Re: [linux-kernel] Re: [PATCH] x86: provide a DMI based port 0x80I/O delay override.

[Christer Weinigel]

On Tue, 08 Jan 2008 13:44:54 -0500
"David P. Reed" <dpreed@xxxxxxxx> wrote:

> Ondrej Zary wrote:
> > On Tuesday 08 January 2008 18:24:02 David P. Reed wrote:
> >   
> >> Windows these days does delays with timing loops or the
> >> scheduler.  It doesn't use a "port".  Also, Windows XP only
> >> supports machines that tend not to have timing problems that use
> >> delays.  Instead, if a device takes a while to respond, it has a
> >> "busy bit" in some port or memory slot that can be tested.
> >>     
> There is no need to use io writes to supposedly/theoretically "unused 
> ports" to make drivers work on any bus.
> ISA included!  You can, for example, wait for an ISA bus serial
> adapter to put out its next character by looping reading the port
> that has the output buffer full flag in a tight loop, with no delay
> code at all.  And if you need to time things, just call a timing loop
> subroutine that you calibrate at boot time.

Now you're totally confusing things.  You're talking about looking at
bits in a register to see if a transmit register is empty.  
That's easy.

The delays needed for the Intel M8259 and M8253 say that you're not
even allowed to access the registers _at_ _all_ for some time after a
register access.  If you do a write to a register immediately followed
by any access, including a read of the status register, you can corrupt
the state of the chip.

And the Intel chips are not the only ones with that kind of brain
damage.  But what makes the 8259 and 8253 a big problem is that every
modern PC has a descendant of those chips in them.  The discrete Intel
chips or clones got aggregated into Super I/O chips, and the Super I/O
chips were put on a LPC bus (an ISA bus with another name) or
integrated into the southbrige.  And the "if it ain't broken, don't fix
it" mantra probably means that some modern chipsets are still using
exactly the same internal design as the 25 year old chips and will
still be subject to some of those ancient limitations.

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