linux-kernel - Re: RTC subsystem and fractions of seconds

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Sun, 7 Jan 2007 11:02:56 +0100
From:	Philippe De Muyter <phdm@...qel.be>
To:	David Brownell <david-b@...bell.net>
Cc:	Linux Kernel list <linux-kernel@...r.kernel.org>
Subject: Re: RTC subsystem and fractions of seconds

On Sat, Jan 06, 2007 at 03:52:43PM -0800, David Brownell wrote:
> On Saturday 06 January 2007 3:26 pm, Philippe De Muyter wrote:
> > On Fri, Jan 05, 2007 at 07:49:00PM -0800, David Brownell wrote:
> > > >  	Those rtc's actually have a 1/100th of second
> > > > register.  Should the generic rtc interface not support that?
> > > 
> > > Are you implying a new userspace API, or just an in-kernel update?
> > 
> > My only concern at the moment is initializing linux's timeofday from the rtc
> > quickly and with a good precision. 
> 
> There will necessarily be a bit of fuzz there since it can take time to
> get that RTC's mutex, and the task setting that time can be preempted.
> Plus, there can also be delays at the I2C or SPI transaction level.
> 
> 
> > The way it is done currently 
> > in drivers/rtc/hctosys.c is 0.5 sec off.  We could obtain a much better
> > precision by looping there until the next change (next second for old clocks,
> > next 0.01 second for m41t81, maybe even better for other ones).
> 
> Hmm ... "looping" fights against "quickly"; as would "wait for next
> update IRQ" (on RTCs that support that).  But it would improve precision,
> at least in the sense of having the system clock and that RTC spending
> less time with the lowest "seconds" digit disagreeing.
> 
> This is something you could write a patch for, n'est-ce pas?

That would require changing the interface provided by the rtc class, but
I'll look at it.

> 
> - Dave
> 
> 

-- 
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/