Re: RFC: outb 0x80 in inb_p, outb_p harmful on some modern AMD64 with MCP51 laptops

Rene Herman
2007-12-11T13:18:39Z

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On 11-12-07 13:08, David Newall wrote:

> Rene Herman wrote:
>> On 11-12-07 08:40, Paul Rolland wrote:
>>
>>> Well, if the delay is so much unspecified, what about _reading_ port
>>> 0x80 ?
>>> Will the delay be shorter ?
>>
>> The delay is completely and fully specified in terms of the ISA/LPC clock
>
> That would be the delay on the i386 (sic) architecture. In general,
> though, the delay is:

This particular discussion isn't about anything in general but solely about
the delay an outb_p gives you on x86 since what is under discussion is not
using an output to port 0x80 on that platform to generate it.

> Thinking that _p gives a pause is perhaps too PC-centric. Why, if a delay
> is needed, wouldn't you use a real delay; one that says how long it
> should be?

Because any possible outb_p delay should be synced to the bus-clock, not to
any wall-clock. Drivers that want to sync to wall-clock need to use an outb,
delay pair as you'd expect.

In the real world, driver authors aren't perfect and will have used outb_p
as a wall-clock delay which they have gotten away with since it's a nicely
specified delay in terms of the ISA/LPC clock and the ISA/LPC clock being
fairly (old) to very (new) constant.

The delay it gives is very close to 1 us on a spec ISA/LPC bus (*) and as
such, even though it may not be the right thing to do from an theoretical
standpoint, generally a udelay(1) is going to be a fine replacement from a
practical one -- as soon as we _can_ use udelay(), as I also wrote.

Rene.

(*) some local testing shows it to be almost exactly that for both out and
in on my own PC -- a little over. If anyone cares, see attached little test
program. The "little over" I don't worry about. 0 us delay is also fine for
me and if any code was _that_ fragile it would have broken long ago.

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

#include <sys/io.h>

int main(void)
{
unsigned long cycles;

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

asm ( "cli \n\t"
"rdtsc \n\t"
"movl %%eax, %%ecx \n\t"
"outb %%al, $0x80 \n\t"
"rdtsc \n\t"
"subl %%ecx, %%eax \n\t"
"sti "

: "=a" (cycles) : : "ecx", "edx");

printf("out = %lu\n", cycles);

asm ( "cli \n\t"
"rdtsc \n\t"
"movl %%eax, %%ecx \n\t"
"inb $0x80, %%al \n\t"
"rdtsc \n\t"
"subl %%ecx, %%eax \n\t"
"sti "

: "=a" (cycles) : : "ecx", "edx");

printf("in = %lu\n", cycles);

return EXIT_FAILURE;
}