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

["H. Peter Anvin" <hpa@zytor.com>]

David Newall wrote:
> H. Peter Anvin wrote:
> > David Newall wrote:
> > > Where did the 8us delay come from?  The documentation and source is 
> > > careful not to say how long the delay is.  Would changing it to, say 
> > > 1us, be technically wrong?  Is code that requires 8us correct?
> >
> > I think a single ISA bus transaction is 1 µs, so two of them back to 
> > back should be 2 µs, not 8 µs...
>
> Exactly.  You think it's 2us, but the documentation doesn't say.  The _p 
> functions are generic inasmuch as they provide an unspecified delay.  
> Drivers which work across platforms, and which use _p, therefore have 
> different delays on different platforms.  Should the length of the delay 
> be unimportant?  I wouldn't have thought so.  If it is important, does 
> that mean that such drivers are buggy on some platforms?

What it specifically does is it generates a delay which is proportional 
to the ISA/LPC clock.

> I really *hate* the idea that access to non-present hardware is used to 
> generate a delay.  That sucks so badly.  It's worthy of a school-aged 
> hacker, not of a world-leading operating system.  It's so not 
> best-practice that it's worst-practice.

Perhaps you do, but it's the de facto standard on the platform.  Every 
BIOS uses the same technique, because it works.

*Now*, the real question is how many drivers actually need these delays. 
 My guess is most don't at all.

	-hpa

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/