linux-kernel - Re: [RFC][PATCH 2/3] PM / sleep: Mechanism to avoid resuming runtime-suspended devices unnecessarily

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Date:	Fri, 16 May 2014 08:20:55 -0700
From:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux PM list <linux-pm@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Aaron Lu <aaron.lu@...el.com>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kevin Hilman <khilman@...aro.org>,
	Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: [RFC][PATCH 2/3] PM / sleep: Mechanism to avoid resuming
 runtime-suspended devices unnecessarily

On Thu, 15 May 2014 11:58:55 -0400 (EDT)
Alan Stern <stern@...land.harvard.edu> wrote:

> On Thu, 15 May 2014, Jacob Pan wrote:
> 
> > On Thu, 15 May 2014 10:29:42 -0400 (EDT)
> > Alan Stern <stern@...land.harvard.edu> wrote:
> > 
> > > On Thu, 15 May 2014, Jacob Pan wrote:
> > > 
> > > > > > should we respect ignore_children flag here? not all parent
> > > > > > devices create children with proper .prepare() function.
> > > > > > this allows parents override children.
> > > > > > I am looking at USB, a USB device could have logical
> > > > > > children such as ep_xx, they don't go through the same
> > > > > > subsystem .prepare().
> > > > > 
> > > > > Well, I'm not sure about that.  Let me consider that for a
> > > > > while.
> > > > OK. let me be more clear about the situation i see in USB.
> > > > Correct me if I am wrong, a USB device will always has at least
> > > > one endpoint/ep_00 as a kid for control pipe, it is a logical
> > > > device. So when device_prepare() is called, its call back is
> > > > NULL which makes .direct_complete = 0. Since children device
> > > > suspend is called before parents, the parents .direct_complete
> > > > flag will always get cleared.
> > > > 
> > > > What i am trying to achieve here is to see if we avoid resuming
> > > > built-in (hardwired connect_type) non-hub USB devices based on
> > > > this new patchset. E.g. we don't want to resume/suspend USB
> > > > camera every time in system suspend/resume cycle if they are
> > > > already rpm suspended. We can save ~100ms resume time for the
> > > > devices we have tested.
> > > 
> > > This is a good point, but I don't think it is at all related to 
> > > ignore_children.
> > > 
> > > Instead, it seems that the best way to solve it would be to add a 
> > > ->prepare() handler for usb_ep_device_type that would always turn 
> > > on direct_complete.
> > > 
> > yeah, that would solve the problem with EP device type. But what
> > about other subdevices. e.g. for USB camera, uvcvideo device? We
> > can add .prepare(return 1;) for each level but would it be better
> > to have a flag similar to ignore_children if not ignore_children
> > itself.
> 
> Something like that could always be added.
or, how about if a device's .prepare() is NULL, we could
assume .direct_resume() should be set. i.e.


--- a/drivers/base/power/main.c
+++ b/drivers/base/power/main.c
@@ -1539,7 +1539,7 @@ static int device_prepare(struct device *dev,
pm_message_t state) pm_runtime_put(dev);
                return ret;
        }
-       dev->power.direct_complete = ret > 0 && state.event ==
PM_EVENT_SUSPEND
+       dev->power.direct_complete = (!callback || ret > 0) &&
state.event == PM_EVENT_SUSPEND && pm_runtime_suspended(dev);
        dev_dbg(dev, "%s:direct_complete %d, info %s\n", __func__,
dev->power.direct_complete, info);

> 
> > Actually, I don't understand why this is not related to
> > ignore_children. Could you explain?
> 
> It's hard to explain why two things are totally separate.  Much
> better for you to describe why you think they _are_ related, so that
> I can explain how you are wrong.
> 
> > If the parent knows it can ignore children and already rpm
> > suspended, why do we still ask children?
> 
> The "ignore_children" flag doesn't mean that the parent can ignore
> its children.  It means that the PM core is allowed to do a runtime
> suspend of the parent while leaving the children at full power.
> 
> In particular, it doesn't mean that the children's ->suspend()
> callback will work correctly if it is called while the parent is
> runtime suspended.
that explains my question about ignore_chilren flag. thanks.
> 
> Alan Stern
> 

[Jacob Pan]
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