RFC: Selecting an NVMEM cell for Power State Change Reason (PSCR) recording

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

Hi all,

I'm seeking consensus on a minimal, upstream-acceptable way to identify the
single NVMEM cell used to persist a Power State Change Reason (PSCR). Typical
targets are battery-backed RTC scratchpads or small EEPROM. The aim is to have
a tiny breadcrumb available before userspace, across full power cuts, and
shared by bootloader/kernel/userspace.

DT vs Userspace vs ACPI

* DeviceTree (preferred): Describing where the storage lives under a real
  NVMEM provider (RTC/EEPROM) is early, robust, and OS-agnostic.

* Userspace (fallback): Possible via module/cmdline/sysfs, but leaves an
  early-boot window unconfigured and reduces usefulness for embedded devices.

* ACPI: No existing shared mechanism for this use case at present (not
  proposing an ACPI path right now).

What implementations were tried

* A PSCR consumer node in DT -> NACKed as not a HW node.

* Kernel/module parameters or sysfs selection -> tried earlier, but rejected
  for new designs and cannot guarantee early availability.

* Name-based lookups in NVMEM -> considered fragile and not scalable.

Other options which came in question (seeking guidance)

* cell-level `compatible` on a fixed-cell child (analogous to `mac-base`) to
  nominate the PSCR cell under the existing NVMEM provider. DT remains purely
  descriptive (location/size); encoding is documented outside DT and shared
  across components.

* `/chosen` phandle pointing to the nominated fixed-cell (simple to discover;
  unsure about policy concerns).

* pstore integration (not tried): a backend that uses a nominated NVMEM cell if
  such a nomination is acceptable.

* nvmem-layout usage (not tried): provider-side markup of the region to
  indicate it carries PSCR, if that pattern is acceptable for this purpose.

* Open to any established precedent for nominating a specific NVMEM cell for a
  system role without introducing software/virtual DT nodes.

Ask

* Is a cell-level `compatible` on a `fixed-cell` child an acceptable way to
  nominate the PSCR cell?

* If not, is a `/chosen` phandle acceptable here, or is there a preferred
  alternative?

Thanks for guidance - once the selection mechanism is agreed, I can respin the
PSCR series accordingly.

Latest patch version: https://lore.kernel.org/all/aHTZTFxfS6Bn4yhz@pengutronix.de/

Best Regards,
Oleksij
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