Messages in this thread |  | | | Date | Tue, 11 Dec 2007 14:16:01 +0100 | | From | Rene Herman <> | | Subject | 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; } |  |