Message-ID: <20080405175437.GA17016@elf.ucw.cz>
Date:	Sat, 5 Apr 2008 19:54:37 +0200
From:	Pavel Machek <pavel@....cz>
To:	John Sigler <linux.kernel@...e.fr>
Cc:	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Linux 11-minute mode (RTC update)

Hi!

>> I run Linux kernel 2.6.22.1-rt9 and ntpd 4.2.4p0
>>
>> # adjtimex --print
>>          mode: 0
>>        offset: 77
>>     frequency: -1309904
>>      maxerror: 493576
>>      esterror: 50
>>        status: 1
>> time_constant: 6
>>     precision: 1
>>     tolerance: 33554432
>>          tick: 10000
>>      raw time:  1207230744s 183249us = 1207230744.183249
>>
>> In my setup, STA_UNSYNC (0x0040, clock unsynchronized) is 0.
>>
>> Thus, ntp_synced() returns 1.
>>
>> Thus the kernel should write the system time to the RTC every
>> 11 minutes; but it does not.
>>
>> The relevant code is in sync_cmos_clock()
>>
>> http://lxr.linux.no/linux/kernel/time/ntp.c#L188
>>
>> I've added several printk() to this function, and it appears
>> that it is never called.
>>
>> The relevant timer is defined with the following macro.
>>
>> static DEFINE_TIMER(sync_cmos_timer, sync_cmos_clock, 0, 0);
>>
>> which expands to
>>
>> static struct timer_list sync_cmos_timer =
>> {
>>   .function = sync_cmos_clock,
>>   .expires  = 0,
>>   .data     = 0,
>>   .base     = &boot_tvec_bases
>> };
>>
>> The problem seems to be that this timer is never armed, to bootstrap
>> the process. It seems there should be a call to mod_timer() somewhere.
>>
>> do_adjtimex() calls notify_cmos_timer() unconditionally.
>>
>> static void notify_cmos_timer(void)
>> {
>>   if (no_sync_cmos_clock)
>>     mod_timer(&sync_cmos_timer, jiffies + 1);
>> }
>>
>> What are the semantics of notify_cmos_timer?
>> What is it supposed to do?
>>
>> And what is 'no_sync_cmos_clock' supposed to mean?
>> /* Disable the cmos update - used by virtualization and embedded */
>> int no_sync_cmos_clock  __read_mostly;
>> Why would we (re)arm the timer when 'no_sync_cmos_clock' is true?
>
> For those wondering what this "11-minute mode" is, I'll quote the
> man page for hwclock. ( http://linux.die.net/man/8/hwclock )
>
> <quote>
>
> Automatic Hardware Clock Synchronization By the Kernel
>
> You should be aware of another way that the Hardware Clock is kept 
> synchronized in some systems. The Linux kernel has a mode wherein it copies 
> the System Time to the Hardware Clock every 11 minutes. This is a good mode 
> to use when you are using something sophisticated like ntp to keep your 
> System Time synchronized. (ntp is a way to keep your System Time 
> synchronized either to a time server somewhere on the network or to a radio 
> clock hooked up to your system. See RFC 1305).
>
> This mode (we'll call it "11 minute mode") is off until something turns it 
> on. The ntp daemon xntpd is one thing that turns it on. You can turn it off 
> by running anything, including hwclock --hctosys, that sets the System Time 
> the old fashioned way.
>
> To see if it is on or off, use the command adjtimex --print and look at the 
> value of "status". If the "64" bit of this number (expressed in binary) 
> equal to 0, 11 minute mode is on. Otherwise, it is off.
>
> If your system runs with 11 minute mode on, don't use hwclock --adjust or 
> hwclock --hctosys. You'll just make a mess. It is acceptable to use a 
> hwclock --hctosys at startup time to get a reasonable System Time until 
> your system is able to set the System Time from the external source and 
> start 11 minute mode.

Does that even belong to kernel?

If it is only useful with ntpd already running, ntpd should be able to
do the RTC sync, too...

Plus, that automatically means that 11-minutes updates stop when ntpd
dies for some reason...

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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