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SubjectRe: setup.S: A20 enable sequence (once again)
Followup to:  <00110618083400.11022@rob>
By author: Robert Kaiser <>
In newsgroup:
> Hi all,
> a while ago, I posted a request here to add the "fast A20" enable
> sequence to setup.S in order to enable booting Linux on boards that
> don't have a keyboard controller. I was happy to see that this has
> been added into 2.4.0-test10 already. However, on some embedded
> boards, booting 2.4.0-test10 does work, but it takes several minutes
> which the board spends in complete silence (it took me almost a day
> to realize that I only had to wait long enough for the system to
> boot ...)
> The reason for this is that the A20 enable sequence in setup.S is still
> accessing the (in my case not existing) keyboard controller. It times out
> eventually, but the timeout takes *very* long, especially on a slowish
> embedded 386EX@25MHz.
> The attached patch fixes this by doing "fast A20" enable first and
> then checking if A20 already is enabled. If it is, the keyboard
> controller sequence is skipped. This works for me, so, could people
> please have a look at this.

This doesn't really work. Neither the fast A20 gate nor the KBC is
guaranteed to have immediate effect (on most systems they won't.)

What's worse, once you have done an "out" to the KBC you need to
finish the sequence. I need to think about this for a bit.

(Arguably, what you're doing is running on completely nonstandard
hardware, which may need a CONFIG_ option. However, if we can avoid
it I guess it's better.)


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