On 2019-10-30 11:18 a.m., Hannes Reinecke wrote:
On 10/30/19 4:12 PM, Bart Van Assche wrote:
On 10/30/19 1:30 AM, Hannes Reinecke wrote:
On 10/28/19 9:38 PM, Bart Van Assche wrote:
If the residual is changed from signed into unsigned, how is a SCSI LLD
expected to report the difference between residual overflow and residual
underflow to the SCSI core?
You don't have to. To quote RFC 3720 page 122:
bit 5 - (O) set for Residual Overflow. In this case, the Residual
Count indicates the number of bytes that were not transferred
because the initiator's Expected Data Transfer Length was not
sufficient. For a bidirectional operation, the Residual Count
contains the residual for the write operation.
IE the 'overflow' setting in the iSCSI command response is an indicator that
there _would_ be more data if the command request _would_ have specified a
larger buffer.
But as it didn't, the entire buffer was filled, and the overflow counter is set.
Which, of course, is then ignored by the linux SCSI stack as the request got
all data, and the residual is set to zero.
Then it's left to the caller to re-send with a larger buffer if required. But
it's nothing the SCSI stack can nor should be attempting on its own.
Hi Hannes,
I do not agree that reporting a residual overflow by calling
scsi_set_resid(..., 0) is acceptable. For reads a residual overflow means that
the length specified in the CDB (scsi_bufflen()) exceeds the data buffer size
(length of scsi_sglist()). I think it's dangerous to report to the block layer
that such requests completed successfully and with residual zero.
But that is an error on submission, and should be aborted before it even got
send to the drive.
However, this does not relate to the residual, which is handled after the
command completes (and which sparked this entire thread ...).
Seen from a pass-through perspective, the resid is just about the near-end
data transfer between the HBA and pass-through, or as cam3r03 says:
− cam_resid;
The data residual length member contains the difference in twos complement
form of the number of data bytes transferred by the HA for the SCSI
command compared with the number of bytes requested by the CCB
cam_dxfer_len member. This is calculated by the total number of bytes
requested to be transferred by the CCB minus the actual number of bytes
transferred by the HA.
[where "HA" is HBA (or the initiator)]
That makes overflow (negative resid) a bit interesting as it is only
reasonable that the pass-though user allocated a buffer big enough to
receive dxfer_len bytes. One would hope that in the READ case of overflow,
the HBA would have sent the residual bytes to the bit bucket (i.e.
/dev/null) rather than overfill the data buffer provided by the pass-through.
Handling discrepancies between page length (e.g. of VPD, LOG and MODE pages)
and the ALLOCATION LENGTH field are defined in the SCSI standards.
That leaves the difficult cases: when there is a discrepancy between what the
SCSI command (and the storage device) implied as a data length and the
dxfer_len allocated at the near-end. In some, but not all, cases that is
detectable before the command is issued.
Overflow can happen, for example if the RDPROTECT field in a READ(10) is
accidentally set (e.g. because it uses bits previously used for a SPI
LUN) and the storage device has protection information. That will result
in an extra 8 bytes per logical block being returned.
Doug Gilbert
