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Date:	Fri, 07 Dec 2007 08:34:04 +0100
From:	Rene Herman <>
To:	"David P. Reed" <>
CC:	Robert Hancock <>,,
	Thomas Gleixner <>,
	Ingo Molnar <>,
	"H. Peter Anvin" <>
Subject: Re: RFC: outb 0x80 in inb_p, outb_p harmful on some modern AMD64
 with MCP51 laptops

On 07-12-07 08:17, Rene Herman wrote:

> On 07-12-07 06:54, David P. Reed wrote:
>> Pardon my ignorance, but is port 0xed really safe to push an out cycle 
>> at across the entire x86_64 family?
> Please do not top-post. Who knows, probably not. You just experienced 
> that 0x80 is apparently not safe for you and that one's the conventional 
> choice so it's likely that someone somewhere will have problems with 
> 0xed as well.
> That's why I adviced you'd test and see what blows up and suggested that 
> in the absence of better fixes a 0x80/0xed port choice might be able to 
> hang off machine types as retrievable from DMI or something.
> The better fix would probably be to simply udelay(1) but you need 
> calibrated timers before you can do that and some googling leads me to 
> believe that's why it's not today. There's also a possible issue in that 
> an I/O access might serve as a method of flushing forwarding buffers on 
> a PCI bridge but I have no idea if that's a real issue (and if it is, an 
> inb() should suffice as well).
>> How long must real _p pauses be in reality?
> 8 ISA bus cycles is the intended delay it seems which at a typical ISA 
> bus speed of 8 MHz amounts to 1 us.
>> (and who cares about what the code calls "really slow i/o").
> Paranoid programmers and those that need to delay for 4 us.
>> Why are we waiting at all?  I read the comments in io_64.h, and am a 
>> bit mystified.  Does Windoze or DOS do this magical mystery wait?
> The CMOS example at hand is the standard example. It's accessed through 
> an index/data register pair. You need to be sure that the RTC has 
> switched the  correct internal register to the data register before you 
> poke at it or you may read/write the wrong one.
> Now, as said, I can't say I've ever in fact caught _any_ piece of 
> hardware with its pants down like that and needing this for actual 
> RTC/CMOS could as far as I'm aware be more of a left-over from The Olden 
> Days with a bus more or less directly tied to the 8086 than sensible for 
> anything on which Linux could run. Hard to test though and certainly for 
> generic outb_p use.
> Yes, DOS or at least many programs that ran under it did very similar 
> things but DOS ofcourse originated on those first PCs.
>> Anyway, the virtualization hooks in 32-bit x86 almost make it possible 
>> to isolate this simply - maybe - after the merge of 32/64 being 
>> contemplated.
>> And anyone who knows what the chipset might be doing with the 80 port 
>> rather than POST codes, perhaps could contribute.  Any nvidia folks 
>> who know what's happening under NDA?  Any Phoenix BIOS types?
> It's fairly surprising that 0x80 is given you trouble. It's a very well 
> known legacy port. Even though it may not be all that sensible a thing 
> today I'd say that if your machine put anything other than an actual 
> integrated POST monitor on port 0x80 it in fact fucked up.

This is a good thread to read:

maybe you have some LPC device that gets confused by aborts on the bus as well.

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