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

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Date:	Tue, 25 Jul 2006 20:18:15 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	David Lang <dlang@...italinsight.com>
Cc:	"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

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.
Neil

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