linux-kernel - Re: [PATCH] x86: provide a DMI based port 0x80 I/O delay override.

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Date:	Sun, 16 Dec 2007 20:09:29 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Rene Herman <rene.herman@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>, Paul Rolland <rol@...917.net>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Pavel Machek <pavel@....cz>, "David P. Reed" <dpreed@...d.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	rol@...be.net
Subject: Re: [PATCH] x86: provide a DMI based port 0x80 I/O delay override.

Rene Herman wrote:
> On 17-12-07 03:05, H. Peter Anvin wrote:
> 
>> Incidentally, I had the thought earlier today that port 0xf0 might be 
>> a suitable delay port.  It is used only by the 387-emulating-a-287 
>> hack for IRQ 13, which Linux doesn't use on 486+.
> 
> rene@...e4:~/src/port80$ su -c ./port80
> cycles: out 2400, in 2400
> rene@...e4:~/src/port80$ su -c ./portf0
> cycles: out 2400, in 2400
> 
> (Duron 1300)
> 
> I suppose you mean using it instead of port 0x80 always and not just as 
> an alternate port? For the latter 0xed is alright I guess...
> 

FWIW, the criterion used in the kernel for when to use IRQ 13 is:

         /*
          * External FPU? Set up irq13 if so, for
          * original braindamaged IBM FERR coupling.
          */
         if (boot_cpu_data.hard_math && !cpu_has_fpu)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                 setup_irq(FPU_IRQ, &fpu_irq);

In that case we can't actually use port 0xF0 (it is, however, safe to 
use it during setup, specifically before we can take our first FPU 
exception.)

	-hpa
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