linux-kernel - Re: [PATCH] RTC: Add mmap method to rtc character driver

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Date:	Tue, 25 Jul 2006 17:39:01 -0400
From:	Jim Gettys <jg@...top.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Neil Horman <nhorman@...driver.com>,
	Dave Airlie <airlied@...il.com>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	linux-kernel@...r.kernel.org, a.zummo@...ertech.it,
	jg@...edesktop.org, Keith Packard <keithp@...thp.com>
Subject: Re: [PATCH] RTC: Add mmap method to rtc character driver

Keith's the expert (who wrote the smart scheduler): I'd take a wild ass
guess that 10ms is good enough.

Maybe people can keep him on the cc list this time...
                             - Jim


On Tue, 2006-07-25 at 14:18 -0700, H. Peter Anvin wrote:
> Jim Gettys wrote:
> > On Tue, 2006-07-25 at 14:04 -0700, H. Peter Anvin wrote:
> > 
> >> That's why I'm suggesting adding a cheap, possibly low-res, gettimeofday 
> >> virtual system call in case there is no way for the kernel to provide 
> >> userspace with a cheap full-resolution gettimeofday.  Obviously, if a 
> >> high-quality gettimeofday is available, then they can be linked together 
> >> by the kernel.
> > 
> > Low res is fine: X Timestamps are 1 millisecond values, and wrap after a
> > few hundred days.  What we do care about is monotonically increasing
> > values (until it wraps). On machines of the past, this was very
> > convenient; we'd just store a 32 bit value for clients to read, and not
> > bother with locking.  I guess these days, you'd at least have to protect
> > the store with a memory barrier, maybe....
> > 
> > It was amusing years ago to find toolkit bugs after applications had
> > been up for that long (32 bits of milliseconds)...  Yes, there are
> > applications and machines that stay up that long, really there are....
> > 
> 
> Do you need 1 ms resolution, or is 10 ms good enough?
> 
> 	-hpa
> 
-- 
Jim Gettys
One Laptop Per Child


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