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.orgSigned-off-by: Martin K. Petersen <martin.petersen@oracle.com>

include/uapi/linux/ioprio.h

@@ -58,4 +58,53 @@ enum {
 #define IOPRIO_NORM	4
 #define IOPRIO_BE_NORM	IOPRIO_NORM
 
+/*
+ * The 10 bits between the priority class and the priority level are used to
+ * optionally define I/O hints for any combination of I/O priority class and
+ * level. Depending on the kernel configuration, I/O scheduler being used and
+ * the target I/O device being used, hints can influence how I/Os are processed
+ * without affecting the I/O scheduling ordering defined by the I/O priority
+ * class and level.
+ */
+#define IOPRIO_HINT_SHIFT		IOPRIO_LEVEL_NR_BITS
+#define IOPRIO_HINT_NR_BITS		10
+#define IOPRIO_NR_HINTS			(1 << IOPRIO_HINT_NR_BITS)
+#define IOPRIO_HINT_MASK		(IOPRIO_NR_HINTS - 1)
+#define IOPRIO_PRIO_HINT(ioprio)	\
+	(((ioprio) >> IOPRIO_HINT_SHIFT) & IOPRIO_HINT_MASK)
+
+/*
+ * Alternate macro for IOPRIO_PRIO_VALUE() to define an I/O priority with
+ * a class, level and hint.
+ */
+#define IOPRIO_PRIO_VALUE_HINT(class, level, hint)		 \
+	((((class) & IOPRIO_CLASS_MASK) << IOPRIO_CLASS_SHIFT) | \
+	 (((hint) & IOPRIO_HINT_MASK) << IOPRIO_HINT_SHIFT) |	 \
+	 ((level) & IOPRIO_LEVEL_MASK))
+
+/*
+ * I/O hints.
+ */
+enum {
+	/* No hint */
+	IOPRIO_HINT_NONE = 0,
+
+	/*
+	 * Device command duration limits: indicate to the device a desired
+	 * duration limit for the commands that will be used to process an I/O.
+	 * These will currently only be effective for SCSI and ATA devices that
+	 * support the command duration limits feature. If this feature is
+	 * enabled, then the commands issued to the device to process an I/O with
+	 * one of these hints set will have the duration limit index (dld field)
+	 * set to the value of the hint.
+	 */
+	IOPRIO_HINT_DEV_DURATION_LIMIT_1 = 1,
+	IOPRIO_HINT_DEV_DURATION_LIMIT_2 = 2,
+	IOPRIO_HINT_DEV_DURATION_LIMIT_3 = 3,
+	IOPRIO_HINT_DEV_DURATION_LIMIT_4 = 4,
+	IOPRIO_HINT_DEV_DURATION_LIMIT_5 = 5,
+	IOPRIO_HINT_DEV_DURATION_LIMIT_6 = 6,
+	IOPRIO_HINT_DEV_DURATION_LIMIT_7 = 7,
+};
+
 #endif /* _UAPI_LINUX_IOPRIO_H */