[sane-devel] FreeBSD and Microtek Scanmaker II

Matto Marjanovic maddog@mir.com
Sat, 21 Jun 2003 08:53:58 -0400

 >> What does the patch do?  (And how did it fix up the Microtek scanning?)
 >depending on the status returned by the OS. The sense handler is only
 >called, if ccb->ccb_h.status & CAM_AUTOSNS_VALID is true, which should
 >guarantee that useful sense data is available.
 >Without the patch, the sense handler of backend is called for every
 >failing command, but because no sense data is available, the sense
 >handler decides to return SANE_STATUS_GOOD, hence the backend makes
 >wrong assuptions about the status of the scanner. 
 >What I don't really understand in hindsight is the CAM status value 16
 >in Martin's log data. It indicates that the FreeBSD CAM system tried to
 >issue a REQUEST SENSE for one or another failing command (where
 >"failing" means in this case that the scanner accepted the command, and
 >returned CHECK CONDITION), and that the REQUEST SENSE was not

A-ha --- ok, the sense handler is not called, an error condition is returned,
 and then - I imagine - the retry logic in the backend takes over.

With regards to the mystery status value:  in this situation under Linux
 (i.e., the scanner is busy, won't accept next command), a REQUEST SENSE
 *is* issued and the scanner *does* respond to it.  However, the returned
 data does not conform to SCSI-2.  In particular, the bit defined by
 Microtek to mean "this is valid sense data" is the bit which, in SCSI-2,
 means "this is *not* valid sense data".
Because of this, the mid-level Linux SCSI driver *zeros out* the sense data,
 yet still indicates that REQUEST SENSE has been issued.  Thus the backend's
 sense handler is called with absolutely nothing and decides "No error".

I suspect a similar thing is happening in the FreeBSD CAM system --- some
 midlevel driver is unnecessarily killing the sense data.  Your patch checks
 to see if the system thought the sense data is/was valid, and since it was
 declared invalid, issues a general error instead.

It's just a theory... (I've never tried nor read FreeBSD code),
-matt m.