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

["H. Peter Anvin" <hpa@zytor.com>]

Bodo Eggert wrote:
> Christer Weinigel <christer@weinigel.se> wrote:
> 
>> How do you find out the speed of the ISA bus?  AFAIK there is no
>> standardized way to do that.  On the Geode SC2200 the ISA bus speed is
>> usually the PCI clock divided by 4 giving 33MHz/4=8.3MHz or
>> 30/4=7.5MHz, but with no external ISA devices it's possible to
>> overclock the ISA bus to /3 to run it at 11MHz or so.  But without
>> poking at some CPU and southbridge specific registers to find out the
>> PCI bus speed and the ISA bus divisor you can't really tell.
> 
> If you overclock, you are on your own. IIRC I've used 13,3 MHz for some time
> and used a lower PIO mode to compensate.
> 
>> So if you do udelay based on a 6MHz clock (I think you can safely
>> assume that any 386 based system runs the ISA bus at least that fast)
>> you'll waste at least 30% and maybe even 100% more time for the delay
>> after every _p call.
> 
> Defaulting to 8 MHz and offering an option to set another clock speed
> (like idebus=) should be OK.
> 

The formalization of the ISA bus which was part of the EISA 
specification settled on 8.33 MHz maximum nominal frequency.  There 
were, however, some earlier designs which used up to 12 MHz nominal; I'm 
not sure if that applied to 386s though.

	-hpa