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

From: Neil Horman
Date: Tue Jul 25 2006 - 20:20:03 EST

On Tue, Jul 25, 2006 at 04:29:27PM -0700, David Lang wrote:
> On Tue, 25 Jul 2006, Neil Horman wrote:
> >>
> >>Quick hacks are frowned upon in the Linux universe. The kernel-user
> >>space interface is supposed to be stable, and thus a hack like this has
> >>to be maintained indefinitely.
> >>
> >>Putting temporary hacks like this in is not a good idea.
> >>
> >Only if you make the mental leap that this is a hack; its not. Its a new
> >feature for a driver. mmap on device drivers is a well known and
> >understood
> >interface. There is nothing hackish about it. And there is no need for
> >it to
> >be temporary either. Why shouldn't the rtc driver be able to export a
> >monotonic
> >counter via the mmap interface? mmtimer does it already, as do many other
> >drivers. Theres nothing unstable about this interface, and it need not be
> >short
> >lived. It can live in perpituity, and applications can choose to use it,
> >or
> >migrate away from it should something else more efficient become available
> >(a
> >gettimeofday vsyscall). More importantly, it can continue to be used in
> >those
> >situations where a vsyscall is not feasable, or simply maps to the nominal
> >slow
> >path kernel trap that one would find to heavy-weight to use in comparison
> >to an
> >mmaped page.
> given that this won't go into 2.6.18 at this point, isn't there time to
> figure out the gettimeofday vsyscall before the 2.6.19 merge window? (in a
> month or so). even if you have to wait until 2.6.20 it's unlikly that any
> apps could be released with an interface to /dev/rtc rather then waiting a
> little bit for the better interface.
> David Lang

My primary concern is my skill level. I normally work in the kernel, and I'm
not too familiar with glibc, and completely unfamiliar with vdso
implementations. I'm interested to do it, but I have no idea how long it will
take to understand vsyscall implementations, code one up, and get it right. If
you think a month is sufficient, I'll take your word for it, but I'm starting
from zero in this area.

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