Re: [PATCH] x86: provide a DMI based port 0x80 I/O delay override.

[Rene Herman]

On 01-01-08 22:15, H. Peter Anvin wrote:

I have mentioned this before... I think writing zero to port 0xf0 would 
be an acceptable pause interface (to the extent where we need an I/O 
port) except on 386 with 387 present; on those systems we can fall back 
to 0x80.
PII 400 / Intel 440 BX (PIIX4):

[EMAIL PROTECTED]:~/port80$ su -c ./portime
out 0x80: 544 cycles
in  0x80: 254 cycles
in  0x61: 254 cycles
out 0xf0: 544 cycles

The Intel PIIX/PIIX3 datasheet specifically mentions that both reads and 
writes at 0xf0 "flow through to the ISA bus".
However, more complete, it says:

"Writing to this register causes the PIIX/PIIX3 to assert IGNNE#. The 
PIIX/PIIX3 also negates IRQ13 (internal to the PIIX). Note that IGNNE# is 
not asserted unless FERR# is active. Reads/writes flow through to the ISA bus".
We don't want the side-effects, do we?

Rene.
/* gcc -W -Wall -O2 -o portime portime.c */

#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>

#include <sys/io.h>

#define LOOPS 10000

inline uint64_t rdtsc(void)
{
        uint32_t hi, lo;

        asm ("rdtsc": "=d" (hi), "=a" (lo));

        return (uint64_t)hi << 32 | lo;
}

inline void serialize(void)
{
        asm ("cpuid": : : "eax", "ebx", "ecx", "edx");
}

int main(void)
{
        uint64_t tsc0, tsc1, tsc2, tsc3, tsc4, tsc5;
        uint64_t out80, in80, in61, outf0;
        int i;

        if (iopl(3) < 0) {
                perror("iopl");
                return EXIT_FAILURE;
        }

        asm ("cli");
        tsc0 = rdtsc();
        for (i = 0; i < LOOPS; i++) {
                serialize();    
                serialize();
        }
        tsc1 = rdtsc();
        for (i = 0; i < LOOPS; i++) {
                serialize();
                asm ("outb %al, $0x80");
                serialize();
        }
        tsc2 = rdtsc();
        for (i = 0; i < LOOPS; i++) {
                serialize();
                asm ("inb $0x80, %%al": : : "al");
                serialize();
        }
        tsc3 = rdtsc();
        for (i = 0; i < LOOPS; i++) {
                serialize();
                asm ("inb $0x61, %%al": : : "al");
                serialize();
        }
        tsc4 = rdtsc();
        for (i = 0; i < LOOPS; i++) {
                serialize();
                asm ("outb %b0, $0xf0": : "a" (0));
                serialize();
        }
        tsc5 = rdtsc();
        asm ("sti");

        out80 = ((tsc2 - tsc1) - (tsc1 - tsc0)) / LOOPS;
        in80  = ((tsc3 - tsc2) - (tsc1 - tsc0)) / LOOPS;
        in61  = ((tsc4 - tsc3) - (tsc1 - tsc0)) / LOOPS;
        outf0 = ((tsc5 - tsc4) - (tsc1 - tsc0)) / LOOPS;

        printf("out 0x80: %llu cycles\n", out80);
        printf("in  0x80: %llu cycles\n", in80);
        printf("in  0x61: %llu cycles\n", in61);
        printf("out 0xf0: %llu cycles\n", outf0);

        return EXIT_SUCCESS;
}