Message-ID: <477AD9EB.9050306@keyaccess.nl>
Date: Wed, 02 Jan 2008 01:25:15 +0100
From: Rene Herman <rene.herman@keyaccess.nl>
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To: "H. Peter Anvin" <hpa@zytor.com>
CC: Alan Cox <alan@lxorguk.ukuu.org.uk>, "David P. Reed" <dpreed@reed.com>,
       Ingo Molnar <mingo@elte.hu>, Paul Rolland <rol@as2917.net>,
       Pavel Machek <pavel@ucw.cz>, Thomas Gleixner <tglx@linutronix.de>,
       linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
       rol@witbe.net
Subject: Re: [PATCH] x86: provide a DMI based port 0x80 I/O delay override.
References: <4762C551.5070003@zytor.com>	<20071214210652.GB28793@elf.ucw.cz>	<4763001A.1070102@zytor.com>	<20071214232955.545ab809@the-village.bc.nu>	<20071215080831.404cdb32@tux.DEF.witbe.net>	<47638C8C.2090604@gmail.com>	<476438B4.2020600@zytor.com>	<476462BE.3030701@gmail.com>	<4764687D.6080609@zytor.com>	<476524DB.7020806@gmail.com>	<20071216152250.GA21245@elte.hu>	<4765D43E.1010800@gmail.com>	<4765D95C.4010404@zytor.com>	<4765DCB0.8030901@gmail.com>	<4765EE7F.80002@zytor.com>	<47667366.7010405@gmail.com>	<4766AE88.4080904@zytor.com>	<4766D175.7040807@reed.com>	<20071217212509.5edaa372@the-village.bc.nu>	<477A634C.8040000@reed.com> <20080101161557.3ce2d5f8@the-village.bc.nu> <477AAD7B.5040405@zytor.com> <477AB204.3070904@keyaccess.nl> <477AB433.5070506@zytor.com> <477AC02A.40108@keyaccess.nl> <477AC130.3050901@zytor.com> <477AC8BA.2050503@keyaccess.nl>
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On 02-01-08 00:11, Rene Herman wrote:

> On 01-01-08 23:39, H. Peter Anvin wrote:
> 
>>>> Yes, we do.  It's exactly this side effect which makes this safer 
>>>> than either 0x80 or 0xED -- it's a port that *guaranteed* can't be 
>>>> reclaimed for other purposes without breaking MS-DOS compatibility.
>>>
>>> I see that with CR0.NE set (*) we indeed don't care about IGNNE#...
>>>
>>> However, I'm worried about this comment in arch/x86/kernel/i8259_32.c
>>>
>>> ===
>>> /*
>>>  * New motherboards sometimes make IRQ 13 be a PCI interrupt,
>>>  * so allow interrupt sharing.
>>>  */
>>> ===
>>>
>>> Is it really safe to just blindly negate IRQ13 on everything out 
>>> there, from regular PC through funky embedded thingies?
>>
>> It's not any IRQ 13, it's IRQ 13 from the FPU.
> 
> Well, on the PIIX it is and I guess on anything where it's _not_ fully 
> internal an 0xf0 write wouldn't have any effect on IRQ13...
> 
> When you earlier mentioned this it seemed 0xed switched on DMI would be 
> good enough, but well.
> 
> Alan, do you have an opinion on the port 0xf0 write? It should probably 
> still be combined with a replacement/deletion for new machines due to 
> the bus-locking "bad for real-time" thing you mentioned earlier but in 
> the short run it could be a fairly low-impact replacement on anything 
> except a 386+387
> 
> We should do a another timing measurement survey and it makes for 
> sligtly worse code if we indeed feel it's not safe enough to write 
> anything other than 0, but otherwise it's quite minimal.

Thinking about this, my main worry about 0xf0 as a 0x80 replacement would be 
systems that have elected to _not_ let port 0xf0 writes flow through to ISA 
changing the timing-characteristics. Given that it's a known port, someone 
may have elected to just keep it fully internal.

Upto now the datasheets I've read do put it on ISA...

Rene.

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