[BUG] x86_64 pci_map_sg modifies sg list - fails multiple map/unmaps

["Justin T. Gibbs" <gibbs@scsiguy.com>]

Berkley Shands recently tripped over this problem.  The 2.6.X pci_map_sg
code for x86_64 modifies the passed in S/G list to compact it for mapping
by the GART.  This modification is not reversed when pci_unmap_sg is
called.  In the case of a retried SCSI command, this causes any attempt
to map the command a second time to fail with a BUG assertion since the
nseg parameter passed into the second map call is state.  nseg comes from
the "use_sg" field in the SCSI command structure which is never touched
by the HBA drivers invoking pci_map_sg.

DMA-API.txt doesn't seem to cover this issue.  Should the low-level DMA
code restore the S/G list to its original state on unmap or should the
SCSI HBA drivers be changed to update "use_sg" with the segment count
reported by the pci_map_sg() API?  If the latter, this seems to contradict
the mandate in DMA-API that the nseg parameter passed into the unmap call
be the same as that passed into the map call.  Most of the kernel assumes
that an S/G list can be mapped an unmapped multiple times using the same
arguments.  This doesn't seem to me to be an unreasonable expectation.

--
Justin