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

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Date:	Thu, 15 May 2014 11:58:55 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
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, 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.

> 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.

Alan Stern

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