Debugging CAM_REQ_CMP_ERR errors / bulk transfer failures

[Shrikanth Kamath]

I have the following USB devices on a machine with FreeBSD 6.x base OS
and Freescale e500v2 PowerPC core

umass0: STMicroelectronics ST72682  High Speed Mode, rev 2.00/2.10, addr 2
umass0: SCSI over Bulk-Only; quirks = 0x0000
umass0:0:0:-1: Attached to scbus0

umass1: Unigen Corporation PQS4000B8-JNP, rev 2.00/11.00, addr 3
umass1: SCSI over Bulk-Only; quirks = 0x4000
umass1:1:1:-1: Attached to scbus1


Seeing the following errors and bulk transfer is failing to complete,
the failure is followed by a panic (panic: vinvalbuf: dirty bufs)

umass0: Invalid CSW: sig 0x9063c482 should be 0x53425355
(da0:umass-sim0:0:0:0): Request completed with CAM_REQ_CMP_ERR
(da0:umass-sim0:0:0:0): Retrying Command
(da0:umass-sim0:0:0:0): Request completed with CAM_REQ_CMP_ERR
(da0:umass-sim0:0:0:0): Retrying Command
(da0:umass-sim0:0:0:0): Request completed with CAM_REQ_CMP_ERR
(da0:umass-sim0:0:0:0): error 5
(da0:umass-sim0:0:0:0): Retries Exausted

umass0: BBB reset failed, IOERROR
umass0: BBB bulk-in clear stall failed, IOERROR
umass0: BBB bulk-out clear stall failed, IOERROR

As an experiment the same transfer was tried with a Sandisk device and
the transfer was successful

umass1: SanDisk Cruzer, rev 2.00/1.26, addr 3
umass1: SCSI over Bulk-Only; quirks = 0x4000
umass1:1:1:-1: Attached to scbus1

Logs seen during the transfer which is successful,
usbd_transfer_cb: short transfer 49152<65536
(da1:umass-sim1:1:0:0): Request completed with CAM_REQ_CMP_ERR
(da1:umass-sim1:1:0:0): Retrying Command
(da1:umass-sim1:1:0:0): Request completed with CAM_REQ_CMP_ERR
(da1:umass-sim1:1:0:0): Retrying Command
(da1:umass-sim1:1:0:0): Request completed with CAM_REQ_CMP_ERR
(da1:umass-sim1:1:0:0): Retrying Command

How can I debug this further?