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

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Message-Id: <1153861094.1230.20.camel@localhost.localdomain>
Date:	Tue, 25 Jul 2006 16:58:14 -0400
From:	Jim Gettys <jg@...top.org>
To:	Neil Horman <nhorman@...driver.com>
Cc:	"H. Peter Anvin" <hpa@...or.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

On Tue, 2006-07-25 at 16:47 -0400, Neil Horman wrote:

> > 
> > The i386 vdso right now has only two entry points, as far as I can
> tell: 
> > system call and signal return.
> > 
> > There is no reason it couldn't have more than that.  A
> low-resolution 
> > and a high-resolution gettimeofday might be a good idea.
> > 
> >       -hpa
> 
> Agreed.  How about we take the /dev/rtc patch now (since its an added
> feature
> that doesn't hurt anything if its not used, as far as tickless kernels
> go), and
> I'll start working on doing gettimeofday in vdso for arches other than
> x86_64.
> That will give the X guys what they wanted until such time until all
> the other
> arches have a gettimeofday alternative that doesn't require kernel
> traps.
> 

Some of us want/need both tickless and smart scheduling in the X server.

To give a bit of data: on my machine, a 1x1 rectangle can go almost 2
million rectangles/second; a 500x500 rectangle is only 2800/second.  You
can see the tremendous variation (and this is on accelerated hardware;
the variation can in fact be much larger than this, if the operation has
to be done in software fall-backs). 

This is why the X server needs to know the time so much, so cheaply; we
have to be able to tell how much time a given client has been using, and
it can't be computed from anything but the time; otherwise individual
clients can "starve" other clients, and interactive feel goes to pot.
                               - Jim

-- 
Jim Gettys
One Laptop Per Child


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