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

[Douglas Gilbert <dgilbert@interlog.com>]

On 2021-04-22 4:42 a.m., Hannes Reinecke wrote:
> On 4/21/21 11:58 PM, Douglas Gilbert wrote:
>> On 2021-04-21 5:10 p.m., Bart Van Assche wrote:
>>> On 4/21/21 10:47 AM, Hannes Reinecke wrote:
>>>> +static inline bool scsi_result_is_good(struct scsi_cmnd *cmd)
>>>> +{
>>>> +    return (cmd->result == 0);
>>>> +}
>>>
>>> Do we really need an inline function to compare an integer with zero? How 
>>> about open-coding this comparison in the callers of this function?
>>>
>>
>> Please don't open code it. Please fix it!
>> Spot the difference:
>>
>> 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                  /* FIXME: this is obsolete in SAM-3 */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));
>> }
>>
>> In sg3_utils' library I ignore the last three SAM_STATs. Not sure if ignoring
>> bit 0 is still required.
>>
>> Without considering SAM_STAT_CONDITION_MET a good status, someone will be
>> scratching their head wondering why so many PRE-FETCH commands fail.
>>
>> That command can be used when a sequence of READs to consecutive LBAs is
>> followed by a different (i.e. non-consecutive) READ. That last READ could
>> be preceded by PRE-FETCH(new_LBA, IMMED). Assuming there is processing
>> of the data from the consecutive LBAs to be done, when the time comes
>> for READ(new_LBA) the probability of its data being in the disk's cache is
>> increased.
>>
> That would be a change in behaviour.
> Current code doesn't check for CONDITION_MET, so this change shouldn't do it, 
> neither. Idea was that this patchset shouldn't change the current behaviour.
> 
> While your argument might be valid, it definitely is a different story and would 
> need to be address with a different patchset.

Okay. May I suggest a "FIX_ME" comment? And again, please don't open code it.

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.

Doug Gilbert