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:	Thu, 27 Jun 2013 01:51:36 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	rtc-linux@...glegroups.com
CC:	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 27.06.2013 00:07, schrieb Greg KH:
> On Wed, Jun 26, 2013 at 11:34:35PM +0200, Alexander Holler wrote:
>>>> +	/*
>>>> +	 * The HID device has to be registered to read the clock.
>>>> +	 * Because rtc_device_register() might read the time, we have to delay
>>>> +	 * rtc_device_register() to a work in order to finish the probe before.
>>>> +	 */
>>>> +	time_state->workts = kmalloc(sizeof(struct hid_time_workts),
>>>> +		GFP_KERNEL);
>>>> +	if (time_state->workts == NULL) {
>>>> +		sensor_hub_remove_callback(hsdev, HID_USAGE_SENSOR_TIME);
>>>> +		return -ENOMEM;
>>>>  	}
>>>> +	time_state->workts->time_state = time_state;
>>>> +	INIT_WORK(&time_state->workts->work,
>>>> +			hid_time_register_rtc_work);
>>>> +	schedule_work(&time_state->workts->work);
>>>
>>> This seems unreliable.  The scheduled work can run one nanosecond
>>> later, on this or a different CPU.  What guarantees that the HID device
>>> will then be fully registered?
>>
>> Nothing, but schedule_delayed_work() is as unreliable as without delay
>> and I don't know of any callback after registration has happened. I have
>> to dig through the hid-(sensor-)code, maybe I will find a callback I can
>> (mis)use to register the rtc driver after the hid driver was registered.
> 
> Why not use the deferred_probe code, which is there just for this type
> of thing (i.e. your other drivers/devices aren't present/initialized
> yet.)?  Just return -EPROBE_DEFER from your probe function if you don't
> find everything already set up properly, the driver core will call you
> again later after it has initialized everything it has found.

Hmm, didn't know about the deferred_probe stuff. Thanks.

Unfortunately I currently don't see how this might help here. The
rtc-device will not be probed, so it can't be deferred. And even if I
will find or implement a way to add a probe for the rtc device, I still
have to search how to find out if the HID device is registered.

Anyway, back to reading to sources. Maybe there already is some callback
from hid-sensor-hub or the hid-subsystem I can use. I haven't searched
in deep for such because using a work worked just fine here on several
machines (besides that it was a quick hack which got only necessary with
the changed hctosys which reads the time in rtc_device_register()).

I already wondered why using a work (even without delay) did work, but I
explained it with some (maybe imaginary) locality of works, such that
they get called after the scheduling thread gives up his timeslice which
happily happened after the hid-device was registered. So I hoped (hope
dies last ;) ) to only have to fix it, if it actually doesn't work
somewhere or sometimes after the foreseen discussion about hctosys has
come to an end.

Regards,

Alexander Holler
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