Asahi Linux Progress Report: Linux 7.2

Asahi Linux has released version 7.2. This release has brought several noteworthy improvements, and we are excited to share the details with you. The Apple Silicon platform's power management infrastructure is intricate, involving multiple hardware blocks like the SMC, PMGR, and PMP. However, improving battery life has been challenging due to the complexities of application cores.

ARM CPU cores can be put "to sleep" in various ways, with the Wait For Interrupt (WFI) instruction being the most basic method. WFI pauses a core until it receives an interrupt from another source, saving power by allowing the core to resume work quickly. Apple cores feature a deeper WFI mode, which shuts down additional core components but loses its state. The downstream cpuidle driver manages this by setting WFI in deep mode and issuing a WFI loop.

To address vendor-specific power management quirks, the Power State Coordination Interface (PSCI) was developed. PSCI offers a standardized interface for CPU core power management, with the Linux kernel relying on PSCI for firmware calls. However, implementing PSCI for Apple Silicon cores has been a challenge, as Apple's cores do not support EL3 execution. This limitation makes it difficult to utilize PSCI for power management.

One potential solution is loading the kernel into EL1 and hosting PSCI functionality within EL2. Although this approach could theoretically work, it would also disrupt various architectural features, such as virtualization. An alternative solution is to utilize UEFI Runtime Services, a component similar to the BIOS interrupts. Sven has been working on implementing a UEFI Runtime Service-based PSCI conduit, which would enable the kernel to call back into the kernel for PSCI services.

This modification to m1n1 has already been initiated, and the corresponding kernel patches are available for review.