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

[Segher Boessenkool <segher@kernel.crashing.org>]

>> Not really.  This introduces a potentially very difficult support
>> user-visible interface.  Consider a tickless kernel -- you might  
>> end up
>> taking tick interrupts ONLY to update this page, since you don't have
>> any way of knowing when userspace wants to look at it.
>>
> Well, you do actually know when they want to look at it.  The rtc  
> driver only
> unmasks its interrupt when a user space process has opened the  
> device and sent
> it a RTC_UIE ON or RTC_PIE_ON (or other shuch ioctl).  So if you  
> open /dev/rtc,
> and memory map the page, but never enable a timer method, then  
> every read of the
> page returns zero.  The only overhead this patch is currently  
> adding, execution
> time-wise is the extra time it takes to write to a the shared page  
> variable.  If
> the timer tick interrupt is executing, its because someone is  
> reading tick data,
> or plans to very soon.

But userland cannot know if there is a more efficient option to
use than this /dev/rtc way, without using VDSO/vsyscall.


Segher