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

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

On 15/04/2025 18:21, Bart Van Assche wrote:
> Using blk_mq_tag_to_rq() to translate a tag of a reserved
> request into a request pointer only works after the block layer has
> set tags->rqs[tag_of_reserved_request] first. That pointer is set by
> blk_mq_get_driver_tag(). blk_mq_get_driver_tag() is called by the
> block layer code that submits a request to a block driver. Hence, to
> ensure that blk_mq_get_driver_tag() is called for reserved requests
> the reserved requests would have to pass through scsi_queue_rq().

Ah, yes, we had the same discussion when trying to implement SCSI 
reserved commands previously.

> For reserved requests sdev == NULL as explained in a previous email.

which mail exactly? I could not see any specific mention in this thread.

> There are many statements in the SCSI command submission and completion
> path in which it is assumed that sdev != NULL. I don't think that the
> SCSI maintainer (Martin) would agree with adding "if (sdev)" statements
> in many places in the SCSI core.

Sure, so, IIRC, Hannes solved this by using a shost sdev in 
https://lore.kernel.org/linux-scsi/20211125151048.103910-2-hare@suse.de/

> 
> Letting UFS reserved requests being processed by another function than
> scsi_queue_rq() doesn't seem feasible to me either.

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.

> 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.

Thanks,
John