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

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Message-ID: <44C6A7B7.8010604@zytor.com>
Date:	Tue, 25 Jul 2006 16:22:31 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Neil Horman <nhorman@...driver.com>
CC:	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

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.
> 

The reason it is a hack is because you're hard-coding the fact that 
you're taking a global, periodic interrupt.  Yes, it can be dealt with 
scheduler hacks in tickless case, but that seems really heavyweight.

	-hpa
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