Messages in this thread | | | From | Segher Boessenkool <> | Subject | Re: [PATCH] RTC: Add mmap method to rtc character driver | Date | Tue, 25 Jul 2006 21:31:32 +0200 |
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>> 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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