Re: [PATCH v1 2/4] ata: libata-scsi: Generate ATA PT sense data when ATA ERR/DF are set

[Niklas Cassel <cassel@kernel.org>]

On Fri, Jun 14, 2024 at 07:18:33PM +0000, Igor Pylypiv wrote:
> SCSI/ATA Translation-5 (SAT-5) Table 209 — "ATA command results"
> specifies that SATL shall generate sense data for ATA PASS-THROUGH
> commands when either CK_COND is set or when ATA_ERR or ATA_DF status
> bits are set.
> 
> ata_eh_analyze_tf() sets AC_ERR_DEV bit in qc->err_mask when ATA_ERR
> or ATA_DF bits are set. It looks like qc->err_mask can be used as
> an error indicator but ata_eh_link_autopsy() clears AC_ERR_DEV bit
> when ATA_QCFLAG_SENSE_VALID is set. This effectively clears the error
> indication if no other bits were set in qc->err_mask.

The reason why libata clears the err_mask when having sense data,
is because the upper layer, i.e. SCSI, should determine what to do
with the command, if there is sense data.

For a non-passthrough command, this will be done by
scsi_io_completion_action():
https://github.com/torvalds/linux/blob/v6.10-rc4/drivers/scsi/scsi_lib.c#L1084-L1087


However, if there is any bits set in cmd->result,
scsi_io_completion_nz_result() will be called:
https://github.com/torvalds/linux/blob/v6.10-rc4/drivers/scsi/scsi_lib.c#L1052-L1053

which will do the following for a passthrough command:
https://github.com/torvalds/linux/blob/v6.10-rc4/drivers/scsi/scsi_lib.c#L969-L978
which will set blk_stat.

After that, scsi_io_completion() which check blk_stat and if it is a
scsi_noretry_cmd():
https://github.com/torvalds/linux/blob/v6.10-rc4/drivers/scsi/scsi_lib.c#L1073-L1078

A passthrough command will return true for scsi_noretry_cmd(), so
scsi_io_completion_action() should NOT get called for a passthough command.

So IIUC, for a non-passthrough command, scsi_io_completion_action() will
decide what to do depending on the sense data, but a passthrough command will
get finished with the sense data, leaving the user to decide what to do.


> 
> ata_scsi_qc_complete() should not use qc->err_mask for ATA PASS-THROUGH
> commands because qc->err_mask can be zero (i.e. "no error") even when
> the corresponding command has failed with ATA_ERR/ATA_DF bits set.
> 
> Additionally, the presence of valid sense data (ATA_QCFLAG_SENSE_VALID)
> should not prevent SATL from generating sense data for ATA PASS-THROUGH.
> 
> Signed-off-by: Igor Pylypiv <ipylypiv@google.com>
> ---
>  drivers/ata/libata-scsi.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
> index 032cf11d0bcc..79e8103ef3a9 100644
> --- a/drivers/ata/libata-scsi.c
> +++ b/drivers/ata/libata-scsi.c
> @@ -1632,8 +1632,8 @@ static void ata_scsi_qc_complete(struct ata_queued_cmd *qc)
>  		!(qc->flags & ATA_QCFLAG_SENSE_VALID);
>  
>  	/* For ATA pass thru (SAT) commands, generate a sense block if
> -	 * user mandated it or if there's an error.  Note that if we
> -	 * generate because the user forced us to [CK_COND =1], a check
> +	 * user mandated it or if ATA_ERR or ATA_DF bits are set. Note that
> +	 * if we generate because the user forced us to [CK_COND=1], a check
>  	 * condition is generated and the ATA register values are returned
>  	 * whether the command completed successfully or not. If there
>  	 * was no error, we use the following sense data:
> @@ -1641,7 +1641,7 @@ static void ata_scsi_qc_complete(struct ata_queued_cmd *qc)
>  	 * asc,ascq = ATA PASS-THROUGH INFORMATION AVAILABLE
>  	 */
>  	if (((cdb[0] == ATA_16) || (cdb[0] == ATA_12)) &&
> -	    ((cdb[2] & 0x20) || need_sense))
> +	    ((cdb[2] & 0x20) || (qc->result_tf.status & (ATA_ERR | ATA_DF))))

qc->result_tf can only be used if qc->flags has ATA_QCFLAG_RESULT_TF set,
otherwise it can contain bogus data.
You don't seem to check if ATA_QCFLAG_RESULT_TF is set.

ata_scsi_pass_thru() does set ATA_QCFLAG_RESULT_TF.


>  		ata_gen_passthru_sense(qc);
>  	else if (need_sense)
>  		ata_gen_ata_sense(qc);
> -- 
> 2.45.2.627.g7a2c4fd464-goog
>