ata: libata-scsi: do not overwrite SCSI ML and status bytes

ata: libata-scsi: do not overwrite SCSI ML and status bytes

For SCSI ML byte:
In the case where a command is completed via libata EH:
irq -> ata_qc_complete() -> ata_qc_schedule_eh()
irq done
... -> ata_do_eh() -> ata_eh_link_autopsy() -> ata_eh_finish() ->
ata_eh_qc_complete() -> __ata_eh_qc_complete() -> __ata_qc_complete() ->
qc->complete_fn() (ata_scsi_qc_complete()) -> ata_qc_done() ->
qc->scsidone() (empty stub)
... -> scsi_eh_finish_cmd() -> scsi_eh_flush_done_q() ->
scsi_finish_command()

ata_eh_link_autopsy() will call ata_eh_analyze_tf(), which calls
scsi_check_sense(), which sets the SCSI ML byte.

Since ata_scsi_qc_complete() is called after scsi_check_sense() when
a command is completed via libata EH, we cannot simply overwrite the
SCSI ML byte that was set earlier in the call chain.

For SCSI status byte:
When a SCSI command is prepared using scsi_prepare_cmd(), it sets
cmd->result to 0. (SAM_STAT_GOOD is defined as 0x0).
Likewise, when a command is requeued from SCSI EH, scsi_queue_insert()
is called, which sets cmd->result to 0.

A SCSI command thus always has a GOOD status by default when being
sent to libata.

If libata fetches sense data from the device, it will call
ata_scsi_set_sense(), which will set the status byte to
SAM_STAT_CHECK_CONDITION, if the caller deems that the status should be
a check condition.

ata_scsi_qc_complete() should therefore never overwrite the existing
status byte, because if it is != GOOD, it was set by libata itself,
for a reason.

For the host byte:
When libata abort commands, because of a NCQ error, it will schedule
SCSI EH for all QCs using blk_abort_request(), which will all end up in
scsi_timeout(), which will call scsi_abort_command(). scsi_timeout()
sets DID_TIME_OUT regardless if a command was aborted or timed out.
If we don't clear the DID_TIME_OUT byte for the QC that caused the
NCQ error, that QC will be reported as a timed out command, instead
of being reported as a NCQ error.

For a command that actually timed out, DID_TIME_OUT would be fine to
keep, but libata has its own way of detecting that a command timed out
(see ata_scsi_cmd_error_handler()), and sets AC_ERR_TIMEOUT if that is
the case. libata will retry timed out commands.

We could clear DID_TIME_OUT only for the QC that caused the NCQ error,
but since libata has its own way of detecting timeouts, simply clear it
always.

Note that the existing ata_scsi_qc_complete() code does:
cmd->result = SAM_STAT_CHECK_CONDITION or cmd->result = SAM_STAT_GOOD.
This WILL clear the host byte. So us clearing the host byte
unconditionally is in line with the existing libata behavior.

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>