On arm and arm64, PSCI is one of the possible firmware interfaces
used for power management. This includes both turning CPUs on and off,
and suspending them (entering idle states).

This patch adds a PSCI checker module that enables basic testing of
PSCI operations during startup. There are two main tests: CPU
hotplugging and suspending.

In the hotplug tests, the hotplug API is used to turn off and on again
all CPUs in the system, and then all CPUs in each cluster, checking
the consistency of the return codes.

In the suspend tests, a high-priority thread is created on each core
and uses low-level cpuidle functionalities to enter suspend, in all
the possible states and multiple times. This should allow a maximum
number of CPUs to enter the same sleep state at the same or slightly
different time.

In essence, the suspend tests use a principle similar to that of the
intel_powerclamp driver (drivers/thermal/intel_powerclamp.c), but the
threads are only kept for the duration of the test (they are already
gone when userspace is started) and it does not require to stop/start
the tick.

While in theory power management PSCI functions (CPU_{ON,OFF,SUSPEND})
could be directly called, this proved too difficult as it would imply
the duplication of all the logic used by the kernel to allow for a
clean shutdown/bringup/suspend of the CPU (the deepest sleep states
implying potentially the shutdown of the CPU).

Note that this file cannot be compiled as a loadable module, since it
uses a number of non-exported identifiers (essentially for
PSCI-specific checks and direct use of cpuidle) and relies on the
absence of userspace to avoid races when calling hotplug and cpuidle
functions.

For now at least, CONFIG_PSCI_CHECKER is mutually exclusive with
CONFIG_TORTURE_TEST, because torture tests may also use hotplug and
cause false positives in the hotplug tests.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Kevin Hilman <khilman@kernel.org>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Acked-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> [torture test config]
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
[lpieralisi: added cpuidle locking, reworded commit log/kconfig entry]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>