scsi: block: Introduce ioprio hints

scsi: block: Introduce ioprio hints

I/O priorities currently only use 6-bits of the 16-bits ioprio value: the
3-upper bits are used to define up to 8 priority classes (4 of which are
valid) and the 3 lower bits of the value are used to define a priority
level for the real-time and best-effort class.

The remaining 10-bits between the I/O priority class and level are unused,
and in fact, cannot be used by the user as doing so would either result in
the value being completely ignored, or in an error returned by
ioprio_check_cap().

Use these 10-bits of an ioprio value to allow a user to specify I/O
hints. An I/O hint is defined as a 10-bitsvalue, allowing up to 1023
different hints to be specified, with the value 0 being reserved as the "no
hint" case. An I/O hint can apply to any I/O that specifies a valid
priority class other than NONE, regardless of the I/O priority level
specified.

To do so, the macros IOPRIO_PRIO_HINT() and IOPRIO_PRIO_VALUE_HINT() are
introduced in include/uapi/linux/ioprio.h to respectively allow a user to
get and set a hint in an ioprio value.

To support the ATA and SCSI command duration limits feature, 7 hints are
defined: IOPRIO_HINT_DEV_DURATION_LIMIT_1 to
IOPRIO_HINT_DEV_DURATION_LIMIT_7, allowing a user to specify which command
duration limit descriptor should be applied to the commands serving an
I/O. Specifying these hints has for now no effect whatsoever if the target
block devices do not support the command duration limits feature. However,
in the future, block I/O schedulers can be modified to optimize I/O issuing
order based on these hints, even for devices that do not support the
command duration limits feature.

Given that the 7 duration limits hints defined have no effect on any block
layer component, the actual definition of the duration limits implied by
these hints remains at the device level.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Link: https://lore.kernel.org/r/20230511011356.227789-3-nks@flawful.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>