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

[Damien Le Moal]

On 6/20/22 16:03, Hannes Reinecke wrote:
> On 6/20/22 08:28, Christoph Hellwig wrote:
>> On Mon, Jun 20, 2022 at 08:24:24AM +0200, Hannes Reinecke wrote:
>>> So my idea for SATA is simply _not_ to use reserved tags.
>>> Any TMF functions (or the equivalent thereof) should always be sent as
>>> non-NCQ commands. And when doing so we're back to QD=1 on SATA anyway, so
>>> there _must_ be tags available. Consequently the main reason for having
>>> reserved tags (namely to guarantee that tags are available for TMF) doesn't
>>> apply here.
>>
>> At least in the non-elevator case (which includes all passthrough I/O)
>> request have tags assigned as soon as they are allocated.  So, we
>> absolutely can have all tags allocated and then need to do a TMF.
> 
> SATA internals may come to the rescue here; if there's an error all NCQ 
> commands are aborted. So we'll get at least one command tag back.
> As for the command duration limits I'm still waiting for clarification
> from Damien if we can reuse tags there.

We cannot easily reuse tags as the CDL case is *not* an error. So no queue
abort come with it. If we had the queue abort, things would be easy as
that would essentially be a regular eh.

Simply using scsi_execute_req() does not work since we have no guarantees
we can get a tag: all 32 commands being executed may complete with needing
sense data, and so scsi_execute_req() would hang waiting for a tag. I
could try hacking scsi_execute_req() to reuse a tag instead of allocating
a new one... Calling scsi_execute_req() from libata is really ugly though.

> 
> But I do agree it's ugly.
> 
> Cheers,
> 
> Hannes


-- 
Damien Le Moal
Western Digital Research