timer counter chip access mystery]

[Petri Helenius]

This access happens over ISA bus and thus happens at the speed the bus
operates at. Use TSC or ACPI for faster gettimeofday.

Pete

----- Original Message ----- 
From: "Jin Guojun [DSD]" <j_guojun at lbl.gov>
To: <freebsd-performance at freebsd.org>
Sent: Thursday, July 17, 2003 12:37 AM
Subject: [Fwd: timer counter chip access mystery]


> i386/isa/clock.c, line 1207
> i8254_get_timecount(...)
> {
>         outb(TIMER_MODE, TIMER_SEL0 | TIMER_LATCH);
> 
>         low = inb(TIMER_CNTR0);
>         high = inb(TIMER_CNTR0);
> ...
>         inb(IO_ICU1);
> return counter.
> }
> 
> This routine takes about 4000 ns. It makes gettimeofday()
> cost over 4000 ns.
> 
> measure_i8254_get_timecount: Two_TIMER_CNTR0 2362 ns
> measure_i8254_get_timecount: IO_ICU1 936 ns
> and outb() looks like to take another 700 ns.
> 
> The Linux uses the same process to get the time counter (see below).
> It also comments that this is from Steve McCanne's microtime-i386 for BSD.
> However, gettimeofday() under Linux costs 900 ns.
> See attached GIF file for a table of comparison.
> 
> Any idea why linux is 4 times faster than FreeBSD in
> reading the same time counter chip?
> 
>     -Jin
> 
> ----------------- Linux source code for the same routine -----------
> 
> arch/x86_64/kernel/time.c (line 68)
> void do_gettimeofday(struct timeval *tv)
> {
>         unsigned long flags, t;
>         unsigned int sec, usec;
> 
>         read_lock_irqsave(&xtime_lock, flags);
>         spin_lock(&time_offset_lock);
> 
>         sec = xtime.tv_sec;
>         usec = xtime.tv_usec;
> 
>         t = (jiffies - wall_jiffies) * (1000000L / HZ) + do_gettimeoffset();
>         if (t > timeoffset) timeoffset = t;
>         usec += timeoffset;
> 
>         spin_unlock(&time_offset_lock);
>         read_unlock_irqrestore(&xtime_lock, flags);
> 
>         tv->tv_sec = sec + usec / 1000000;
>         tv->tv_usec = usec % 1000000;
> }
> 
> arch/i386/kernel/time.c (line 127)
> 
> /* This function must be called with interrupts disabled
>  * It was inspired by Steve McCanne's microtime-i386 for BSD.  -- jrs
>  *
>  * However, the pc-audio speaker driver changes the divisor so that
>  * it gets interrupted rather more often - it loads 64 into the
>  * counter rather than 11932! This has an adverse impact on
>  * do_gettimeoffset() -- it stops working! What is also not
>  * good is that the interval that our timer function gets called
>  * is no longer 10.0002 ms, but 9.9767 ms. To get around this
>  * would require using a different timing source. Maybe someone
>  * could use the RTC - I know that this can interrupt at frequencies
>  * ranging from 8192Hz to 2Hz. If I had the energy, I'd somehow fix
>  * it so that at startup, the timer code in sched.c would select
>  * using either the RTC or the 8253 timer. The decision would be
>  * based on whether there was any other device around that needed
>  * to trample on the 8253. I'd set up the RTC to interrupt at 1024 Hz,
>  * and then do some jiggery to have a version of do_timer that
>  * advanced the clock by 1/1024 s. Every time that reached over 1/100
>  * of a second, then do all the old code. If the time was kept correct
>  * then do_gettimeoffset could just return 0 - there is no low order
>  * divider that can be accessed.
>  *
>  * Ideally, you would be able to use the RTC for the speaker driver,
>  * but it appears that the speaker driver really needs interrupt more
>  * often than every 120 us or so.
>  *
>  * Anyway, this needs more thought....          pjsg (1993-08-28)
>  *
>  * If you are really that interested, you should be reading
>  * comp.protocols.time.ntp!
>  */
> 
> static unsigned long do_slow_gettimeoffset(void)
> {
>         int count;
> 
>         static int count_p = LATCH;    /* for the first call after boot */
>         static unsigned long jiffies_p = 0;
> 
>         /*
>          * cache volatile jiffies temporarily; we have IRQs turned off.
>          */
>         unsigned long jiffies_t;
> 
>         /* gets recalled with irq locally disabled */
>         spin_lock(&i8253_lock);
>         /* timer count may underflow right here */
>         outb_p(0x00, 0x43);     /* latch the count ASAP */
> 
>         count = inb_p(0x40);    /* read the latched count */
> 
>         jiffies_t = jiffies;
> 
>         count |= inb_p(0x40) << 8;
> 
>         /* VIA686a test code... reset the latch if count > max + 1 */
>         if (count > LATCH) {
>                 outb_p(0x34, 0x43);
>                 outb_p(LATCH & 0xff, 0x40);
>                 outb(LATCH >> 8, 0x40);
>                 count = LATCH - 1;
>         }
> 
>         spin_unlock(&i8253_lock);
> ...
>         return count;
> }


--------------------------------------------------------------------------------


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