sys/i386/i386/microtime.s

/*-
 * Copyright (c) 1993 The Regents of the University of California.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      from: Steve McCanne's microtime code
 *      $Id$
 */

#include "asm.h"
#include "../isa/isa.h"
#include "../isa/timerreg.h"

/*
 * Use a higher resolution version of microtime if HZ is not
 * overridden (i.e. it is 100Hz).
 */
#ifndef HZ
ENTRY(microtime)
        pushl %edi
        pushl %esi
        pushl %ebx

        movl $_time,%ebx

        cli                             # disable interrupts

        movl    (%ebx),%edi             # sec = time.tv_sec
        movl    4(%ebx),%esi            # usec = time.tv_usec

        movl    $(TIMER_SEL0|TIMER_LATCH),%eax
        outb    %al,$TIMER_MODE         # latch timer 0's counter

        #
        # Read counter value into ebx, LSB first
        #
        inb     $TIMER_CNTR0,%al
        movzbl  %al,%ebx
        inb     $TIMER_CNTR0,%al
        movzbl  %al,%eax
        sall    $8,%eax
        orl     %eax,%ebx

        #
        # Now check for counter overflow.  This is tricky because the
        # timer chip doesn't let us atomically read the current counter
        # value and the output state (i.e., overflow state).  We have
        # to read the ICU interrupt request register (IRR) to see if the
        # overflow has occured.  Because we lack atomicity, we use
        # the (very accurate) heuristic that we only check for
        # overflow if the value read is close to the interrupt period.
        # E.g., if we just checked the IRR, we might read a non-overflowing
        # value close to 0, experience overflow, then read this overflow
        # from the IRR, and mistakenly add a correction to the "close
        # to zero" value.
        #
        # We compare the counter value to heuristic constant 11890.
        # If the counter value is less than this, we assume the counter
        # didn't overflow between disabling interrupts above and latching
        # the counter value.  For example, we assume that the above 10 or so
        # instructions take less than 11932 - 11890 = 42 microseconds to
        # execute.
        #
        # Otherwise, the counter might have overflowed.  We check for this
        # condition by reading the interrupt request register out of the ICU.
        # If it overflowed, we add in one clock period.
        #
        # The heuristic is "very accurate" because it works 100% if
        # we're called from an ipl less than the clock.  Otherwise,
        # it might not work.  Currently, only gettimeofday and bpf
        # call microtime so it's not a problem.
        #
        cmpl    $11890,%ebx
        jle     2f
        movl    $0x0a,%eax      # tell ICU we want IRR
        outb    %al,$IO_ICU1

        inb     $IO_ICU1,%al    # read IRR in ICU
        testb   $1,%al          # is a timer interrupt pending?
        je      1f
        addl    $-11932,%ebx    # yes, subtract one clock period
1:
        movl    $0x0b,%eax      # tell ICU we want ISR
        outb    %al,$IO_ICU1    #   (rest of kernel expects this)
2:
        sti                     # enable interrupts

        movl    $11932,%eax     # subtract counter value from 11932 since
        subl    %ebx,%eax       #   it is a count-down value
        imull   $1000,%eax,%eax
        movl    $0,%edx         # zero extend eax for div
        movl    $1193,%ecx
        idivl   %ecx            # convert to usecs: mult by 1000/1193

        addl    %eax,%esi       # add counter usecs to time.tv_usec
        cmpl    $1000000,%esi   # carry in timeval?
        jl      3f
        subl    $1000000,%esi   # adjust usec
        incl    %edi            # bump sec
3:
        movl    16(%esp),%ecx   # load timeval pointer arg
        movl    %edi,(%ecx)     # tvp->tv_sec = sec
        movl    %esi,4(%ecx)    # tvp->tv_usec = usec

        popl    %ebx            # restore regs
        popl    %esi
        popl    %edi
        ret
#endif