Re: [PATCH 14/42] scsi: add scsi_result_is_good()

[Douglas Gilbert <dgilbert@interlog.com>]

On 2021-04-22 12:52 p.m., Bart Van Assche wrote:
> On 4/22/21 8:56 AM, Douglas Gilbert wrote:
>> In driver manuals Seagate often list the PRE-FETCH command as optional. As
>> in: pay us some extra money and we will put it in. That suggests to me some
>> big OEM likes PRE-FETCH. Where I found it supported in WDC manuals, they
>> didn't support the IMMED bit which sort of defeats the purpose of it IMO.
> 
> Since the sd driver does not submit the PRE-FETCH command, how about
> moving support for CONDITION MET into the sg code and treating CONDITION
> MET as an error inside the sd, sr and st drivers? I think that would
> allow to simplify scsi_status_is_good(). The current definition of that
> function is as follows:
> 
> static inline int scsi_status_is_good(int status)
> {
> 	/*
> 	 * FIXME: bit0 is listed as reserved in SCSI-2, but is
> 	 * significant in SCSI-3.  For now, we follow the SCSI-2
> 	 * behaviour and ignore reserved bits.
> 	 */
> 	status &= 0xfe;
> 	return ((status == SAM_STAT_GOOD) ||
> 		(status == SAM_STAT_CONDITION_MET) ||
> 		/* Next two "intermediate" statuses are obsolete in*/
> 		/* SAM-4 */
> 		(status == SAM_STAT_INTERMEDIATE) ||
> 		(status == SAM_STAT_INTERMEDIATE_CONDITION_MET) ||
> 		/* FIXME: this is obsolete in SAM-3 */
> 		(status == SAM_STAT_COMMAND_TERMINATED));
> }

The whole stack needs to treat SAM_STAT_CONDITION_MET as a non-error.
However the complex multi-layer return values are represented,
reducing them to a comparison with zero, spread all over the
stack just seems bad software engineering. IMO a predicate function
(i.e. returning bool) is needed.

I would argue that in the right circumstances, the sd driver should
indeed by using PRE-FETCH. It would need help from the upper layers.
It is essentially "read-ahead" in the case where the next LBA
does not follow the last read LBA. A smarter read-ahead ...

Doug Gilbert