> When a tape drive is exported via LIO using the pscsi module, a read that
> requests more bytes per block than the tape can supply returns an empty
> buffer. This is because the pscsi pass-through target module sees the
> "ILI" illegal length bit set and thinks there is no reason to return
> the data.
>
> This is a long-standing transport issue, since it assumes that no data
> need be returned under a check condition, which isn't always the case
> for tape.
>
> Add in a check for tape reads with the ILI, EOM, or FM bits set,
> with a sense code of NO_SENSE, treating such cases as if the read
> succeeded. The layered tape driver then "does the right thing" when
> it gets such a response.
I tested the patch. Using the st driver to handle the exported tape drive on
the initiator side, everything works fine.
But the st driver masks a problem that still persists. A real tape drive, if
the READ is longer than the data block to read, will transfer only the amount
of data from the tape block.
The exported tape drive with this patch transfers the full amount of data
as requested by the READ. The over scoring part of the transmitted data is
padded with NUL bytes.
Sending READ commands via sg with sg_raw I was able to test this behavior.
I also verified it using a FibreChannel analyzer.
I'd suggest to call target_complete_cmd_with_length() instead of
target_complete_cmd() in the descibed situation. That should fix the length
of the transmitted data, I think.
BR, Bodo
>
> Changes from v2:
> - Cleaned up subject line and bug text formatting
> - Removed unneeded inner braces
> - Removed ugly goto
> - Also updated the "queue full" path to handle this case
>
> Changes from RFC:
> - Moved ugly code from transport to pscsi module
> - Added checking EOM and FM bits, as well as ILI
> - fixed malformed patch
> - Clarified description a bit
>
> Signed-off-by: Lee Duncan <lduncan@xxxxxxxx>
> ---
> drivers/target/target_core_pscsi.c | 23 +++++++++++++++++++-
> drivers/target/target_core_transport.c | 39 +++++++++++++++++++++++++++++-----
> include/target/target_core_base.h | 1 +
> 3 files changed, 57 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/target/target_core_pscsi.c b/drivers/target/target_core_pscsi.c
> index 0d99b242e82e..a9656368a96d 100644
> --- a/drivers/target/target_core_pscsi.c
> +++ b/drivers/target/target_core_pscsi.c
> @@ -689,8 +689,29 @@ static void pscsi_complete_cmd(struct se_cmd *cmd, u8 scsi_status,
> }
> after_mode_select:
>
> - if (scsi_status == SAM_STAT_CHECK_CONDITION)
> + if (scsi_status == SAM_STAT_CHECK_CONDITION) {
> transport_copy_sense_to_cmd(cmd, req_sense);
> +
> + /*
> + * hack to check for TAPE device reads with
> + * FM/EOM/ILI set, so that we can get data
> + * back despite framework assumption that a
> + * check condition means there is no data
> + */
> + if (sd->type == TYPE_TAPE &&
> + cmd->data_direction == DMA_FROM_DEVICE) {
> + /*
> + * is sense data valid, fixed format,
> + * and have FM, EOM, or ILI set?
> + */
> + if (req_sense[0] == 0xf0 && /* valid, fixed format */
> + req_sense[2] & 0xe0 && /* FM, EOM, or ILI */
> + (req_sense[2] & 0xf) == 0) { /* key==NO_SENSE */
> + pr_debug("Tape FM/EOM/ILI status detected. Treat as normal read.\n");
> + cmd->se_cmd_flags |= SCF_TREAT_READ_AS_NORMAL;
> + }
> + }
> + }
> }
>
> enum {
> diff --git a/drivers/target/target_core_transport.c b/drivers/target/target_core_transport.c
> index 74b646f165d4..a15a9e3dce11 100644
> --- a/drivers/target/target_core_transport.c
> +++ b/drivers/target/target_core_transport.c
> @@ -2084,12 +2084,24 @@ static void transport_complete_qf(struct se_cmd *cmd)
> goto queue_status;
> }
>
> - if (cmd->se_cmd_flags & SCF_TRANSPORT_TASK_SENSE)
> + /*
> + * Check if we need to send a sense buffer from
> + * the struct se_cmd in question. We do NOT want
> + * to take this path of the IO has been marked as
> + * needing to be treated like a "normal read". This
> + * is the case if it's a tape read, and either the
> + * FM, EOM, or ILI bits are set, but there is no
> + * sense data.
> + */
> + if (!(cmd->se_cmd_flags & SCF_TREAT_READ_AS_NORMAL) &&
> + cmd->se_cmd_flags & SCF_TRANSPORT_TASK_SENSE)
> goto queue_status;
>
> switch (cmd->data_direction) {
> case DMA_FROM_DEVICE:
> - if (cmd->scsi_status)
> + /* queue status if not treating this as a normal read */
> + if (cmd->scsi_status &&
> + !(cmd->se_cmd_flags & SCF_TREAT_READ_AS_NORMAL))
> goto queue_status;
>
> trace_target_cmd_complete(cmd);
> @@ -2194,9 +2206,15 @@ static void target_complete_ok_work(struct work_struct *work)
>
> /*
> * Check if we need to send a sense buffer from
> - * the struct se_cmd in question.
> + * the struct se_cmd in question. We do NOT want
> + * to take this path of the IO has been marked as
> + * needing to be treated like a "normal read". This
> + * is the case if it's a tape read, and either the
> + * FM, EOM, or ILI bits are set, but there is no
> + * sense data.
> */
> - if (cmd->se_cmd_flags & SCF_TRANSPORT_TASK_SENSE) {
> + if (!(cmd->se_cmd_flags & SCF_TREAT_READ_AS_NORMAL) &&
> + cmd->se_cmd_flags & SCF_TRANSPORT_TASK_SENSE) {
> WARN_ON(!cmd->scsi_status);
> ret = transport_send_check_condition_and_sense(
> cmd, 0, 1);
> @@ -2238,7 +2256,18 @@ static void target_complete_ok_work(struct work_struct *work)
> queue_rsp:
> switch (cmd->data_direction) {
> case DMA_FROM_DEVICE:
> - if (cmd->scsi_status)
> + /*
> + * if this is a READ-type IO, but SCSI status
> + * is set, then skip returning data and just
> + * return the status -- unless this IO is marked
> + * as needing to be treated as a normal read,
> + * in which case we want to go ahead and return
> + * the data. This happens, for example, for tape
> + * reads with the FM, EOM, or ILI bits set, with
> + * no sense data.
> + */
> + if (cmd->scsi_status &&
> + !(cmd->se_cmd_flags & SCF_TREAT_READ_AS_NORMAL))
> goto queue_status;
>
> atomic_long_add(cmd->data_length,
> diff --git a/include/target/target_core_base.h b/include/target/target_core_base.h
> index 9f9f5902af38..922a39f45abc 100644
> --- a/include/target/target_core_base.h
> +++ b/include/target/target_core_base.h
> @@ -143,6 +143,7 @@ enum se_cmd_flags_table {
> SCF_ACK_KREF = 0x00400000,
> SCF_USE_CPUID = 0x00800000,
> SCF_TASK_ATTR_SET = 0x01000000,
> + SCF_TREAT_READ_AS_NORMAL = 0x02000000,
> };
>
> /*
> --
> 2.16.3
>
> --
