[PATCHv3] coresight: etm4x: Fix etm4_count race by moving cpuhp callbacks to init

[Stephen Boyd]

Quoting Sai Prakash Ranjan (2020-07-28 22:13:10)
> etm4_count keeps track of number of ETMv4 registered and on some systems,
> a race is observed on etm4_count variable which can lead to multiple calls
> to cpuhp_setup_state_nocalls_cpuslocked(). This function internally calls
> cpuhp_store_callbacks() which prevents multiple registrations of callbacks
> for a given state and due to this race, it returns -EBUSY leading to ETM
> probe failures like below.
> 
>  coresight-etm4x: probe of 7040000.etm failed with error -16
> 
> This race can easily be triggered with async probe by setting probe type
> as PROBE_PREFER_ASYNCHRONOUS and with ETM power management property
> "arm,coresight-loses-context-with-cpu".
> 
> Prevent this race by moving cpuhp callbacks to etm driver init since the
> cpuhp callbacks doesn't have to depend on the etm4_count and can be once
> setup during driver init. Similarly we move cpu_pm notifier registration
> to driver init and completely remove etm4_count usage. Also now we can
> use non cpuslocked version of cpuhp callbacks with this movement.
> 
> Fixes: 9b6a3f3633a5 ("coresight: etmv4: Fix CPU power management setup in probe() function")
> Fixes: 58eb457be028 ("hwtracing/coresight-etm4x: Convert to hotplug state machine")
> Suggested-by: Suzuki K Poulose <suzuki.poulose at arm.com>
> Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan at codeaurora.org>
> ---

Reviewed-by: Stephen Boyd <swboyd at chromium.org>
Tested-by: Stephen Boyd <swboyd at chromium.org>