Re: [linux-kernel] Re: [PATCH] x86: provide a DMI based port 0x80I/O delay override.
From: David P. Reed
Date: Tue Jan 08 2008 - 13:45:40 EST
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.
Windows XP can run on a machine with ISA slot(s) and has built-in drivers for
some plug&play ISA cards - e.g. the famous 3Com EtherLink III. I think that
there's a driver for NE2000-compatible cards too and it probably works.
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.
I wrote DOS drivers for NE2000's on the ISA bus when they were brand new
designs from Novell without such kludges as writes to I/O port 80. I
don't remember writing a driver for the 3com devices - probably didn't,
because 3Com's cards were expensive at the time.
In any case, Linux *did* adopt this port 80 strategy - I'm sure all
concerned thought it was frightfully clever at the time. Linus
expressed his skepticism in the comments in io.h. The problem is to
safely move away from it toward a proper strategy that doesn't depend on
"bus aborts" which would trigger machine checks if they were properly
enabled.
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