Re: [PATCH] [RFC] notify userspace about time changes

From: Kay Sievers
Date: Thu Aug 05 2010 - 18:22:51 EST

On Thu, Aug 5, 2010 at 23:38, Greg KH <gregkh@xxxxxxx> wrote:
> On Thu, Aug 05, 2010 at 02:11:05PM -0700, john stultz wrote:
>> On Thu, 2010-08-05 at 15:33 +0300, Alexander Shishkin wrote:
>> > On 4 August 2010 18:58, john stultz <johnstul@xxxxxxxxxx> wrote:
>> > > Is there a actual use case that you need this for? ÂI don't really have
>> > > an issue with the code I just really want to make sure the feature would
>> > > be useful enough to justify the API and code maintenance going forward.
>> >
>> > Yes. What we have here is an application which takes care of different means
>> > of time synchronization (trusted time servers, different GSM operators, etc)
>> > and also different kinds of time-based events/notifications (like "dentist
>> > appointment next thursday"). When it encounters a time change that is
>> > made by some other application, it basically wants to disable automatic
>> > time adjustment and trigger the events/notifications which are due at this
>> > (new) time.
>> Ok. Something specific is always more helpful then theoretical uses.
>> I think the filtering is still a bit controversial, so you might want to
>> respin it without that. But otherwise I'm ok with it as long as no one
>> else objects to any of the minor details of the interface
>> GregKH: Does /sys/kernel/time_notify seem ok by you?
> Um, it depends, what is that file going to do? ÂI don't see a
> Documentation/ABI/ entry here that describes it fully :)

I think that's really awkward interface, to pass file descriptor
numbers around and write them to magic sysfs files.

I would very much prefer a file that contains the current time, and
wakes up possible users with a POLL_ERR on changes caused by some
other process. That works very well for things like /proc/mounts, is
easy to get, and does not need a full page of weird instructions to
get stuff done. :)

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