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

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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. 
#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;
}
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