On Tue, 14 Nov 2006, David Brownell wrote:
> 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".
We could carry on this argument indefinitely. Yes, the device_may_wakeup
stuff does work without sysfs. But it doesn't do anything significant; it
amounts to no more than device_can_wakeup(). AFAIK there's no way to
change the setting of the may_wakeup flag other than via sysfs. That's
what I meant by "tied".
> > > 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.
Things work differently in uhci-hcd. I still haven't submitted the patch
to add device_may_wakeup support (although it was written quite a while
ago and may have been posted to linux-usb-devel; I can't remember).
However even when it is added and may_wakeup is off, autostop will still
function. It won't rely on interrupts or other wakeup events, though --
instead the root-hub status polling mechanism will be used.
Alan Stern
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