On 4/22/21 7:33 PM, Douglas Gilbert wrote: > 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 /* FIXME: this is obsolete in SAM-3 */ 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 ... > Might, but again not something we should attempt in this patchset. Using PRE-FETCH might be worthwhile for larger I/O, which we could easily prepend with a PRE-FETCH command for the entire range. But then error handling for PRE-FETCH is decidedly tricky, and we might end up incurring quite some regressions just because we didn't get the error handling right in the first go. Worth a shot, but please in another patchset. The other use-case for using PRE-FETCH after cryptographic erase definitely is required, but as that's triggered by userspace I would expect userspace to do it properly, too. The only valid use-case I see would be for issuing PRE-FETCH after UNMAP, but that would need to be plugged into the 'discard' machinery, which is already fragile as hell; I'd rather not attempt that one. Cheers, Hannes -- Dr. Hannes Reinecke Kernel Storage Architect hare@xxxxxxx +49 911 74053 688 SUSE Software Solutions Germany GmbH, 90409 Nürnberg GF: F. Imendörffer, HRB 36809 (AG Nürnberg)