Message-ID: <525FA4F3.1030206@intel.com>
Date:	Thu, 17 Oct 2013 16:50:59 +0800
From:	Lan Tianyu <tianyu.lan@...el.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
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 2013年10月17日 10:40, 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? I will try to
> resolve it by other way.
> 

How about create generic power domain for power resource and adding
physical devices and dependent devices to the domain? When the domain
powers on, resume all the devices.


>>>
>>> Thanks,
>>> Rafael
>>>
>>
>>
> 
> 


-- 
Best regards
Tianyu Lan
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