Re: RFC: outb 0x80 in inb_p, outb_p harmful on some modern AMD64 with MCP51 laptops

["linux-os (Dick Johnson)" <linux-os@analogic.com>]

On Tue, 11 Dec 2007, David P. Reed wrote:

>
>
> Alan Cox wrote:
>>
>> The vga driver is somewhat misnamed. In console mode we handle everything
>> back to MDA/HGA and some HGA adapters do need delays.
>>
>>
> No they don't.   I really, really, really know this for a fact.  I wrote
> ASM drivers for every early video adapter and ran them all through Lotus
> QA and Software Arts QA.   Personally.  The only delay needed is caused
> by not having dual-ported video frame buffers on the original CGA in
> high res character mode. This caused "snow" when a memory write was done
> concurrently with the read being done by the scanline character
> generator. And that delay was done by waiting for a bit in the I/O port
> space to change.  There was NO reason to do waits between I/O
> instructions.  Produce a spec sheet, or even better a machine.   I may
> have an original PC-XT still lying around in the attic, don't know if I
> can fire it up, but it had such graphics cards.  I also have several
> early high-speed clones that were "overclocked".
>
>>> I do remind all that 0x80 is a BIOS-specific standard, and is per BIOS -
>>> other ports have been used in the history of the IBM PC family by some
>>> vendors, and some machines have used it for DMA port mapping!!  And
>>>
>>
>> All do -thats why it is suitable.
>>
> Not true.  Again, I can produce machines that don't use 0x80.  Perhaps
> that is because I am many years older than you are, and have been
> writing code for PC compatibles since 1981.  (not a typo - this was
> before the first IBM PC was released to the public).

Hmmm,
I didn't know you worked in Boca Raton for Don Estrage on
the IBM 5150. I must have missed you --somehow.

[Snipped...]

You do remember that the X86 can do back-to-back port
instructions faster than the ISA bus capacity can be
charged, don't you? You do remember the admonishment
about:
 	intel asm

 	mov	dx, port	; One of two adjacent ports
 	mov	al,ffh		; All bits set
 	out	dx,al		; Output to port, bits start charging bus
 	inc	al		; Al becomes 0
 	inc	dx		; Next port
 	out	dx,al		; Write 0 there, data bits discharged

When the port at 'port' gets its data, it will likely
be 0, not 0xff, because the intervening instructions
can execute faster than a heavily-loaded ISA bus.

So, with a true ISA/EISA bus, not an emulated one off
a bridge, you have to worry about this. In the IBM/PC
BIOS listing, supplied with every early real PC, it
was called "bus settle time." Remember? If not, you
never wrote code for that platform.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.22.1 on an i686 machine (5588.29 BogoMips).
My book : http://www.AbominableFirebug.com/
_


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