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

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Date:	Sun, 30 Jul 2006 01:18:15 +0200
From:	Edgar Toernig <froese@....de>
To:	Keith Packard <keithp@...thp.com>
Cc:	Neil Horman <nhorman@...driver.com>, keithp@...thp.com,
	Bill Huey <billh@...ppy.monkey.org>,
	Jim Gettys <jg@...top.org>, "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

Keith Packard wrote:
>
> A jiffy counter is sufficient for the X server; all I need is some
> indication that time has passed with a resolution of 10 to 20 ms. I
> check this after each X request is processed as that is the scheduling
> granularity. An X request can range in time from .1us to 100 seconds, so
> I really want to just check after each request rather than attempt some
> heuristic.

That's exactly what the mmap interface of /dev/itimer does.
See: http://marc.theaimsgroup.com/?m=115412412427996

Example code:

    volatile unsigned long *counter;
    ...
    fd=open("/dev/itimer", O_RDWR);
    write(fd, "20/1000\n", 8);
    counter = mmap(0, sizeof(*counter), PROT_READ, MAP_PRIVATE, fd, 0);
    close(fd);

Now, "*counter" is incremented every 20 ms by the kernel.

Ciao, ET.
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