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

From: Alan Stern
Date: Fri Jun 26 2015 - 16:28:32 EST


On Fri, 26 Jun 2015, Alan Stern wrote:

> On Fri, 26 Jun 2015, Tom Gundersen wrote:
>
> > This policy used to be unconditionally applied by udev, but there
> > is no reason to make userspace be involved in this and in the future
> > udev will not be doing it by default.
> >
> > See: <https://github.com/systemd/systemd/pull/353>.
> >
> > Signed-off-by: Tom Gundersen <teg@xxxxxxx>
> > Cc: Jiri Kosina <jkosina@xxxxxxx>
> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > ---
> >
> > Hi,
> >
> > I don't have the right hardware for this, so it has only been compile-tested.
> > I'm therefore sending it as an RFC only. Mainly I want to bring it to people's
> > attention that it would be great to get this feature into the kernel as we want
> > to drop it from udev.
> >
> > Cheers,
> >
> > Tom
> >
> > drivers/hid/usbhid/hid-core.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/hid/usbhid/hid-core.c b/drivers/hid/usbhid/hid-core.c
> > index bfbe1be..af80700 100644
> > --- a/drivers/hid/usbhid/hid-core.c
> > +++ b/drivers/hid/usbhid/hid-core.c
> > @@ -1358,6 +1358,9 @@ static int usbhid_probe(struct usb_interface *intf, const struct usb_device_id *
> > setup_timer(&usbhid->io_retry, hid_retry_timeout, (unsigned long) hid);
> > spin_lock_init(&usbhid->lock);
> >
> > + if (dev->removable == USB_DEVICE_FIXED)
> > + usb_enable_autosuspend(dev);
>
> 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

Oops -- I meant "compound", not "composite". A compound USB device
actually has multiple devices (one of which is a hub) in a single
package.

> 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.

Another problem is that the set_usb_port_removable() routine in
drivers/usb/core/hub.c (the routine that sets dev->removable in the
first place) contains this code:

hub = usb_hub_to_struct_hub(udev->parent);

wHubCharacteristics = le16_to_cpu(hub->descriptor->wHubCharacteristics);

if (!(wHubCharacteristics & HUB_CHAR_COMPOUND))
return;

I guess the assumption was that all of a hub's ports would be
unpluggable if the hub wasn't part of a compound device. But that's
not correct for root hubs. This test should be deleted.

Alan Stern

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