linux-kernel - Re: [PATCH v12 1/4] iommu/arm-smmu: Add pm

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Date:   Mon, 16 Jul 2018 12:23:12 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Vivek Gautam <vivek.gautam@...eaurora.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Tomasz Figa <tfiga@...omium.org>,
        Mark Rutland <mark.rutland@....com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Linux PM <linux-pm@...r.kernel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Will Deacon <will.deacon@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Rob Herring <robh+dt@...nel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        freedreno <freedreno@...ts.freedesktop.org>
Subject: Re: [PATCH v12 1/4] iommu/arm-smmu: Add pm_runtime/sleep ops

Hi,

On Mon, Jul 16, 2018 at 12:11 PM, Vivek Gautam
<vivek.gautam@...eaurora.org> wrote:
> HI Rafael,
>
>
>
> On 7/16/2018 2:21 PM, Rafael J. Wysocki wrote:
>>
>> On Thu, Jul 12, 2018 at 12:57 PM, Vivek Gautam
>> <vivek.gautam@...eaurora.org> wrote:

[cut]

>>>> Although, given the PM
>>>> subsystem internals, the suspend function wouldn't be called on SMMU
>>>> implementation needed power control (since they would have runtime PM
>>>> enabled) and on others, it would be called but do nothing (since no
>>>> clocks).
>>>>
>>>>> Honestly, I just don't know. :-)
>>>>>
>>>>> It just looks odd the way it is done.  I think the clock should be
>>>>> gated during system-wide suspend too, because the system can spend
>>>>> much more time in a sleep state than in the working state, on average.
>>>>>
>>>>> And note that you cannot rely on runtime PM to always do it for you,
>>>>> because it may be disabled at a client device or even blocked by user
>>>>> space via power/control in sysfs and that shouldn't matter for
>>>>> system-wide PM.
>>>>
>>>> User space blocking runtime PM through sysfs is a good point. I'm not
>>>> 100% sure how the PM subsystem deals with that in case of system-wide
>>>> suspend. I guess for consistency and safety, we should have the
>>>> suspend callback.
>>>
>>> Will add the following suspend callback (same as
>>> arm_smmu_runtime_suspend):
>>>
>>>   static int __maybe_unused arm_smmu_pm_suspend(struct device *dev)
>>>   {
>>>           struct arm_smmu_device *smmu = dev_get_drvdata(dev);
>>>
>>>           clk_bulk_disable(smmu->num_clks, smmu->clks);
>>>
>>>           return 0;
>>>   }
>>
>> I think you also need to check if the clock has already been disabled
>> by runtime PM.  Otherwise you may end up disabling it twice in a row.
>
>
> Should I rather call a pm_runtime_put() in suspend callback?

That wouldn't work as runtime PM may be effectively disabled by user
space via sysfs.  That's one of the reasons why you need the extra
system-wide suspend callback in the first place. :-)

> Or an expanded form something similar to:
> https://elixir.bootlin.com/linux/v4.18-rc5/source/drivers/slimbus/qcom-ctrl.c#L695

Yes, you can do something like that, but be careful to make sure that
the state of the device after system-wide resume is consistent with
its runtime PM status in all cases.