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

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Date:	Tue, 25 Jul 2006 14:18:32 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	jg@...top.org
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
Subject: Re: [PATCH] RTC: Add mmap method to rtc character driver

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

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