linux-kernel - Re: [rtc-linux] Re: [PATCH 3/9 v2] rtc: rtc-hid-sensor-time: delay registering as rtc into a work

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Date:	Sat, 6 Jul 2013 20:21:25 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Alexander Holler <holler@...oftware.de>
Cc:	rtc-linux@...glegroups.com, Greg KH <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Alessandro Zummo <a.zummo@...ertech.it>
Subject: Re: [rtc-linux] Re: [PATCH 3/9 v2] rtc: rtc-hid-sensor-time: delay
 registering as rtc into a work

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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