[RFC PATCH 2/5] block: ioprio hint to low-level device drivers

["Jason B. Akers" <jason.b.akers@intel.com>]

From: Dan Williams <dan.j.williams@intel.com>

The priority in the io_context is consumed by the io scheduler.  For
caching advice we need the request->ioprio field to be up-to-date.  Set
the bio ioprio at submit time.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Jason B. Akers <jason.b.akers@intel.com>
---
 block/bio.c    |    1 +
 block/ioprio.c |   22 ++++++++++++++++------
 2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 3e6e198..e133b5c 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -2009,6 +2009,7 @@ int bio_associate_current(struct bio *bio)
 	/* acquire active ref on @ioc and associate */
 	get_io_context_active(ioc);
 	bio->bi_ioc = ioc;
+	bio_set_prio(bio, ioprio_best(ioc->ioprio, bio_prio(bio)));
 
 	/* associate blkcg if exists */
 	rcu_read_lock();
diff --git a/block/ioprio.c b/block/ioprio.c
index e50170c..fec1202 100644
--- a/block/ioprio.c
+++ b/block/ioprio.c
@@ -159,18 +159,28 @@ int ioprio_best(unsigned short aprio, unsigned short bprio)
 {
 	unsigned short aclass = IOPRIO_PRIO_CLASS(aprio);
 	unsigned short bclass = IOPRIO_PRIO_CLASS(bprio);
+	unsigned short class, data, advice;
 
 	if (aclass == IOPRIO_CLASS_NONE)
 		aclass = IOPRIO_CLASS_BE;
 	if (bclass == IOPRIO_CLASS_NONE)
 		bclass = IOPRIO_CLASS_BE;
 
-	if (aclass == bclass)
-		return min(aprio, bprio);
-	if (aclass > bclass)
-		return bprio;
-	else
-		return aprio;
+	/* best priority */
+	if (aclass == bclass) {
+		class = aclass;
+		data = min(IOPRIO_PRIO_DATA(aprio), IOPRIO_PRIO_DATA(bprio));
+	} else if (aclass > bclass) {
+		class = bclass;
+		data = IOPRIO_PRIO_DATA(bprio);
+	} else {
+		class = aclass;
+		data = IOPRIO_PRIO_DATA(aprio);
+	}
+
+	/* best cache advice, assumes invalid advice is zero */
+	advice = max(IOPRIO_ADVICE(aprio), IOPRIO_ADVICE(bprio));
+	return IOPRIO_ADVISE(class, data, advice);
 }
 
 SYSCALL_DEFINE2(ioprio_get, int, which, int, who)