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

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Date:	Tue, 25 Jul 2006 16:29:27 -0700 (PDT)
From:	David Lang <dlang@...italinsight.com>
To:	Neil Horman <nhorman@...driver.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, 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
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