Interesting anomoly with a 2940UW

[Doug Ledford]

--------
> >To be quite honest, I think most of the races you have mentioned in 
> >relation to my usage of the CMDOUTCNT variable exist not only in my code, 
> >but in yours as well.
> 
> Nope.  Since I only drop the count to 0 once I'm sure that the sequencer
> has put fifodepth commands into the QOUTFIFO (as I have removed that many
> from the FIFO), I'm guaranteed that CMDOUTCNT is equal to fifodepth.  This
> means that the sequencer is either in the spin lock or executing before
> reaching the lock.  This ensures that when I set CMDOUTCNT to 0, the FIFO
> is truely empty and that the sequencer will increment CMDOUTCNT before it
> places another entry in the FIFO.

Ahh, well see, that's why I didn't do it that way.  In that case, we would 
be hitting the spin lock right and left and the sequencer would spend way 
too much time locked down.  Finding an acceptable solution to the isr 
handling the completion stuff would help, but it's tricky under linux.

> >Does that sound any more efficient (and race protected) to you?
> 
> It would be unacceptable to me to run with interrupts disabled for any
> length of time in a disk driver especially when it isn't necessary.

And in the future it may be possible to remove those interrupt off calls, 
but in the meantime, there were race conditions that needed something to 
protect them.  It's been that way since I started working on the driver 
anyway, and we are starting to iron out some areas where we can turn them 
back on, but we haven't gotten around to removing all possible race 
conditions so that they can be unilaterally on without having the potential 
for problems.  However, that is one of the things on my TODO list since it 
appears that the interrupt off situation can also cause problems with IRQ 
forwarding on SMP machines.  But still, that doesn't answer the question.  
Does that solution sound race protected and more efficient than the current 
one I use?

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