basic domain validation patches

[mjacob at freebsd.org]

See http://people.freebsd.org/~mjacob/DV_PATCHES

The ANSI document on Domain Validation gives an algorithm for doing 
Basic Domain Validation as, in part:

  Send an INQUIRY command to a SCSI device 3 times- twice with the
  default transfer agreement and once with the transfer agreement set
  to the fastest supported values. The Basic test fails when the first
  36 bytes of data returned at the negotiated synchronous speed does not
  match the data received at the asynchronous transfer speed. In
  addition, the Basic test fails if a CRC error (or parity error for
  non-DT clocking) or a timeout occurs. If data miscompare occurs, the
  test should be repeated (e.g., this could be due to the target changing
  INQUIRY data during SCSI device initialization). After a finite number
  of retries, if data miscompare recurs then fall-back should be
  attempted (see 5.5.2).

and 5.5.2 has:

  If Basic or Enhanced tests fail, a fall-back setting is set and the
  tests are performed again. The recommended fall-back order is:

     1. fast-160
     2. fast-80
     3. fast-40 (with DT clocking enabled)
     4. fast-40 (with ST clocking enabled)
     5. fast-20
     6. fast-10; and
     7. asynchrnous transfer.

I've done an implementation interwoven into the probe code in cam_xpt 
such that the following occurs..

The normal sequence is:

     ...
     INQUIRY (short)
     INQUIRY (long)
     ...
     ('default' sync settings are enabled)
     ...
     PROBE_TUR_FOR_NEGOTIATION
     <END>

I've changed <END> to be:

     PROBE_TUR_FOR_NEGOTIATION
     PROBE_INQUIRY_BASIC_DV1
     ...
     PROBE_INQUIRY_BASIC_DV2
     ...

The idea here is to leverage the two initial INQUIRY commands done at 
async mode and compare with the last known one and to do *two* 
additional INQUIRY commands (at speed) to see if they compare with the 
last ASYNC long INQUIRY command. If they don't, we can do backoff and 
just re-enter the state machine at PROBE_TUR_FOR_NEGOTIATION.

The backoff doesn't do DT/ST settings, but just simply increments the 
sync_period factor until we hit 0xf (5 MHz) and then drops to async.

I don't mess with PPR settings on the theory that the SIM will do the 
right thing with PPR settings depending on the values of sync period and 
offset.

By doing fault injection (e..g, failed compares) I've done some testing 
and this *appears* to do what one would want.

Any comments? I *know* there are people out there on this list who have 
more experience with DV than I do so I'd appreciate some comments.

-matt