linux-kernel - Re: [PATCH][RFC] usbhid: enable autosuspend for internal devices

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Message-ID: <Pine.LNX.4.44L0.1506262113240.31192-100000@netrider.rowland.org>
Date:	Fri, 26 Jun 2015 21:20:19 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
cc:	Tom Gundersen <teg@...m.no>, <linux-usb@...r.kernel.org>,
	<linux-input@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Jiri Kosina <jkosina@...e.cz>
Subject: Re: [PATCH][RFC] usbhid: enable autosuspend for internal devices

On Fri, 26 Jun 2015, Greg Kroah-Hartman wrote:

> > This doesn't do what the patch title says.  USB_DEVICE_FIXED means that
> > the device can't be unplugged from its upstream port.  It doesn't mean
> > the device is internal to the computer.
> > 
> > As an example, consider a composite Apple keyboard, which has an
> > internal 3-port USB hub where two of the hub's ports are exposed on the
> > edge of the keyboard case and the keyboard controller is permanently
> > attached to the third hub port.  Then the controller device would be
> > marked USB_DEVICE_FIXED, even though the whole thing is external to
> > the computer and can be unplugged.
> > 
> 
> Is that really how those devices are marked?  I can't find any of my
> keyboard with hubs that mark things that way.

My Apple keyboard isn't here at the moment, and I don't remember
exactly what its hub descriptor contains.  In theory, it _should_ mark
the permanently attached port as non-removable.

I can test it next week, if you would like to see the actual values.

> > Also, are you really certain this is safe?  Aren't there a number of 
> > built-in keyboards that will work badly if you allow them to 
> > autosuspend?
> 
> udev has been doing this for a while now by default, with no reports of
> problems, so I think we should be safe.

I thought udev used a whitelist of devices known to work okay with 
autosuspend.  Does it really turn on autosuspend for _every_ USB HID 
device that is marked as removable?

(Come to think of it, given the bug in the hub driver, no device 
attached directly to the root hub will _ever_ be marked as removable 
AFAICS.  So maybe that bug is masking possible regressions.)

Alan Stern

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