Date:	Thu, 17 Oct 2013 13:38:27 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Lan Tianyu <tianyu.lan@...el.com>
Cc:	lenb@...nel.org, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org, Lan Tianyu <lantianyu1986@...il.com>,
	Aaron Lu <aaron.lu@...el.com>
Subject: Re: [PATCH] ACPI/Power: Check physical device's runtime pm status before requesting to resume it

On Thursday, October 17, 2013 10:40:03 AM Lan Tianyu wrote:
> On 2013年10月17日 09:02, Lan Tianyu wrote:
> > On 2013年10月16日 20:42, Rafael J. Wysocki wrote:
> >> On Wednesday, October 16, 2013 05:26:21 PM Lan Tianyu wrote:
> >>> This is a multi-part message in MIME format.
> >>> --------------090400010209000300030201
> >>> Content-Type: text/plain; charset=UTF-8
> >>> Content-Transfer-Encoding: 8bit
> >>>
> >>> On 10/16/2013 05:22 AM, Rafael J. Wysocki wrote:
> >>>> On Tuesday, October 15, 2013 04:58:28 PM Lan Tianyu wrote:
> >>>>> On 2013年10月11日 19:19, Rafael J. Wysocki wrote:
> >>>>>> On Friday, October 11, 2013 04:16:25 PM tianyu.lan@...el.com
> >>>>>> wrote:
> >>>>>>> From: Lan Tianyu <tianyu.lan@...el.com>
> >>>>>>>
> >>>>>>> Currently, when one power resource is turned on, devices owning
> >>>>>>> it will be requested to resume regardless of their runtime pm
> >>>>>>> status. ACPI power resource maybe turn on in some devices'
> >>>>>>> runtime pm resume callback(E.G, usb port) while turning on the
> >>>>>>> power resource will trigger one new resume request of the
> >>>>>>> device. It causes infinite loop between resume and suspend.
> >>>>>>> This has happened on clearing usb port's PM Qos NO_POWER_OFF
> >>>>>>> flag twice. This patch is to add check of physical device's
> >>>>>>> runtime pm status and request resume if the device is
> >>>>>>> suspended.
> >>>>>>>
> >>>>>>> Signed-off-by: Lan Tianyu <tianyu.lan@...el.com> ---
> >>>>>>> drivers/acpi/power.c | 6 ++++-- 1 file changed, 4
> >>>>>>> insertions(+), 2 deletions(-)
> >>>>>>>
> >>>>>>> diff --git a/drivers/acpi/power.c b/drivers/acpi/power.c index
> >>>>>>>  0dbe5cd..228c138 100644 --- a/drivers/acpi/power.c +++
> >>>>>>> b/drivers/acpi/power.c @@ -250,8 +250,10 @@ static void
> >>>>>>> acpi_power_resume_dependent(struct work_struct *work)
> >>>>>>>
> >>>>>>> mutex_lock(&adev->physical_node_lock);
> >>>>>>>
> >>>>>>> -	list_for_each_entry(pn, &adev->physical_node_list, node) -
> >>>>>>> pm_request_resume(pn->dev); +	list_for_each_entry(pn,
> >>>>>>> &adev->physical_node_list, node) { +		if
> >>>>>>> (pm_runtime_suspended(pn->dev)) + pm_request_resume(pn->dev);
> >>>>>>> + }
> >>>>>>
> >>>>>> This is racy, because the status may change right after you check
> >>>>>> it and before you call pm_request_resume().
> >>>>>
> >>>>> Yes, the runtime status may be changed just after the check.
> >>>>>
> >>>>>>
> >>>>>> Besides, pm_request_resume() checks the status of the device and
> >>>>>>  won't try to resume it if it is not suspended.
> >>>>>>
> >>>>>
> >>>>> For this issue, usb port is in the RPM_SUSPENDING state when
> >>>>> pm_request_resume() is called.
> >>>>
> >>>> Why exactly does that happen?
> >>>>
> >>>>> The deferred_resume will be set to true during this procedure and
> >>>>> trigger resume after finishing suspend. USB port runtime resume
> >>>>> callback will turn on its power resource again and the work of
> >>>>> acpi_power_resume_dependent() is scheduled. Because the usb port's
> >>>>>  usage count remains zero, it's to be suspended soon. When
> >>>>> pm_request_resume() of acpi_power_resume_dependent() is called, the
> >>>>> usb port is always in the PRM_SUSPENDING. Fall in the loop of
> >>>>> suspend and resume.
> >>>>>
> >>>>> How about running acpi_power_dependent when turning on power
> >>>>> resource rather than scheduling a work to run it?
> >>>>
> >>>> Is this actually going to help?  Even if
> >>>> acpi_power_resume_dependent() is run synchronously from within the
> >>>> resume callback, it will still see RPM_SUSPENDING as the device's
> >>>> status, won't it?
> >>>>
> >>>>> After this, pm_request_resume() can check device's right status
> >>>>> just after turning on power resource.
> >>>>
> >>>> The status doesn't change until the .runtime_suspend() callback
