Further to the thread titled: "evaluation of scsi_cmnd->resid"
started by Andreas Herrmann on 2005/4/13, the attachment
adds resid handling to the sd driver. The patch is against
lk 2.6.12-rc4 .

It treats a "short" DMA read operation in a similar fashion
to a MEDIUM ERROR. This way the block layer gets as much
valid data as is available. Pathological double errors
are not handled (e.g. a MEDIUM ERROR on a latter sector
in a multi sector read plus a resid that indicates even
less data was read (than the MEDIUM ERROR indicates)).

The resid field in the scsi_cmnd structure is initialized
to zero so older LLDs that don't set it will not trip up
the proposed new code.
[resid = requested_xfer_len - actual_xfer_len]

While testing this patch I noticed MEDIUM ERRORs and
resid>0 on READs (since it mimics the former) do not
cause errors to appear in the log. Mid level logging
does show what is happening but it is not very
practical to have that on for very long. Reporting
and counting these occurrences may be a good idea.


ChangeLog:
   - in the sd driver check if the LLD reports an
     abridged DMA transfer on READ operations.
     If so process in a similar fashion to a
     MEDIUM ERROR condition.


Signed-off-by: Douglas Gilbert <dougg@torque.net>