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

Neil Horman
2006-07-25T18:28:52Z

On Tue, Jul 25, 2006 at 07:57:30PM +0200, Segher Boessenkool wrote:
> > 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.
>
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.

Neil

>
> Segher

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*Neil Horman
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