Re: [RFC] libata new EH document

Luben Tuikov
2005-08-30T14:32:12Z

On 08/30/05 06:26, Tejun Heo wrote:
> Albert Lee wrote:
>
>>>4. Corresponding scmd's result code is set to
>>> SAM_STAT_CHECK_CONDITION and qc->scsidone() callback is called
>>> directly. As we haven't filled sense data,
>>> scsi_determine_disposition() will return FAILED and SCSI EH will
>>> be scheduled. Note that as we directly call qc->scsidone(), qc is
>>> left intact.
>>>
>>>
>>
>>Could we get the sense data before calling qc->scsidone()? (Using the
>>proposed separate
>>EH qc can keep the original qc intact.)
>>
>>The issue:
>>When a DVD drive returns MEDIUM_ERROR in the sense data, libata doesn't
>>retry the command.
>>
>>For libata, when scsi_softirq() calls scsi_decide_disposition() and
>>scsi_check_sense() to determine
>>how to handle the result, scsi_check_sense() always returns "fail" since
>>the sense data is not there
>>yet. The sense data is requested later in the libata error handler. But
>>the command has already been
>>considered as an "error".
>>
>>By having the sense data ready before calling qc->scsidone(), we can
>>make the
>>NEEDS_RETRY work in scsi_softirq(). So, for things like MEDIUM_ERROR,
>>the device has
>>a chance to retry/recover the error. This seems to be important for
>>devices with built-in
>>defect management system.
>
>
> There are two ways a scmd can leave EH - retry by scsi_queue_insert()
> and finish by scsi_finish_cmd(). I think the problem you described can
> be easily solved by choosing the former method when finishing the qc
> from EH. Note that other advanced EH stuff like reconfiguring transport
> speed also requires retrying, so we will surely have a mechanism for
> retrying failed qc's from EH.

What is needed is autosense simulation for ATA, so that SCSI Core doesn't
know that the device doesn't support autosense.

So, before a failed command reaches SCSI Core recovery, it should pass by
ATA layer recovery to get sense.

Note: if you send another command for execution after the failed command
_and_ no autosense is provided, then any sense data is lost -- this is further
subject to more rules set forth in SAM and SPC.

Luben