linux-kernel - Re: [PATCH] usb: xhci: Disable runtime PM suspend for quirky controllers.

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Message-ID: <Pine.LNX.4.44L0.1305291015230.1473-100000@iolanthe.rowland.org>
Date:	Wed, 29 May 2013 10:23:01 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Julius Werner <jwerner@...omium.org>
cc:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	Shawn Nematbakhsh <shawnn@...gle.com>,
	<linux-usb@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: xhci: Disable runtime PM suspend for quirky
 controllers.

On Tue, 28 May 2013, Julius Werner wrote:

> The policy we want to achieve is to disable runtime PM iff there is a
> device connected that doesn't have persist_enabled or a reset_resume()
> handler and whose parent/root hub resets on resume, right? So couldn't

Probably just root hub, not parent.  A non-root hub that resets upon 
resume wouldn't be a good idea.  Also, we know in advance that the hub 
driver _does_ have a reset-resume handler.

> we remove the HCD-specific XHCI_RESET_ON_RESUME and set the (existing)
> generic USB_QUIRK_RESET_RESUME on the root hub instead? Then we could
> handle all of this in the USB core (during device initialization and
> when changing persist_enabled through sysfs) by just checking for
> udev->reset_resume on all parent hubs of the device in question (and
> use pm_runtime_get/put() on said device to prevent its parents from
> suspending as appropriate).

This sounds too intricate to me.  You might want to prevent resets even 
if the device does support reset-resume, because they consume time.  Or 
you might not care about resets even if persist isn't enabled (consider 
a USB mouse, for example).

I agree that setting the RESET_RESUME quirk on the root hub is a good
way to represent the situation.  And perhaps the kernel could implement 
a useful default policy -- but userspace should ultimately remain in 
control.

Alan Stern

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