> >>>> returns and running pm_request_resume() syncrhonously from that
> >>>> callback for the device being suspended just plain doesn't make
> >>>> sense.
> >>>>
> >>>>
> >>>> Can you please explain to me how this is possible that the USB port's
> >>>> power resource is turned "on" while the port is RPM_SUSPENDING?
> >>>>
> >>> [This mail seems not to be sent to maillist. So resend. Try again
> >>> Sorry for noise]
> >>>
> >>>
> >>> Hi Rafael:
> >>>
> >>> No, I mean the acpi_power_resume_dependent() is running while the port's
> >>> status is RPM_SUSPENDING. It runs from a work item and turning on power
> >>> resource adds the work to workqueue. There is a timeslot between running
> >>> acpi_power_resume_dependent() and turning on power resource. In the
> >>> timeslot, the device runtime pm status maybe change.
> >>>
> >>> In this case, changing PM Qos flag will do pm_runtime_get_sync and
> >>> pm_runtime_put usb port. The usb port is without device attached and so
> >>> it will be resumed and suspended soon during changing PM Qos flag.
> >>>
> >>> Resume callback turns on power resource if NO_POWER_OFF flag has been
> >>> cleared and add the work of acpi_power_resume_dependent() to
> >>> workqueue. After resume, the usb port will be suspended while the
> >>> acpi_power_resume_dependent() is running. pm_request_resume() gets
> >>> RPM_SUSPENDING as the usb port's runtime status and set the
> >>> deferred_resume of usb port.
> >>>
> >>> After suspend, usb port will be resumed again and turn on power
> >>> resource. The work of acpi_power_resume_dependent() is also added to
> >>> workqueue. Because the usb port's usage count remains 0 during this
> >>> procedure. it will be suspended soon after resume. While suspending,
> >>> acpi_power_resume_dependent() is running and pm_request_resume()
> >>> sets deferred_resume again. The infinite look starts.
> >>
> >> So the problem is that the whole thing is racy.  There is no guarantee
> >> that the power resource will not be turned off after the
> >> acpi_power_get_inferred_state() check in acpi_power_resume_dependent()
> >> and before rpm_resume() queued by the pm_request_resume() called from
> >> there runs.  Playing with the time windows doesn't make that race go away.
> >>
> >> If we did what you suggested, the race still would be there, because the
> >> queued up resume may still run after the power resource has been turned off.
> > 
> > Yes, the queued up resume will run after the power resource has been
> > turned off but normally the resume should try to turn on the power
> > resource before doing some hardware related operations. If so, there
> > will not be problem, right?
> > 
> 
> Sorry. I think I misunderstood your word. Please ignore the previous
> comment.
> 
> Yes, there is still a racy. I think of one case that there are multi
> devices that share one power resource. One device turns on power
> resource during resuming and queue resumes for other devices. The device
> enter into suspend and power resource turns off soon. When one other
> device do resume, the power resource will turn on again and queue resume
> for the previous device. Just like a Ping-pong.
> 
> >>
> >> Unfortunately, I don't see how we can fix this race in a satisfactory way
> >> and I'm starting to think that the whole resuming of dependent devices may
> >> be a bad idea.
> >>
> >> IIRC, the original concern was that devices may end up in D0-uninitialized
> >> if we don't do that, but then whoever turned the power resource on will
> >> probably turn if off at one point anyway, so they will be in that state
> >> temporarily.  In other words, in addition to the fact that this is racy,
> >> there even is no reason to do it.
> >>
> >> I'll send a patch to rip off that stuff later today.
> 
> Currently, dropping it should be the better choice but I think we still
> need to resolve the D0-uninitialized problem, right?

Why do you think it is a problem in the first place?  Those devices will not
be accessed while in that state (unless there's a bug somewhere).

Thanks!

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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