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

Segher Boessenkool
2006-07-25T19:31:42Z

>> 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