On Mon, 21 May 2007, Oliver Neukum wrote:
> Am Mittwoch, 16. Mai 2007 17:20 schrieb Alan Stern:
> > > @@ -592,8 +604,21 @@ static int hid_get_class_descriptor(stru
> > > int usbhid_open(struct hid_device *hid)
> > > {
> > > struct usbhid_device *usbhid = hid->driver_data;
> > > + int res;
> > >
> > > - hid->open++;
> > > + mutex_lock(&hid_open_mut);
> > > + if (!hid->open++) {
> > > + res = usb_autopm_get_interface(usbhid->intf);
> > > + /* the device must be awake to reliable request remote
> > > wakeup */
> > > + if (res < 0) {
> > > + hid->open--;
> > > + mutex_unlock(&hid_open_mut);
> > > + return -EIO;
> > > + }
> > > + usbhid->intf->needs_remote_wakeup = 1;
> > > + usb_autopm_put_interface(usbhid->intf);
> > > + }
> > > + mutex_unlock(&hid_open_mut);
> > > if (hid_start_in(hid))
> > > hid_io_error(hid);
> > > return 0;
> >
> > Don't you need to do usb_autopm_get_interface() before hid_start_in()?
>
> hid_start_in() takes the spinlock and checks for a suspension. In the
> unlikely case that the device already has been suspended, remote
> wakeup must be active and resume() will call hid_start_in() again.
But this code runs whenever a process opens the device file. If the
device is suspended at that time, there might not be a remote wakeup
request pending. So you'd run into trouble unless you did an
autoresume before calling hid_start_in().
> > > @@ -1154,13 +1184,26 @@ static int hid_suspend(struct usb_interf
> > > struct usbhid_device *usbhid = hid->driver_data;
> > > struct usb_device *udev = interface_to_usbdev(intf);
> > >
> > > -
> > > - spin_lock_irq(&usbhid->inlock);
> > > - set_bit(HID_REPORTED_IDLE, &usbhid->iofl);
> > > - spin_unlock_irq(&usbhid->inlock);
> > > - if (usbhid_wait_io(hid) < 0)
> > > - return -EIO;
> > > -
> > > + if (udev->auto_pm) {
> > > + spin_lock_irq(&usbhid->inlock); /* Sync with error handler
> > > */
> > > + if (!test_bit(HID_RESET_PENDING, &usbhid->iofl)
> > > + && !test_bit(HID_CLEAR_HALT, &usbhid->iofl)
> > > + && !test_bit(HID_OUT_RUNNING, &usbhid->iofl)
> > > + && !test_bit(HID_CTRL_RUNNING, &usbhid->iofl))
> > > + {
> > > + set_bit(HID_REPORTED_IDLE, &usbhid->iofl);
> > > + spin_unlock_irq(&usbhid->inlock);
> > > + } else {
> > > + spin_unlock_irq(&usbhid->inlock);
> > > + return -EBUSY;
> > > + }
> > > + } else {
> > > + spin_lock_irq(&usbhid->inlock);
> > > + set_bit(HID_REPORTED_IDLE, &usbhid->iofl);
> > > + spin_unlock_irq(&usbhid->inlock);
> > > + if (usbhid_wait_io(hid) < 0)
> > > + return -EIO;
> > > + }
> > > del_timer(&usbhid->io_retry);
> > > usb_kill_urb(usbhid->urbin);
> > > flush_scheduled_work();
> >
> > This might be simpler if you inverted the order of the tests. That is,
> > first get the spinlock, then test for
> >
> > udev->auto_pm && (test_bit() || test_bit() || ...)
>
> But that would put code needlessly under lock.
A single test of udev->auto_pm! I think it would be worthwhile for the
improvements in comprehensibility and code size.
> > to see if you need to fail right away. You probably don't even need to
> > check udev->auto_pm again before calling usbhid_wait_io.
> >
> > > @@ -1175,6 +1218,7 @@ static int hid_resume(struct usb_interfa
> > > int status;
> > >
> > > clear_bit(HID_REPORTED_IDLE, &usbhid->iofl);
> > > + usbhid_mark_busy(usbhid);
> > > usbhid->retry_delay = 0;
> > > status = hid_start_in(hid);
> > > usbhid_restart_queues(usbhid);
> >
> > Again, don't you need to call usb_autopm_get_interface() before
> > hid_start_in()?
>
> resume() and suspend() have mutual exclusion, don't they? So
> the device must be awake while resume() is running.
Oh, yes. My mistake.
> > One last thing, about the race between a last-minute URB completion and
> > autosuspend. The USB autosuspend core routine doesn't check the
> > last_busy value after suspending the interface drivers and before
> > suspending the device. Do we need to?
>
> As the lock is per device, the resume() call would wait and proceed
> immediately after suspend() drops the lock. I see no problem.
I guess it's just a theoretical problem. The whole point of the
last_busy field is to prevent autosuspend too soon after any I/O. So
if an URB does complete and nevertheless the device is suspended a few
milliseconds later, then last_busy hasn't fulfilled its role. Like
this:
The autosuspend timer expires.
last_busy is sufficiently old.
hid_suspend() is called.
The interrupt URB completes.
last_busy is set to jiffies.
hid_suspend() kills the
interrupt URB and returns.
usbcore doesn't notice that
last_busy has been changed.
usbcore suspends the device.
It's not actually _wrong_, but it isn't optimal.
Alan Stern
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