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

From: Segher Boessenkool
Date: Tue Jul 25 2006 - 13:55:28 EST

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.


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at