linux-kernel - Re: [PATCH 1/2] Revert "drivers: qcom: rpmh-rsc: Use rcuidle tracepoints for rpmh"

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Date:   Wed, 19 Aug 2020 11:04:23 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Maulik Shah <mkshah@...eaurora.org>, agross@...nel.org,
        bjorn.andersson@...aro.org
Cc:     linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        ulf.hansson@...aro.org, dianders@...omium.org,
        rnayak@...eaurora.org, ilina@...eaurora.org, lsrao@...eaurora.org,
        Maulik Shah <mkshah@...eaurora.org>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        John Stultz <john.stultz@...aro.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH 1/2] Revert "drivers: qcom: rpmh-rsc: Use rcuidle tracepoints for rpmh"

Quoting Maulik Shah (2020-08-19 03:07:49)
> This change was done based on an test results of unmerged series of
> adding RSC power domain and using .power_off callback of genpd to
> invoke rpmh_flush().

Perhaps:

Commit efde2659b0fe ("drivers: qcom: rpmh-rsc: Use rcuidle tracepoints
for rpmh") was written to fix a bug seen in an unmerged series that
implemented a struct generic_pm_domain::power_off() callback calling
rpmh_flush(). See stack trace below.

> 
>      Call trace:
>       dump_backtrace+0x0/0x174
>       show_stack+0x20/0x2c
>       dump_stack+0xc8/0x124
>       lockdep_rcu_suspicious+0xe4/0x104
>       __tcs_buffer_write+0x230/0x2d0
>       rpmh_rsc_write_ctrl_data+0x210/0x270
>       rpmh_flush+0x84/0x24c
>       rpmh_domain_power_off+0x78/0x98
>       _genpd_power_off+0x40/0xc0
>       genpd_power_off+0x168/0x208
> 
> Later the final merged solution is to use CPU PM notification to invoke
> rpmh_flush() and .power_off callback of genpd is not implemented in the
> driver.

I'd also add:

CPU PM notifiers are run with RCU enabled/watching (see cpu_pm_notify()
and how it calls rcu_irq_enter_irqson() before calling the notifiers).

> 
> Remove this change since RCU will not be idle during CPU PM notifications
> hence not required to use _rcuidle tracepoint. Using _rcuidle tracepoint
> prevented rpmh driver to be loadable module as these are not exported
> symbols.
> 
> This reverts commit efde2659b0fe835732047357b2902cca14f054d9.
> 
> Cc: Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
> Cc: John Stultz <john.stultz@...aro.org>
> Cc: Stephen Rothwell <sfr@...b.auug.org.au>
> Signed-off-by: Maulik Shah <mkshah@...eaurora.org>
> ---

Otherwise

Reviewed-by: Stephen Boyd <swboyd@...omium.org>