AMD SC410 boot problems with recent kernels

[Robert Schwebel]

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Hi,

As I reported some days ago in

  http://marc.theaimsgroup.com/?l=linux-kernel&m=100876129529834&q=raw

there are boot problems with new kernels on AMD SC410 processors. Symptom
is that the machine reboots right in the middle of the initialisation of
the serial port (indeed, right in the middle of a printk message).

In the meantime I've tracked down the problem, but cannot fully find the
origin. Here are some facts:

- The problem came in in 2.4.15. Linus has merged in some changes to
  the boot code by H. Peter Anvin (which basically are a Good Thing(TM)).
  They affect arch/i386/boot/setup.S.

- I could narrow it down to the A20 gate routines. My machine's BIOS
  doesn't seem to have the appropiate routine, so the algorithm falls
  back to using the keyboard controller method (which was also used
  in the old code).

- The problem seems to come from the code that waits for A20 gate to
  be _really_ enabled (shortly after a20_kbc:).

Attached is a experimental patch which demonstrates the problem: in line
687 you can change with a jump to "old_wait" or "new_wait" which routine
shall be used. With the old one the machine starts, with the new one it
reboots.

I must say I do not really understand what the problem is. First I thought
that maybe the loop counter overruns, but it doesn't seem to happen. I
have written the counter value to a port with LEDs and it seems to contain
"1" when the waiting loop detects the successful A20 switch.

Any idea would be helpful...

Robert
--
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 | Dipl.-Ing. Robert Schwebel | http://www.pengutronix.de |
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