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?
Regards,
Alexander Holler
Am 06.05.2013 16:02, schrieb Alexander Holler:
>
> 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?
To offer something for discussion I'm thinking about:
When a timestamp with millisecond precision is received, instruct the
kernel to adjust the system time in the next minute for the calculated
offset.
Question here would be what to choose as window to adjust the time (the
"next minute" above"). I think that should depend on the offset. But
what are acceptable values? One minute to adjust the clock for offsets
up to +-500ms? Two minutes if the offset is one second? ...
As I never spend a thought about such before, I just don't have any
pointers what would be acceptable.
Regards,
Alexander
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
-john
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