The current PSCI implementation can apply certain optimizations upon the
assumption that all PSCI participants are cache-coherent.
- Skip performing cache maintenance during power-up.
- Skip performing cache maintenance during power-down:
At present, on the power-down path, CPU driver disables caches and
MMU, and performs cache maintenance in preparation for powering down
the CPU. This means that PSCI must perform additional cache
maintenance on the extant stack for correct functioning.
If all participating CPUs are cache-coherent, CPU driver would
neither disable MMU nor perform cache maintenance. The CPU being
powered down, therefore, remain cache-coherent throughout all PSCI
call paths. This in turn means that PSCI cache maintenance
operations are not required during power down.
- Choose spin locks instead of bakery locks:
The current PSCI implementation must synchronize both cache-coherent
and non-cache-coherent participants. Mutual exclusion primitives are
not guaranteed to function on non-coherent memory. For this reason,
the current PSCI implementation had to resort to bakery locks.
If all participants are cache-coherent, the implementation can
enable MMU and data caches early, and substitute bakery locks for
spin locks. Spin locks make use of architectural mutual exclusion
primitives, and are lighter and faster.
The optimizations are applied when HW_ASSISTED_COHERENCY build option is
enabled, as it's expected that all PSCI participants are cache-coherent
in those systems.
Change-Id: Iac51c3ed318ea7e2120f6b6a46fd2db2eae46ede
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>
This patch adds the PSCI library integration guide for AArch32 ARMv8-A
systems `psci-lib-integration-guide.md` to the documentation. The
patch also adds appropriate reference to the new document in
the `firmware-design.md` document.
Change-Id: I2d5b5c6b612452371713399702e318e3c73a8ee0
Jeenu Viswambharan
Soby Mathew