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From: Clemens Ladisch <clemens@ladisch.de>
Newsgroups: comp.os.linux.misc,comp.os.linux.hardware
Subject: Re: Realtime clock accessible without context-switch?
Date: Wed, 21 May 2014 19:32:46 +0200
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Charles T. Smith wrote:
> On Tue, 20 May 2014 21:56:48 +0200, Clemens Ladisch wrote:
>> Charles T. Smith wrote:
>>> Is there a way to access (say on Intel platforms) the realtime clock
>>> without using a special device driver, but simply with non-synchronized
>>> memory access?
>>
>> On IBM-PC compatible machines, the RTC lives in the good ol' ISA I/O
>> space, which cannot be memory-mapped.
>>
>> When doing ISA I/O accesses, the switch to kernel mode is _not_ the
>> bottleneck.  If you want to avoid it anyway, use iopl and inb/outb.
>
> Still these days?

If it didn't use I/O space, it wouldn't be compatible.

> But aren't there other clocks around?

Lots, but none of those store the _real_ time.

I'd suggest the High Precision Event Timer; see the example program below.

> Newer processors have various debug registers, I think.

Try rdtsc.

Or just forget about all that hardware-dependent hackery and use
clock_gettime().

> Your statement that the "switch to kernel mode is *not* the bottleneck"
> is intriguing.  Can you give a hint or a URL about what you mean?

An I/O access (about one microsecond) is much slower than the SYSCALL
instruction; and with the RTC, you'd need to read several registers.


Regards,
Clemens


--8<---------------------------------------------------------------->8--
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>

int main(void)
{
	int fd = open("/dev/hpet", O_RDONLY);
	if (fd == -1) {
		perror("/dev/hpet");
		return 1;
	}
	const volatile unsigned int *ptr = mmap(NULL, 4096, PROT_READ, MAP_SHARED, fd, 0);
	if (ptr == MAP_FAILED) {
		perror("mmap");
		return 1;
	}
	printf("frequency: %.5f MHz\n", 1e9 / ptr[1]);
	for (;;) {
		printf("\rcounter: %08x", ptr[60]);
		fflush(stdout);
		usleep(123456);
	}
}