Messages in this thread | | | Subject | Re: [PATCH] RTC: Add mmap method to rtc character driver | From | Jim Gettys <> | Date | Wed, 26 Jul 2006 13:25:09 -0400 |
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On Wed, 2006-07-26 at 17:16 +0200, Andi Kleen wrote: > Neil Horman <nhorman@tuxdriver.com> writes: > > > At OLS last week, During Dave Jones Userspace Sucks presentation, Jim > > Geddys and some of the Xorg guys noted that they would be able to stop using gettimeofday > > so frequently, if they had some other way to get a millisecond resolution timer > > in userspace,
I agree with Andi here.
> > No, no, it's wrong. They should use gettimeofday and the kernel's job > is to make it fast enough that they can.
Exactly. On modern machines, doing a procedure call to get the time (as opposed to a system trap) is, I suspect, very tolerable. And who knows, maybe a smart compiler inlines the procedure so it optimizes to just a few instructions.
If behind the scenes there is a mapped page that is used to convey this information efficiently, that's fine.
But I don't think it should be the application programmer's responsibility to know of hackish solutions of mmapping particular devices on particular OS hardware or software platforms. That's a symptom of the disease, rather than a clean solution.
> > Or rather they likely shouldn't use gettimeofday, but clock_gettime() > with CLOCK_MONOTONIC instead to be independent of someone setting the > clock back.
Turns out we already have code to handle the turn back case, but monotonically increasing time is generally appreciated ;-).
> > Memory mapped counters are generally not flexible enough and there > are lots of reasons why the kernel might need to do special things > for time keeping. Don't expose them.
Yup. I agree entirely.
> > -Andi -- Jim Gettys One Laptop Per Child
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