Re: [PATCH v2 1/3] scsi_cmnd: Introduce scsi_transfer_length helper

Michael Christie
2014-06-24T16:08:54Z

On Jun 24, 2014, at 7:53 AM, Martin K. Petersen <martin.petersen@oracle.com> wrote:

>>>>>> "Mike" == Mike Christie <michaelc@cs.wisc.edu> writes:
>
> Mike> The problem is WRITE_SAME requests are setup so that
> Mike> req->__data_len is the value of the entire request when the setup
> Mike> is completed but during the setup process it's value changes
>
> Oh, I see. So things break because iSCSI uses scsi_transfer_length()
> where the scatterlist length was used in the past.
>
> How about this?
>
>
> SCSI: Use SCSI data buffer length to extract transfer size
>
> Commit 8846bab180fa introduced a helper that can be used to query the
> wire transfer size for a SCSI command taking protection information into
> account.
>
> However, some commands do not have a 1:1 mapping between the block range
> they work on and the payload size (discard, write same). After the
> scatterlist has been set up these requests use __data_len to store the
> number of bytes to report completion on. This means that callers of
> scsi_transfer_length() would get the wrong byte count for these types of
> requests.
>
> To overcome this we make scsi_transfer_length() use the scatterlist
> length in the scsi_data_buffer as basis for the wire transfer
> calculation instead of __data_len.
>
> Reported-by: Christoph Hellwig <hch@infradead.org>
> Debugged-by: Mike Christie <michaelc@cs.wisc.edu>
> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
>
> diff --git a/include/scsi/scsi_cmnd.h b/include/scsi/scsi_cmnd.h
> index 42ed789ebafc..e0ae71098144 100644
> --- a/include/scsi/scsi_cmnd.h
> +++ b/include/scsi/scsi_cmnd.h
> @@ -318,7 +318,7 @@ static inline void set_driver_byte(struct scsi_cmnd *cmd, char status)
>
> static inline unsigned scsi_transfer_length(struct scsi_cmnd *scmd)
> {
> - unsigned int xfer_len = blk_rq_bytes(scmd->request);
> + unsigned int xfer_len = scsi_out(scmd)->length;
> unsigned int prot_op = scsi_get_prot_op(scmd);
> unsigned int sector_size = scmd->device->sector_size;

Do we need to check for the data direction. Something like

if (scmd->sc_data_direction == DMA_TO_DEVICE)
xfer_len = scsi_out(scmnd)->length;
else
xfer_len = scsi_in(scmnd)->length;