Interesting anomoly with a 2940UW

[Doug Ledford]

--------
> >Followed by a bunch more timeouts, then eventually a bus reset, then things 
> >went back to normal.  I kind of liked seeing the qoutcnt = 17, that 
> >shouldn't ever happen since I'm running with the spin lock.
> 
> Does your spin lock completely empty the FIFO before setting the sequencer
> variable to 0?  It doesn't sound like it.  The 17 could simply be from
> overflowing the QOUTFIFO as, although the documentation claims insertions
> when full are a no-op, I'm not sure I believe it.

Actually, the way I have the locking code in the kernel, there is a small 
chance of an overflow, although I haven't worked out exactly yet how it 
would occur.  Instead of specifically setting the CMDOUTCNT to 0, I do the 
following:

while ( qoutcnt > 0 )
{
  for(i=0; i<qoutcnt; i++)
  {
    do our stuff here
  }
  if (p->flags & PAGE_ENABLED)
  {
    pause_sequencer(p);  // In case we ever have paging on 2{7,8}4x class 
cards
    outb(qoutcnt - i, p->base + CMDOUTCNT); //In most cases, this should be 0
    unpause_sequencer(p, FALSE);  // Do this before checking qoutcnt again
  }                               // so we might catch a blocked command
  outb(CLRCMDINT, p->base + CLRINT);
  interrupt_cleared++;
  qoutcnt = inb(p->base + QOUTCNT) & p->qcntmask;
}

This way, there is no possibility that as we unblock the sequencer in its 
spin lock that it could write to the QOUTFIFO (although it also couldn't if 
we didn't unpause it) and so that there is a good chance that if it is spin 
locking, we might be able to catch the interrupt and the command on this 
iteration of the while loop.  However, although I don't see it anywhere, I 
guess there is a possibility that we could get into the for test with 
qoutcnt == 0, in which case the post increment to i would cause qoutcnt - i 
== 255 and allow the sequencer to overrun the QOUTFIFO the next time it had 
a lot of commands, but it would have to do so before the next interrupt 
reset the CMDOUTCNT variable.
-- 
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