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

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Message-ID: <20060725192138.GI4608@hmsreliant.homelinux.net>
Date:	Tue, 25 Jul 2006 15:21:38 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	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, Jul 25, 2006 at 12:10:09PM -0700, H. Peter Anvin wrote:
> Neil Horman wrote:
> >
> >In general I agree, but that only works if you operate on a platform that
> >supports virtual syscalls, and has vdso configured.  I'm not overly 
> >familiar
> >with vdso, but I didn't think vdso could be supported on all 
> >platforms/arches.
> >This seems like it might be a nice addition in those cases.
> >
> 
> Not really.  This introduces a potentially very difficult support 
> user-visible interface.  Consider a tickless kernel -- you might end up 
> taking tick interrupts ONLY to update this page, since you don't have 
> any way of knowing when userspace wants to look at it.
> 
Well, you do actually know when they want to look at it.  The rtc driver only
unmasks its interrupt when a user space process has opened the device and sent
it a RTC_UIE ON or RTC_PIE_ON (or other shuch ioctl).  So if you open /dev/rtc,
and memory map the page, but never enable a timer method, then every read of the
page returns zero.  The only overhead this patch is currently adding, execution
time-wise is the extra time it takes to write to a the shared page variable.  If
the timer tick interrupt is executing, its because someone is reading tick data,
or plans to very soon.

Neil

> 	-hpa

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