linux-kernel - Re: [PATCH 3/3] rtc: rtc-hid-sensor-time; add option hctosys to set time at boot

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Message-ID: <5176AD08.8090108@ahsoftware.de>
Date:	Tue, 23 Apr 2013 17:47:20 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org, rtc-linux@...glegroups.com,
	Alessandro Zummo <a.zummo@...ertech.it>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Jonathan Cameron <jic23@....ac.uk>,
	Jiri Kosina <jkosina@...e.cz>
Subject: Re: [PATCH 3/3] rtc: rtc-hid-sensor-time; add option hctosys to set
 time at boot

Am 23.04.2013 12:17, schrieb Alexander Holler:
> Am 23.04.2013 12:13, schrieb Alexander Holler:
>> Am 23.04.2013 12:08, schrieb Alexander Holler:
>>> Am 23.04.2013 10:51, schrieb Alexander Holler:
>>>> Am 23.04.2013 01:38, schrieb Andrew Morton:
>>>>> On Fri, 19 Apr 2013 17:14:12 +0200 Alexander Holler
>>>>> <holler@...oftware.de> wrote:
>>>>>
>>>>>> drivers/rtc/hctosys (CONFIG_RTC_HCTOSYS) doesn't work for
>>>>>> rtc-hid-sensor-time because it will be called in late_init, and thus
>>>>>> before
>>>>>> rtc-hid-sensor-time gets loaded.
>>>>>
>>>>> Isn't that true of all RTC drivers which are built as modules?
>>>>> There's
>>>>> nothing special about hid-sensor-time here?
>>>>>
>>>>> I assume the standard answer here is "your RTC driver should be built
>>>>> into vmlinux".  If we wish to make things work for modular RTC drivers
>>>>> then we should find a solution which addresses *all* RTC drivers?
>>>>
>>>> No. I having rtc-hid-sensor-time, hid-sensor-hub (and USB) statically
>>>> linked in doesn't help. Here is what happens here with such an
>>>> configuration:
>>>>
>>>> --
>>>> [    7.638970] drivers/rtc/hctosys.c: unable to open rtc device (rtc0)
>>>> [    7.645639] Waiting 180sec before mounting root device...
>>>> [   16.598759] HID-SENSOR-2000a0 HID-SENSOR-2000a0.0: rtc core:
>>>> registered hid-sensor-time as rtc0
>>>> [   16.608712] HID-SENSOR-2000a0 HID-SENSOR-2000a0.0: hctosys: setting
>>>> system clock to 2013-04-19 16:45:06 UTC (1366389906)
>>>> --
>>>>
>>>> I havent't looked in detail at why rtc-hid-sensor-time gets loaded that
>>>> late, but I assume it's because the USB stack (and/or the device or the
>>>> communication inbetween) needs some time (and I assume that's why
>>>> rootwait and rootdelay got invented too).
>>>>
>>>>>
>>>>>> To set the time through rtc-hid-sensor-time
>>>>>> at startup, the module now checks by default if the system time is
>>>>>> before
>>>>>> 1970-01-02 and sets the system time (once) if this is the case.
>>>>>>
>>>>>> To disable this behaviour, set the module option hctosys to zero,
>>>>>> e.g. by
>>>>>> using rtc-hid-sensor-time.hctosys=0 at the kernel command line if the
>>>>>> driver is statically linked into the kernel.
>>>>>
>>>>> Is a bit hacky, no?
>>>>
>>>> I didn't have any other idea to prevent an USB (or any other
>>>> hot-pluggable HID) device to change the time while still beeing able
>>>> (by
>>>> default) to set the time by such an device at boot. But I'm open to
>>>> suggestions. (E.g. one of the scenarios I want to prevent is, that a
>>>> computer gets it's time by NTP and someone is able to change the time
>>>> later on by simply plugging in some HID device.)
>>>>
>>>
>>> To add something more: I use the system time as a bool
>>> "time_was_set_once" and have choosen one day just in case something
>>> needs really long to boot (e.g. because of some lengthy fsck or whatever
>>> else).
>>>
>>> A solution to both problems might be to change the logic for hctosys
>>> completly to read the time when the first RTC device appears (or when
>>> the device mentioned in CONFIG_RTC_HCTOSYS_DEVICE appears). But that
>>> would require a change to hctosys or the RTC subsystem, which would
>>> involve more patches and discussion. As rtc-hid-sensor-time currently
>>> seems to be the only RTC with the above problems, I've gone the easy
>>> route and only modified this driver.
>>
>> Oh, damn. I've forgotten my example above with NTP. In that case setting
>> the time when the first RTC appears doesn't work.
>
> So a general solution might be to set the system time when the first RTC
> (or the one mentioned in CONFIG_RTC_HCTOSYS_DEVICE) appears AND nothing
> else did set the time before.

To extend that idea a bit further, I think an option timewait (similiar 
to rootwait) would be a nice to have.

If just time wouldn't be that rare ... ;)

Regards,

Alexander
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