On Sat, 6 Jul 2013, Alexander Holler wrote:
> I've now traced down why the above patch _really_ is needed: it's because of
> how the locking is done in the hid-subsystem. So my intuition to use a work
> was ok, but my assumption that it's because the HID device driver wasn't
> registered before probe() finished was wrong.
>
> What happens without using a work is the following (shortened a lot):
>
> hid_device_probe() // hid subsystem locks hdev->driver_input_lock
> hid_time_probe()
> devm_rtc_device_register() // wants to read the clock
> hid_rtc_read_time() // submits GET_REPORT and waits for the answer
> // (the timestamp from the clock)
> hid_input_report()
>
> And there we have something like a deadlock situation because
> hid_input_report() needs hdev->driver_input_lock to submit the report to the
> HID driver.
>
> So in short, it's currently impossible for a HID driver to read input from a
> HID device from inside it's probe function.
Please see commit c849a6143b ("HID: Separate struct hid_device's
driver_lock into two locks"). It's done exactly in order to allow for
receiving input inside of the probe() function of the driver.
--
Jiri Kosina
SUSE Labs
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