commands

The attached represents an attempt to break the scsi mid-layer of the 
assumption that any device can queue at least one command.

What essentially happens if the host rejects a command with no other 
outstanding commands, it does a very crude countdown (basically counts the 
number of cycles through the scsi request function) until the device gets 
enabled again when the count reaches zero.  I think the iteration in the 
request function is better than a fixed timer because it makes the system more 
responsive to I/O pressure (and also, it's easier to code).

I've tested this by making a SCSI driver artificially reject commands with 
none outstanding (and run it on my root device).  A value of seven seems to 
cause a delay of between half and five seconds before the host starts up again 
(depending on the I/O load).

If this approach looks acceptable, I plan the following enhancements

1. Make device_busy count down in the same fashion
2. give ->queuecommand() a two value return (one for blocking the entire host 
and another for just blocking the device).
3. Make the countdown tuneable from the host template.