  the failsafe PWM, whichever is higher.

Added a unit test that can test this new build flag code path if the
compile flag is defined.

Tested:

Verified on an internal machine that by adding the following to the
bbappend:

EXTRA_OECONF:append = " --enable-strict-failsafe-pwm=yes"

With flag:
ipmitool sensor list
fan_pwm         | 89.768

Without flag:
ipmitool sensor list
fan_pwm         | 99.960

We can see that the fan pwm was limited to the failsafe percentage when
in failsafe mode with the flag. Without the flag, it ran at 100%

Bug-Id: #17
Signed-off-by: Brandon Kim <[email protected]>
Change-Id: I72a1e5aab8d3e5b0e3716f0b3720d704a6f05008
@Krellan
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Krellan commented Jun 6, 2023

I believe this can be closed. The compile-time flag serves the purpose.

When flag is not given (default behavior), the failsafe PWM establishes merely a minimum. The fans are free to go to higher PWM, up to and including 100%, if any thermal PID loops are still working well enough to request a higher RPM that corresponds to a higher PWM.

When flag is given, the failsafe PWM establishes a forced constant value. The PWM will be held at the failsafe value, regardless of any thermal PID loops.

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@Krellan @brandonkimbk