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From:   Ulf Hansson <ulf.hansson@linaro.org>
Date:   Mon, 1 Oct 2018 14:02:11 +0200
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Subject: Re: [PATCH v16 1/5] iommu/arm-smmu: Add pm_runtime/sleep ops
To:     Vivek Gautam <vivek.gautam@codeaurora.org>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        Alex Williamson <alex.williamson@redhat.com>,
        Linux PM <linux-pm@vger.kernel.org>,
        Stephen Boyd <sboyd@kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Will Deacon <will.deacon@arm.com>,
        open list <linux-kernel@vger.kernel.org>,
        "list@263.net:IOMMU DRIVERS <iommu@lists.linux-foundation.org>, Joerg
        Roedel <joro@8bytes.org>," <iommu@lists.linux-foundation.org>,
        "robh+dt" <robh+dt@kernel.org>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        freedreno <freedreno@lists.freedesktop.org>,
        Robin Murphy <robin.murphy@arm.com>
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On 1 October 2018 at 12:32, Vivek Gautam <vivek.gautam@codeaurora.org> wrote:
> On Mon, Oct 1, 2018 at 11:19 AM Vivek Gautam
> <vivek.gautam@codeaurora.org> wrote:
>>
>> HI Ulf,
>>
>> On Fri, Sep 28, 2018 at 5:30 PM Ulf Hansson <ulf.hansson@linaro.org> wrote:
>> >
>> > On 30 August 2018 at 16:45, Vivek Gautam <vivek.gautam@codeaurora.org> wrote:
>> > > From: Sricharan R <sricharan@codeaurora.org>
>> > >
>> > > The smmu needs to be functional only when the respective
>> > > master's using it are active. The device_link feature
>> > > helps to track such functional dependencies, so that the
>> > > iommu gets powered when the master device enables itself
>> > > using pm_runtime. So by adapting the smmu driver for
>> > > runtime pm, above said dependency can be addressed.
>> > >
>> > > This patch adds the pm runtime/sleep callbacks to the
>> > > driver and also the functions to parse the smmu clocks
>> > > from DT and enable them in resume/suspend.
>> > >
>> > > Also, while we enable the runtime pm add a pm sleep suspend
>> > > callback that pushes devices to low power state by turning
>> > > the clocks off in a system sleep.
>> > > Also add corresponding clock enable path in resume callback.
>> > >
>> > > Signed-off-by: Sricharan R <sricharan@codeaurora.org>
>> > > Signed-off-by: Archit Taneja <architt@codeaurora.org>
>> > > [vivek: rework for clock and pm ops]
>> > > Signed-off-by: Vivek Gautam <vivek.gautam@codeaurora.org>
>> > > Reviewed-by: Tomasz Figa <tfiga@chromium.org>
>> > > Tested-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
>> > > ---
>> > >  drivers/iommu/arm-smmu.c | 77 ++++++++++++++++++++++++++++++++++++++++++++++--
>> > >  1 file changed, 74 insertions(+), 3 deletions(-)
>> > > diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
>> >
>> > [...]
>> >
>> > > -static int __maybe_unused arm_smmu_pm_resume(struct device *dev)
>> > > +static int __maybe_unused arm_smmu_runtime_resume(struct device *dev)
>> > >  {
>> > >         struct arm_smmu_device *smmu = dev_get_drvdata(dev);
>> > > +       int ret;
>> > > +
>> > > +       ret = clk_bulk_enable(smmu->num_clks, smmu->clks);
>> > > +       if (ret)
>> > > +               return ret;
>> > >
>> > >         arm_smmu_device_reset(smmu);
>> > > +
>> > >         return 0;
>> > >  }
>> > >
>> > > -static SIMPLE_DEV_PM_OPS(arm_smmu_pm_ops, NULL, arm_smmu_pm_resume);
>> > > +static int __maybe_unused arm_smmu_runtime_suspend(struct device *dev)
>> > > +{
>> > > +       struct arm_smmu_device *smmu = dev_get_drvdata(dev);
>> > > +
>> > > +       clk_bulk_disable(smmu->num_clks, smmu->clks);
>> > > +
>> > > +       return 0;
>> > > +}
>> > > +
>> > > +static int __maybe_unused arm_smmu_pm_resume(struct device *dev)
>> > > +{
>> > > +       if (pm_runtime_suspended(dev))
>> > > +               return 0;
>> >
>> > Looks like you should be able use pm_runtime_force_resume(), instead
>> > of using this local trick. Unless I am missing something, of course.
>> >
>> > In other words, just assign the system sleep callbacks for resume, to
>> > pm_runtime_force_resume(). And vice verse for the system suspend
>> > callbacks, pm_runtime_force_suspend(), of course.
>>
>> Thanks for the review. I will change this as suggested.
>
> Coming back at this - actually Rafael suggested _not_ to use
> pm_runtime_force_suspend/resume() when Marek had suggested
> the same [1].

I see.

> He also mentioned few caveats/limitations of using these APIs
> for system sleep ops.
> Let me know your opinion. Thanks.

>
> [1] https://lkml.org/lkml/2018/7/11/978
> [2] https://lkml.org/lkml/2018/7/23/334

Me and Rafael have been discussing these topics historically as well.
I don't want to get that discussion started again here.

If your device is attached to the PCI bus or the ACPI PM domain (and
also gets runtime PM enabled), then I suggest you to stick to the
currently suggested approach. Otherwise it should be perfectly fine to
switch to the *force helpers.

Kind regards
Uffe