Re: [rtc-linux] Re: adjusting the monotonic system time (from insidethe kernel)

From: Alexander Holler
Date: Sat Jun 08 2013 - 04:24:00 EST

Am 08.06.2013 01:49, schrieb John Stultz:
On 05/06/2013 07:02 AM, Alexander Holler wrote:

I'm looking at how to adjust the (monotonic) system time from inside
the kernel.

Use case is that I have a hw-clock which (not necessarily) regulary
sends a timestamp with millisecond precision which I want to use to
adjust the system time.

It seems the usual solution to do such, is to use NTP which uses it's
own driver (which usually seems to be based on some serial connection):

hw-clock --serial--> kernel --serial-device--> ntpd -> kernel ->

So one solution would be to emulate such a serial device:

hw-clock --> kernel --emulated-serial-device--> ntpd -> kernel ->

Another solution would be to "invent" a ntp-device and write a driver
for ntpd to use it:

hw-clock --> kernel --ntp-device--> ntpd -> kernel -> system-time

But I would prefer the following:

hw-clock --> kernel -> system-time

Problem is that the hw-clock in question doesn't offer something like
a tick. It just might send a timestamp with millisecond precision
whenever it wants.

Because I don't want to reinvent the wheel and because I think there
are some people which already have spend some thoughts on similiar
things, I'm asking here before I try to implement something which then
never might find it's way into the mainline kernel.

Any hints, suggestions, whatever?

Sorry on the delay to reply here, just noticed this in my spam folder
(hopefully I've trained it not to catch your mails now).

You probably want to check out do_adjtimex() in the kernel. It has a
number of ways that allow for the clock to be slewed or jumped.
Otherwise you probably should look into the PPS subsystems to see if it
could be extended to support your needs.

Thanks, I've already digged around in the source for adjtimex to get a clue about the ntp-api and how it works. At first view wasn't that enlightened. ;)

But that now has to wait. If the new hctosys mechanism ends up in the kernel, the millisecond support for rtc-hid-sensor-time will disappear because rtc-hid-sensor-time will not set the time itself afterwards.

So I have to figure out and implement a new way to feed a timestamp with milliseconds to hctosys.

My first idea would be to add a read_timeval() or even read_timespec() to rtc_class_ops which would be used by hctosys instead of read_time() if it exists.


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at