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List:       freebsd-hackers
Subject:    How to sync/mutex with acpi_tz_get_temperature()
From:       Johannes Totz <jo () bruelltuete ! com>
Date:       2022-09-13 15:59:10
Message-ID: 9b87dd66-de57-f363-279e-dc33134c0fa6 () bruelltuete ! com
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Hi there,

I would like to make sure that some of my code does not run concurrently 
with acpi_tz_get_temperature() 
[https://github.com/freebsd/freebsd-src/blob/main/sys/dev/acpica/acpi_thermal.c#L465].
What's the best way to achieve that?

I wrote a small driver for one of those SuperIO hardware monitors that 
measures fan speed, voltage, temperature etc. That works fine.
But looking at my board/BIOS's ASL, invoking _TMP is not safe to do 
concurrently with my driver. [see footnote]

Ultimately, acpi_tz_thread() 
[https://github.com/freebsd/freebsd-src/blob/main/sys/dev/acpica/acpi_thermal.c#L949] 
is calling acpi_tz_get_temperature() with a few hops in-between.
There are a few instances of ACPI_LOCK(thermal) etc but nothing that 
guards acpi_tz_get_temperature() specifically.

Anyone have ideas/pointers?


thanks,

Johannes


footnote: Reading from that chip is stateful, as in you need to do I/O 
to select which register you want to read/write and then read/write from 
another location to access the value of that register. Try to do that 
from 2 threads at the same time, one of them will read/write the wrong 
register, thus you can glitch ACPI into doing something 
disruptive/destructive.

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