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

[Bart Van Assche <bvanassche@acm.org>]

On 4/4/25 3:49 AM, John Garry wrote:
> On 03/04/2025 22:17, Bart Van Assche wrote:
>> From: Hannes Reinecke<hare@suse.de>
>>
>> Quite some drivers are using management commands internally. These 
>> commands
>> typically use the same tag pool as regular SCSI commands. Tags for these
>> management commands are set aside before allocating the block-mq tag 
>> bitmap
>> for regular SCSI commands. The block layer already supports this via the
>> reserved tag mechanism. Add a new field 'nr_reserved_cmds' to the SCSI 
>> host
>> template to instruct the block layer to set aside a tag space for these
>> management commands by using reserved tags. Exclude reserved commands 
>> from
>> .can_queue because .can_queue is visible in sysfs.
>>
>> Signed-off-by: Hannes Reinecke<hare@suse.de>
>> [ bvanassche: modified patch description ]
>> Cc: John Garry<john.g.garry@oracle.com>
>> Signed-off-by: Bart Van Assche<bvanassche@acm.org>
> 
> How is this related to the rest of the series?
> 
> To allocate a reserved-tag request we need to use BLK_MQ_REQ_RESERVED, 
> right? I don't see that used...

Hi John,

Calling blk_mq_alloc_request(..., BLK_MQ_REQ_RESERVED) is not the only 
way to allocated a reserved tag. Another possibility is to let the
driver manage reserved tags. The UFS driver only needs a single reserved
tag and already serializes use of that tag. See also the following
change in patch 21/24:

-	hba->reserved_slot = hba->nutrs - 1;
+	hba->reserved_slot = 0;

Use of hba->reserved_slot is serialized by calling ufshcd_dev_man_lock()
and ufshcd_dev_man_unlock(). More code is serialized by these calls than
only the region in which hba->reserved_slot is used so I don't think
that the UFS driver code would become shorter by using block layer
functions for allocating / freeing reserved tags.

Thanks,

Bart.