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: <51765D8C.1010008@ahsoftware.de>
Date:	Tue, 23 Apr 2013 12:08:12 +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 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.

Regards,

Alexander

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