Re: [PATCH 04/24] scsi: core: Implement reserved command handling

[John Garry <john.g.garry@oracle.com>]

On 16/04/2025 08:39, Hannes Reinecke wrote:
>> JFYI, I was working on another solution (different to Hannes') which 
>> allows reserved requests pass through scsi_queue_rq(), but I stopped 
>> that work when I changed employer.
>>
> Oh, really?
> Do you have the patchset still around?
> I'd be very much interested in that...

Yeah, if you remember, I was working on that for libsas/libata, but 
started to get bogged down in how to handle reserved commands for 
libsas/libata:

https://lore.kernel.org/linux-scsi/1666693096-180008-1-git-send-email-john.garry@huawei.com/T/#m35ad9a27320077f55c2be5549f036628fe79dcf7

It would be a lot simpler to support for HBAs which plug directly into 
the SCSI midlayer - like aacraid.

> 
>>> Although it is easy
>>> to create an additional request queue for reserved requests, that
>>> request queue shares its tag set with the SCSI host and hence also the
>>> request submission function. From scsi_host.h:
>>>
>>> struct Scsi_Host {
>>>      struct blk_mq_tag_set    tag_set;
>>>      [ ... ]
>>> };
>>>
>>>  From blk-mq.h:
>>>
>>> struct blk_mq_tag_set {
>>>      const struct blk_mq_ops    *ops;
>>>      [ ... ]
>>> };
>>>
>>> Unless someone comes up with an elegant proposal, I will keep the
>>> approach where ufshcd_tag_to_cmd() handles reserved tags and regular
>>> tags differently.
>>>
>>> It should be possible to do this without referencing tags->static_rqs[]
>>> directly from the UFS driver.
>>
>> I'm not sure how that will look, but my preference is to fully 
>> implement reserved tags support which can be used by all SCSI LLDs.
>>
> Sure, that was my goal, too.
> But then I got stuck as some drivers (most notably aacraid) use internal
> commands to detect the hardware and allocate SCSI devices, so with my
> approach we need a 'fake' SCSI device for the host to handle that.
> If you have other ideas I'd be very interested in thems.

My solution still involved supporting a fake sdev also for times like 
you describe, e.g. shost needs to send reserved commands.

Thanks,
John