linux-kernel - Re: [PATCH RESEND v3 2/3] drivers: qcom: rpmh-rsc: return if the controller is idle

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Message-ID: <20190306221910.GC10971@codeaurora.org>
Date:   Wed, 6 Mar 2019 15:19:10 -0700
From:   Lina Iyer <ilina@...eaurora.org>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     "Raju P.L.S.S.S.N" <rplsssn@...eaurora.org>, andy.gross@...aro.org,
        david.brown@...aro.org, linux-arm-msm@...r.kernel.org,
        linux-soc@...r.kernel.org, rnayak@...eaurora.org,
        bjorn.andersson@...aro.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, evgreen@...omium.org,
        dianders@...omium.org, mka@...omium.org
Subject: Re: [PATCH RESEND v3 2/3] drivers: qcom: rpmh-rsc: return if the
 controller is idle

On Wed, Mar 06 2019 at 15:12 -0700, Stephen Boyd wrote:
>Quoting Lina Iyer (2019-03-04 09:14:50)
>> On Fri, Mar 01 2019 at 10:58 -0700, Stephen Boyd wrote:
>> >Quoting Lina Iyer (2019-02-27 14:29:13)
>> >> Hi Stephen,
>> >>
>> >> On Tue, Feb 26 2019 at 17:49 -0700, Stephen Boyd wrote:
>> >
>> >Ok, can you explain why it's even a problem for the TCSes to be active
>> >during suspend? I would hope that for suspend/resume, if this is
>> >actually a problem, the RPMh driver itself can block suspend with a
>> >driver suspend callback that checks for idleness.
>> The RSC can transmit TCS executed from Linux and when all the CPUs have
>> powered down, could execute a firmware in the RSC to deliver the sleep
>> state requests. The firmware cannot run when there are active requests
>> being processed. To ensure that case, we bail out of sleep or suspend,
>> when the last CPU is powering down, if there are active requests.
>
>Ok, do we actually bail out or just pick a shallower idle state that
>wouldn't trigger the firmware to run something that may conflict with
>the active requests (i.e. some light CPU sleep mode)? The commit text
>seems to imply we block certain idle states.
>
We bail out of idle and let cpuidle determine the state again. We don't
go into a shallower state.
>>
>> >But I suspect that in
>> >the system wide suspend/resume case, any callers that could make TCS
>> >requests are child devices of the RPMh controller and therefore they
>> >would already be suspended if they didn't have anything pending they're
>> >waiting for a response on or they would be blocking suspend themselves
>> >if they're waiting for the response. So why are we even checking the
>> >TCSes in system suspend path at all? Assume that callers know what
>> >they're doing and will block suspend if they care?
>> >
>> In suspend, they probably would do what you mention above. All CPUs
>> might conincidentally be idle at the same idle, when a request is being
>> processed.
>>
>> >Following that same logic, is this more of an API that is planned for
>> >use by CPU idle? Where the case is much more of a runtime PM design.
>> >Even then, I don't get it. A device that's runtime active and making
>> >RPMh requests might need to block some forms of CPU idle states because
>> >a request hasn't been processed yet that may change the decision for
>> >certain deep idle states?
>> >
>> A process waiting on a RPMH request, may let the CPU go to sleep and
>> therefore this is a possibility.
>>
>
>Ok thanks for the info. Can these details be included in the commit text
>so we don't lose sight of the bigger picture? And can this patch series
>be combined with a larger cpuidle/suspend patch series so we don't have
>to review this in isolation? I don't understand the need to add more
>APIs that aren't used yet.
>
Agreed.

--Lina