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

[David Newall <david@davidnewall.com>]

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?

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.

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