Re: [PATCH v2 03/30] scsi: core: Do not allocate a budget token for reserved commands

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

On 8/20/25 11:42 PM, Hannes Reinecke wrote:
> Ah. But then you can allocate two separate queues & tagsets, one for
> TMFs and one for 'normal' commands (like NVMe does it).

Agreed.

> Then you wouldn't need reserved commands at all; they only make sense
> if TMFs and commands share the same tagspace.

That's wrong. There are other use cases for reserved commands, e.g. a
SCSI LLD driver allocating and submitting a SCSI or non-SCSI command to
the storage controller.

Here is an overview of the types of commands submitted by the UFS driver
to a UFS host controller:
* Task management functions. As explained in the previous email, TMFs
   have their own tag space that is separate from SCSI and non-SCSI
   commands.
* SCSI commands that are the result of a block layer request having been
   translated into a SCSI command, e.g. to implement filesystem I/O.
* SCSI passthrough commands submitted through the SG I/O interface.
* SCSI commands allocated and submitted by the SCSI core, e.g. the
   INQUIRY and MODE SELECT commands.
* SCSI commands allocated by the UFS driver, e.g. START STOP UNIT to
   change the power mode of the UFS device during runtime and system-wide
   power transitions.
* non-SCSI UPIU commands allocated by the UFS driver to perform tasks
   supported by the UFS standard that fall outside the scope of the
   SCSI standard, e.g. to query the supported queue depth or to interact
   with the real-time clock in the UFS device.
* non-SCSI UPIU commands submitted through the BSG interface.

The upstream UFS driver implements its own mechanism for reserving a tag
for non-SCSI UPIU commands (hba->reserved_slot). This patch series
converts that mechanism into allocating a reserved tag
(BLK_MQ_REQ_RESERVED).

Bart.