RE: [PATCH v3 10/29] arm_mpam: Add cpuhp callbacks to probe MSC hardware

[Shaopeng Tan (Fujitsu)]

> Because an MSC can only by accessed from the CPUs in its cpu-affinity set we
> need to be running on one of those CPUs to probe the MSC hardware.
> 
> Do this work in the cpuhp callback. Probing the hardware will only happen
> before MPAM is enabled, walk all the MSCs and probe those we can reach that
> haven't already been probed as each CPU's online call is made.
> 
> This adds the low-level MSC register accessors.
> 
> Once all MSCs reported by the firmware have been probed from a CPU in their
> respective cpu-affinity set, the probe-time cpuhp callbacks are replaced.  The
> replacement callbacks will ultimately need to handle save/restore of the
> runtime MSC state across power transitions, but for now there is nothing to do
> in them: so do nothing.
> 
> The architecture's context switch code will be enabled by a static-key, this can
> be set by mpam_enable(), but must be done from process context, not a cpuhp
> callback because both take the cpuhp lock.
> Whenever a new MSC has been probed, the mpam_enable() work is scheduled
> to test if all the MSCs have been probed. If probing fails, mpam_disable() is
> scheduled to unregister the cpuhp callbacks and free memory.
> 
> CC: Lecopzer Chen <lecopzerc@xxxxxxxxxx>
> Signed-off-by: James Morse <james.morse@xxxxxxx>
> Reviewed-by: Jonathan Cameron <jonathan.cameron@xxxxxxxxxx>
> Reviewed-by: Ben Horgan <ben.horgan@xxxxxxx>
> Reviewed-by: Fenghua Yu <fenghuay@xxxxxxxxxx>
> Reviewed-by: Gavin Shan <gshan@xxxxxxxxxx>
> Tested-by: Fenghua Yu <fenghuay@xxxxxxxxxx>

Reviewed-by: Shaopeng Tan <tan.shaopeng@xxxxxxxxxxxxxx>