linux-kernel - Re: [PATCH 1/2] PM / Domains: Add GENPD_FLAG_SUSPEND

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Date:   Tue, 18 Aug 2020 10:31:38 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Sibi Sankar <sibis@...eaurora.org>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Andy Gross <agross@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Doug Anderson <dianders@...omium.org>,
        linux-kernel-owner@...r.kernel.org,
        Kevin Hilman <khilman@...nel.org>,
        linux-arm-msm-owner@...r.kernel.org
Subject: Re: [PATCH 1/2] PM / Domains: Add GENPD_FLAG_SUSPEND_ON flag

On Mon, 17 Aug 2020 at 18:49, Sibi Sankar <sibis@...eaurora.org> wrote:
>
> On 2020-08-17 14:14, Ulf Hansson wrote:
> > On Thu, 13 Aug 2020 at 19:26, Sibi Sankar <sibis@...eaurora.org> wrote:
> >>
> >> On 2020-08-13 18:04, Ulf Hansson wrote:
> >> > On Wed, 12 Aug 2020 at 19:03, Sibi Sankar <sibis@...eaurora.org> wrote:
> >> >>
> >> >> Uffe,
> >> >> Thanks for taking time to review the
> >> >> series!
> >> >>
> >> >> On 2020-08-12 15:15, Ulf Hansson wrote:
> >> >> > On Tue, 11 Aug 2020 at 21:03, Sibi Sankar <sibis@...eaurora.org> wrote:
> >> >> >>
> >> >> >> This is for power domains which needs to stay powered on for suspend
> >> >> >> but can be powered on/off as part of runtime PM. This flag is aimed at
> >> >> >> power domains coupled to remote processors which enter suspend states
> >> >> >> independent to that of the application processor. Such power domains
> >> >> >> are turned off only on remote processor crash/shutdown.
> >> >> >
> >> >> > As Kevin also requested, please elaborate more on the use case.
> >> >> >
> >> >> > Why exactly must the PM domain stay powered on during system suspend?
> >> >> > Is there a wakeup configured that needs to be managed - or is there a
> >> >> > co-processor/FW behaviour that needs to be obeyed to?
> >> >>
> >> >> Yes this is a co-processor behavior that
> >> >> needs to be obeyed. Specifically application
> >> >> processor notifies the Always on Subsystem
> >> >> (AOSS) that a particular co-processor is up
> >> >> using the power domains exposed by AOSS QMP
> >> >> driver. AOSS uses this information to wait
> >> >> for the co-processors to suspend before
> >> >> starting its sleep sequence. The application
> >> >> processor powers off these power domains only
> >> >> if the co-processor has crashed or powered
> >> >> off.
> >> >
> >> > Thanks for clarifying!
> >> >
> >> > Although, can you please elaborate a bit more on the actual use case?
> >> > What are the typical co-processor and what drivers are involved in
> >> > managing it?
> >>
> >> The co-processors using the power domains
> >> exposed by qcom_aoss driver are modem,
> >> audio dsp, compute dsp managed using
> >> qcom_q6v5_mss and qcom_q6v5_pas driver.
> >>
> >> >
> >> > As you may know, runtime PM becomes disabled during system suspend of
> >> > a device. Which means, if the driver tries to power off the
> >> > coprocessor (via calling pm_runtime_put() for example), somewhere in
> >> > the system suspend phase of the corresponding device, its attached PM
> >> > domain stays powered on when managed by genpd.
> >>
> >> The drivers aren't really expected
> >> do anything during suspend/resume
> >> pretty much because the co-processors
> >> enter low-power modes independent to
> >> that of the application processor. On
> >> co-processor crash the remoteproc core
> >> does a pm_stay_awake followed by a
> >> pm_relax after crash recovery.
> >
> > Okay, thanks again for clarifying. You have convinced me about the
> > need for a new flag to cope with these use cases.
> >
> > Would you mind updating the commit message with some of the
> > information you just provided?
> >
> > Additionally, to make it clear that the flag should be used to keep
> > the PM domain powered on during system suspend, but only if it's
> > already powered on - please rename the flag to GENPD_FLAG_NO_SUSPEND,
> > and update the corresponding description of it in the header file.
>
> Thanks, naming it ^^ makes more sense :)
>
> https://lore.kernel.org/lkml/340a7aafcf0301ff3158a4e211992041@codeaurora.org/
>
> Also we wouldn't want to power on
> runtime suspended power domains with
> the NO_SUSPEND flag set, on resume as
> explained ^^. Do you agree with that
> as well?

Actually no.

Instead, I think that deserves a separate flag, as it may very well
turn out that resuming can be skipped for other cases than
"NO_SUSPEND".

Therefore, please add a GENPD_FLAG_NO_RESUME for this.

Kind regards
Uffe