netdev - Re: [PATCH 1/1] posix clocks: introduce syscall for clock tuning.

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Message-ID: <20100827112406.GB11657@riccoc20.at.omicron.at>
Date:	Fri, 27 Aug 2010 13:24:06 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	Arnd Bergmann <arnd@...db.de>, linux-api@...r.kernel.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] posix clocks: introduce syscall for clock tuning.

On Mon, Aug 23, 2010 at 01:41:13PM -0700, john stultz wrote:
> As I mentioned in the previous mail, I agree the new functionality
> (adjusting the time by an offset instantaneously) is useful, but I'd
> prefer it be done initially within the existing adjtimex() interface.

But the adjtimex does not support nanosecond resolution.
 
> Then if the posix-time clock_id multiplexing version of adjtimex is
> found to be necessary, the new syscall should be introduced, using the
> same API (not all clock_ids need to support all the adjtimex modes, but
> the new interface should be sufficient for NTPd to use).

Would the new syscall need to take a struct timex?

If so, I think it not worth the effort of adding a syscall. Instead,
we can just add "clockid" flags into the mode field.

> There are some other conceptual issues this new syscall introduces:
> 
> 1) While clock_adjtimex(CLOCK_REALTIME,...) would be equivalent to
> adjtimex(), would clock_adjtimex(CLOCK_MONOTONIC,...) make sense? 
> 
> Given CLOCK_MONOTONIC and CLOCK_REALTIME are both based off the same
> notion of time, but offset from each other, any adjustment to one clock
> would be reflected in the other. However, the API would make it seem
> like they could be adjusted independently.

You could adjust the frequency of either one. As a side effect, the
other clock would also be adjusted.

You can only change the time offset on CLOCK_REALTIME, and that would
have no effect on CLOCK_MONOTONIC.

> 2) The same issue in #1 exists for CLOCK_REALTIME/MONOTONIC_COARSE
> variants. 
> 
> 3) Freq steering for MONOTONIC_RAW would defeat the purpose of the
> clock_id.

If I understand correctly, MONOTONIC_RAW is just access to the
hardware counter?
 
> 4) Does adjustments to CPU_TIME clock_ids make sense?

Don't think so.


Thanks,
Richard
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