[RFC PATCH 05/14] arm64: kernel: cpu_{suspend/resume} implementation

On Wed, Aug 28, 2013 at 12:35:57PM +0100, Lorenzo Pieralisi wrote:

Kernel subsystems like CPU idle and suspend to RAM require a generic
mechanism to suspend a processor, save its context and put it into
a quiescent state. The cpu_{suspend}/{resume} implementation provides
such a framework through a kernel interface allowing to save/restore
registers, flush the context to DRAM and suspend/resume to/from
low-power states where processor context may be lost.

Different SoCs might require different operations to be carried out
before a power down request is committed. For this reason the kernel
allows the caller of cpu_suspend to provide a function pointer (fn in
the cpu_suspend prototype below),

int cpu_suspend(unsigned long arg, int (*fn)(unsigned long));

called suspend finisher, that is executed after the context is saved and
flushed to DRAM, so that SoC/platform specific operations can be carried out
before issuing power down commands.

Context memory is allocated on the stack, whose address is stashed in a
per-cpu variable to keep track of it and passed to core functions that
save/restore the registers required by the architecture.

Even though, upon successful execution, the cpu_suspend function shuts
down the suspending processor, the warm boot resume mechanism, based
on the cpu_resume function, makes the resume path operate as a
cpu_suspend function return, so that cpu_suspend can be treated as a C
function by the caller, which simplifies coding the PM drivers that rely
on the cpu_suspend API.

Upon context save, the minimal amount of memory is flushed to DRAM so
that it can be retrieved when the MMU is off and caches are not searched.

The suspend finisher, depending on the required operations (eg CPU vs
Cluster shutdown) is in charge of flushing the cache hierarchy either
implicitly (by calling firmware implementations like PSCI) or explicitly
by executing the required cache maintainance functions.

As we discussed, I would like the finisher argument to cpu_suspend() to be
removed and just use the default cpu_operations.cpu_suspend (currently
smp_operations) which is populated from DT and uses PSCI as the default
finisher. The cpuidle drivers can still pass arguments that would make
their way into the PSCI CPU_SUSPEND call (like how deep to go) but I
would like to avoid each cpuidle driver implementing a finisher that
does the PSCI call.

If PSCI is not available, the cpuidle driver can register a different
cpu_suspend method.

-- 
Catalin