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

From: H. Peter Anvin
Date: Tue Jul 25 2006 - 19:21:17 EST

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.

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