Re: [PATCH RFC v2 03/18] scsi: core: Implement reserved command handling

[John Garry]

On 13/06/2022 10:06, Damien Le Moal wrote:
> > > We cannot have more than 32 tags.
> > We may have 32 regular tags and 1 reserved tag for SATA.
> Right. But that is the messy part though. That extra 1 tag is actually not
> a tag since all internal commands are non-NCQ commands that do not need a
> tag...

But apart from SATA, libsas LLDDs do need a real tag for the libata 
internal command.

> I am working on command duration limits support currently. This feature
> set has a new horrendous "improvement": a command can be aborted by the
> device if it fails its duration limit, but the abort is done with a good
> status + sense data available bit set so that the device queue is not
> aborted entirely like with a regular NCQ command error.
>
> For such aborted commands, the command sense data is set to
> "COMPLETED/DATA UNAVAILABLE". In this case, the host needs to go read the
> new "successful NCQ sense data log" to check that the command sense is
> indeed "COMPLETED/DATA UNAVAILABLE". And to go read that log page without
> stalling the device queue, we would need an internal NCQ (queuable) command.
>
> Currently, that is not possible to do cleanly as there are no guarantees
> we can get a free tag (there is a race between block layer tag allocation
> and libata internal tag counting). So a reserved tag for that would be
> nice. We would end up with 31 IO tags at most + 1 reserved tag for NCQ
> commands + ATA_TAG_INTERNAL for non-NCQ. That last one would be rendered
> rather useless. But that also means that we kind-of go back to the days
> when Linux showed ATA drives max QD of 31...

So must the ATA_TAG_INTERNAL qc always be available for non-NCQ action 
like EH, and that is why you cannot reuse for this internal NCQ 
(queuable) command?

> I am still struggling with this particular use case and trying to make it
> fit with your series. Trying out different things right now.

ok

> > > I think keeping can_queue as the max queue depth with at most
> > > nr_reserved_cmds tags reserved is better.
> > Maybe the wording in the comment can be improved as it originally
> > focused on SAS HBAs where there are no special rules for tagset depth or
> > how the tagset should be carved up to handle regular and reserved commands.
> Indeed. And that would be for HBAs that do*not*  use libsas/libata.
> Otherwise, the NCQ vs non-NCQ reserved tag mess is there.

Thanks,
John