Re: [PATCH v7 7/7] Documentation: Add sysfs documentation for PSCRR reboot reason tracking

[Oleksij Rempel <o.rempel@pengutronix.de>]

On Tue, Mar 18, 2025 at 01:17:38PM +0200, Matti Vaittinen wrote:
> On 18/03/2025 11:47, Oleksij Rempel wrote:
> > Add documentation for the Power State Change Reason Recorder (PSCRR)
> > sysfs interface, which allows tracking of system shutdown and reboot
> > reasons. The documentation provides details on available sysfs entries
> > under `/sys/kernel/pscrr/`, explaining their functionality, example usage,
> > and how they interact with different backend storage options (e.g., NVMEM).
> > 
> > Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
> > ---
> > changes v7:
> > - document expected values
> > ---
> >   .../ABI/testing/sysfs-kernel-reboot-pscrr     | 79 +++++++++++++++++++
> >   1 file changed, 79 insertions(+)
> >   create mode 100644 Documentation/ABI/testing/sysfs-kernel-reboot-pscrr
> > 
> > diff --git a/Documentation/ABI/testing/sysfs-kernel-reboot-pscrr b/Documentation/ABI/testing/sysfs-kernel-reboot-pscrr
> > new file mode 100644
> > index 000000000000..9aa3df8f2fc7
> > --- /dev/null
> > +++ b/Documentation/ABI/testing/sysfs-kernel-reboot-pscrr
> > @@ -0,0 +1,79 @@
> > +What:		/sys/kernel/pscrr/reason
> > +Date:		April 2025
> > +KernelVersion:  6.15
> > +Contact:	Oleksij Rempel <o.rempel@pengutronix.de>
> > +Description:
> > +		This file provides access to the last recorded power state
> > +		change reason. The storage backend is configurable and, if
> > +		supported, the reason may be stored persistently in an
> > +		NVMEM cell or another backend.
> > +
> > +		Reading this file returns an integer representing the last
> > +		recorded shutdown or reboot cause.
> > +
> > +		Writing an integer value to this file sets the reason to be
> > +		stored and recorded for system analysis.
> > +
> > +		Example usage (values are for illustration and may not reflect
> > +		actual reasons used in a given system):
> > +		  Read:
> > +			$ cat /sys/kernel/pscrr/reason
> > +			3   # (Example: Power loss event, may differ per system)
> > +
> > +		  Write:
> > +			$ echo 5 > /sys/kernel/pscrr/reason
> > +			# Sets the reason to 5 (Example: User-triggered reboot,
> > +			# this may not be a real value in your system)
> 
> nit:
> Now that the 'number' <=> 'reason' relation is fixed, we might drop the "may
> differ" etc. Perhaps just:
> 
> 		Example usage:
> 		  Read:
> 			$ cat /sys/kernel/pscrr/reason
> 			3   # (Example: Power loss event)
> 
> 		  Write:
> 			$ echo 5 > /sys/kernel/pscrr/reason
> 			# Sets the reason to 5 (Example: User-triggered reboot)

ack

> Maybe adding a note that not all values are supported on all systems.

Yes, there is after supported values.

> 
> > +
> > +		Values are defined in:
> > +		  - `include/linux/reboot.h` (enum psc_reason)
> > +
> > +		Supported Values:
> > +		Defined in `include/linux/reboot.h` (enum psc_reason):
> > +
> > +		+-------+---------------------------+--------------------------+
> > +		| Value | Symbol                    | Description              |
> > +		+-------+---------------------------+--------------------------+
> > +		| 0     | PSCR_UNKNOWN              | Unknown or unspecified   |
> > +		|       |                           | power state change reason|
> > +		+-------+---------------------------+--------------------------+
> > +		| 1     | PSCR_UNDER_VOLTAGE        | Supply voltage drop below|
> > +		|       |                           | safe threshold.          |
> > +		+-------+---------------------------+--------------------------+
> > +		| 2     | PSCR_OVER_CURRENT         | Excessive current draw,  |
> > +		|       |                           | potential short circuit. |
> > +		+-------+---------------------------+--------------------------+
> > +		| 3     | PSCR_REGULATOR_FAILURE    | Failure in voltage       |
> > +		|       |                           | regulator, preventing    |
> > +		|       |                           | stable power delivery.   |
> > +		+-------+---------------------------+--------------------------+
> > +		| 4     | PSCR_OVER_TEMPERATURE     | Unsafe system temperature|
> > +		|       |                           | detected by sensors.     |
> > +		+-------+---------------------------+--------------------------+
> > +		| 5     | PSCR_EC_PANIC             | Shutdown/reboot triggered|
> > +		|       |                           | by Embedded Controller   |
> > +		|       |                           | (EC) panic.              |
> > +		+-------+---------------------------+--------------------------+
> > +
> > +		(Note: The actual reason codes used on a specific system depend
> > +		on hardware capabilities and configuration.)
> 
> I like this. Nice and clean.
> 
> A side note which you can consider if you feel like:
> This, as far as I understand, will also make the ABI out of this. So,
> perhaps there should be comments in enum psc_reason and the defines in
> Defined in 'include/linux/reboot.h' that they are part of an ABI, and must
> not be changed? I suppose user-space tools may get unhappy if these values
> change between the kernel versions.

Yes, there is already: "Importantly, the order of these values **must
remain stable**, as bootloaders, user-space tools, or post-mortem
investigation utilities may rely on their numerical representation for
consistent behavior." :)

> > +
> > +What:		/sys/kernel/pscrr/reason_boot
> > +Date:		April 2025
> > +KernelVersion:  6.15
> > +Contact:	Oleksij Rempel <o.rempel@pengutronix.de>
> > +Description:
> > +		This file provides the last recorded power state change reason
> > +		from before the current system boot. If a supported backend
> > +		(e.g., NVMEM) is configured, this value is retained across
> > +		reboots.
> > +
> > +		Example usage (values are for illustration and may not reflect
> > +		actual reasons used in a given system):
> > +		  Read:
> > +			$ cat /sys/kernel/pscrr/reason_boot
> > +			2   # (Example: Over-temperature shutdown, may differ per system)
> > +
> > +		Supported Values:
> > +		Same as `/sys/kernel/pscrr/reason` (see above).
> > +
> 
> All in all, this looks very good to me.
> 
> Reviewed-by: Matti Vaittinen <mazziesaccount@gmail.com>
> 
> Yours,
> 	-- Matti
> 
> 

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