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

Neil Horman
2006-07-25T19:08:10Z

On Tue, Jul 25, 2006 at 08:56:14PM +0200, Segher Boessenkool wrote:
> >>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.
>
> That's why I said "a better way forward", not "this already
> works everywhere".
>
> >I'm not overly familiar
> >with vdso, but I didn't think vdso could be supported on all
> >platforms/arches.
>
> Oh? Which can not, and why?
>
I'm sorry, I shouldn't say that vdso itself can't be supported, but rather a
vsyscall that doesn't just wind up trapping into the kernel anyway. Older
systems without a hpet timer to map into user space jump immediately to mind.
Arjan had mentioned a calibration on rdtsc as another alternative, which I had
not considered, so this may all be moot, but I was worried that a vdso solution
wouldn't always give the X guys what they were really after.

Regards
Neil

>
> Segher

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*Neil Horman
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