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

David Lang
2006-07-25T23:30:17Z

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