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 10:29:42 -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:

> > > 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.

Alan Stern

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