some scanner driver docs

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diff --git a/sys/scsi/README.ss b/sys/scsi/README.ss
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+$OpenBSD: README.ss,v 1.1 1997/03/08 21:21:37 kstailey Exp $
+
+If you think SCSI tape drives are quirky you haven't seen anything.
+
+There are many SCSI scanners that don't use the SCSI Scanner protocol
+with CDB's like SET_WINDOW.  Instead they send Esc-code sequences over
+the SCSI bus using the READ and WRITE operations.  True brain death at
+its finest.  The HP ScanJet, the Sharp JX-600S and the Epson ES-300C
+are among these.
+
+Ricoh, UMAX, Mustek, Fujitsu, Microtek on the other hand all try to
+use the SCSI scanner protocol, but scanning, unlike reading a block
+from a disk, is a multi-step operation.  Certain steps must be
+performed in sequence and the protocol does not define this.  In
+addition vendors are permitted by the SCSI spec. to have unique
+additional features that the spec. does not fully define.  Last but not
+least vendors make mistakes in implementing the spec.
+
+My SCSI scanner driver architecture is designed to support scanners
+two ways.
+
+The first way is used if a scanner uses CDB's like SET_WINDOW. The
+driver code should be in ss.c and quirk tables and sequence strings
+etc. can battle it out.  This part is not fully implemented yet.
+Work is being done on it from time to time.
+
+The other way is used when the driver is used with an
+Esc-code-over-SCSI case like a ScanJet it installs an "operations
+switch" so that parts of the code in ss.c can be bypassed.  This
+feature is implemented for ScanJets.  Currently some Mustek scanners
+use this approach, as the Mustek scanners use MODE_SELECT and not
+SET_WINDOW to send parameter data.  However it is possible that too
+much code was farmed out to ss_mustek.c; it may be that common code
+for ssread() in ss.c could be used.
+