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:	Sun, 07 Jul 2013 09:35:57 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	rtc-linux@...glegroups.com
CC:	Jiri Kosina <jkosina@...e.cz>,
	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

Am 06.07.2013 20:21, schrieb Jiri Kosina:
> 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.

So that would have been likely my next discovery. ;)

Very nice, this will reduce the whole patch to one line.

I will test it and send an updated patch.

Thanks a lot,

Alexander Holler

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