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Date:	Tue, 14 Nov 2006 13:18:10 -0800
From:	David Brownell <>
To:	Alan Stern <>
Subject: Re: [linux-usb-devel] 2.6.19-rc5 regression: can't disable OHCI wakeup via sysfs

On Monday 13 November 2006 9:15 am, Alan Stern wrote:
> On Mon, 13 Nov 2006, David Brownell wrote:
> > It's a *driver model* API, which is also accessible from sysfs ... to support
> > per-device policies, for example the (a) workaround.  The mechanism exists
> > even on kernels that don't include sysfs ... although on such systems, there
> > is no way for users to do things like say "ignore the fact that this mouse
> > claims to issue wakeup events, its descriptors lie".
> Yes, it is separate from sysfs -- but it is _tied_ to the sysfs API.

I can't agree.  If you deconfigure sysfs, it still works.
Since it's independent like that, there's no way it's "tied".

> > > and therefore administrative  
> > > in nature, but now you say it's also being used to record hardware quirks.
> > 
> > No; I'm saying the driver model is used to record that the hardware mechanism
> > isn't available.   The fact that it's because of an implementation artifact
> > (bad silicon, or board layout, etc) versus a design artifact (silicon designed
> > without that feature) is immaterial ... in either case, the system can't use
> > the mechanism.
> But the information is being recorded in the wrong spot.  The correct test
> should use device_can_wakeup, not device_may_wakeup.  The can_wakeup flag
> is the one which records whether or not the hardware mechanism is actually
> available.

Go look again.  "may" implies (i) can , and (ii) should.  So if there's a
hardware quirk registered, (i) always fails.  And in the not-uncommon case
where the device misbehavior isn't known to the kernel, userspace has the
option of making (ii) kick in (the workaround mentioned above).  This is a
generic approach, it works on all wakeup-capable devices.

So "may" is correct, and "can" is insufficient.

> Okay.  I'll write a patch to eliminate autostop and those routines when
> CONFIG_PM is off.
> But that doesn't answer the question above: Should autostop check 
> device_can_wakeup rather than device_may_wakeup?

See above, and the definition of may_wakeup().

> Also: Does the quirk/bug detection logic clear can_wakeup, as it should?  
> Or does it only affect may_wakeup?

See above.  Quirks directly recognized by the kernel clear can_wakeup.
Ones that are reported via userspace clear should_wakeup.  Either suffices
to ensure that the may_wakeup() predicate fails.

- Dave
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