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

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Message-Id: <03BCDC7F-13D9-42FC-86FC-30C76FD3B3B8@kernel.crashing.org>
Date:	Tue, 25 Jul 2006 19:57:30 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Neil Horman <nhorman@...driver.com>
Cc:	linux-kernel@...r.kernel.org, a.zummo@...ertech.it,
	jg@...edesktop.org
Subject: Re: [PATCH] RTC: Add mmap method to rtc character driver

> 	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, one that they could perhaps read from a memory mapped  
> page.  I was
> right behind them and though that seemed like a reasonable  
> request,  so I've
> taken a stab at it.  This patch allows for a page to be mmaped  
> from /dev/rtc
> character interface, the first 4 bytes of which provide a regularly  
> increasing
> count, once every rtc interrupt.  The frequency is of course  
> controlled by the
> regular ioctls provided by the rtc driver. I've done some basic  
> testing on it,
> and it seems to work well.

Similar functionality is already available via VDSO on
platforms that support it (currently PowerPC and AMD64?) --
seems like a better way forward.


Segher

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