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
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