linux-kernel - Re: [PATCH v5 5/5] intel

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Message-ID: <CAGAzgspRz4vx-enfX1De_ffzRXLxFKUNxKc2NKZWC-04K9-3LQ@mail.gmail.com>
Date:   Thu, 13 Jul 2017 15:49:37 -0700
From:   "dbasehore ." <dbasehore@...omium.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Rajneesh Bhardwaj <rajneesh.bhardwaj@...el.com>,
        x86@...nel.org,
        Platform Driver <platform-driver-x86@...r.kernel.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>,
        Linux-pm mailing list <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v5 5/5] intel_idle: Add S0ix validation

On Wed, Jul 12, 2017 at 10:11 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Wed, 12 Jul 2017, dbasehore . wrote:
>> On Wed, Jul 12, 2017 at 3:16 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>> > There are more issues with this: If there is a hrtimer scheduled on that
>> > last CPU which enters the idle freeze state and that timer is 10 minutes
>> > away, then the check timer can't be programmed and the system will happily
>> > stay for 10 minutes in some shallow C state without notice. Not really
>> > useful.
>>
>> Are hrtimers not suspended after timekeeping_suspend is called?
>
> They are. As I said I forgot about the inner workings and that check for
> state != shutdown confused me even more, as it just looked like this might
> be a valid state.

Okay. I'll add a comment to clarify this part.

>
>> > You know upfront whether the i915 power wells (or whatever other machinery)
>> > is not powered off to allow the system to enter a specific power state. If
>> > you think hard enough about creating infrastructure which allows you to
>> > register power related facilities and then check them in that idle freeze
>> > enter state, then you get immediate information WHY this happens and not
>> > just the by chance notification about the fact that it happened.
>>
>> It's not always something that can be checked by software. There was
>> one case where an ordering for powering down audio hardware prevented
>> proper PC10 entry, but there didn't seem to be any way to check that.
>> Hardware watchdogs also have the same lack of clarity, but most if not
>> all desktop and mobile processors ship with one. Overall, this seems
>> to be the best that can be done at this point in freeze, and we can't
>> really rely on every part of the system properly validating it's state
>> in its suspend operation.
>
> So if I understand correctly, this is the last resort of catching problems
> which can't be detected upfront or are caused by a software bug.
>
> I'm fine with that, but please explain and document it proper. The current
> explanation is confusing at best.

Will do.

>
> Thanks,
>
>         tglx