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

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Date:	Tue, 25 Jul 2006 15:07:47 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Segher Boessenkool <segher@...nel.crashing.org>
Cc:	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, Jul 25, 2006 at 08:56:14PM +0200, Segher Boessenkool wrote:
> >>Similar functionality is already available via VDSO on
> >>platforms that support it (currently PowerPC and AMD64?) --
> >>seems like a better way forward.
> >>
> >In general I agree, but that only works if you operate on a  
> >platform that
> >supports virtual syscalls, and has vdso configured.
> 
> That's why I said "a better way forward", not "this already
> works everywhere".
> 
> >I'm not overly familiar
> >with vdso, but I didn't think vdso could be supported on all  
> >platforms/arches.
> 
> Oh?  Which can not, and why?
> 
I'm sorry, I shouldn't say that vdso itself can't be supported, but rather a
vsyscall that doesn't just wind up trapping into the kernel anyway.  Older
systems without a hpet timer to map into user space jump immediately to mind.
Arjan had mentioned a calibration on rdtsc as another alternative, which I had
not considered, so this may all be moot, but I was worried that a vdso solution
wouldn't always give the X guys what they were really after.

Regards
Neil

> 
> Segher

-- 
/***************************************************
 *Neil Horman
 *Software Engineer
 *gpg keyid: 1024D / 0x92A74FA1 - http://pgp.mit.edu
 ***************************************************/
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