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

[Rene Herman <rene.herman@keyaccess.nl>]

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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):

rene@6bap:~/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.

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/* 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;
}