S0ix idle standby power states are valuable for energy savings and thus important to ensure such states are properly being achieved when portions of the SoC can power-down. While Intel's S0ix handling on Linux has become fairly mature, on AMD APU/SoC systems there still are some cases of issues around these sub-states to ACPI's S0.
Queued in the x86 platform driver's "for-next" branch this week were a set of patches from AMD for enhancing their amd-pmc driver. Notable on the S0ix debugging front are exposing SMU metrics via DebugFS so S0ix debug statistics are easily obtainable from user-space. The time in various states is reported, the hint count, S0i3 cycle count, and other information.
A follow up patch added additional logging around S0ix counters for ensuring the S0ix states are actually being reached.
Beyond debugging improvements, the AMD-PMC driver improvements for Linux 5.15 also add a new ACPI ID that will be used by future AMD PMC controllers. That added ACPI ID is identified with "AMD_CPU_ID_YC" and comes after AMD_CPU_ID_CZN "Cezanne". This presumably is in reference then to Yellow Carp .
Ultimately it's good to see improved debuggability around S0ix come for AMD platforms in ensuring proper power management for AMD laptops as well as other AMD-powered devices like Steam Deck.