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

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Date:	Sat, 08 Jun 2013 10:23:08 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	rtc-linux@...glegroups.com
CC:	John Stultz <john.stultz@...aro.org>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [rtc-linux] Re: adjusting the monotonic system time (from inside
 the kernel)

Am 08.06.2013 01:49, schrieb John Stultz:
> On 05/06/2013 07:02 AM, Alexander Holler wrote:
>> Hello,
>>
>> 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 ->
>> system-time
>>
>> So one solution would be to emulate such a serial device:
>>
>> hw-clock --> kernel --emulated-serial-device--> ntpd -> kernel ->
>> system-time
>>
>> 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.

Regards,

Alexander
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