From 22c40e134c4c7a828ac09d25a5a8597b1e45c031 Mon Sep 17 00:00:00 2001
From: Joe Thornber <ejt@redhat.com>
Date: Thu, 26 Jan 2023 10:14:26 +0000
Subject: [PATCH] dm cache: Add some documentation to
 dm-cache-background-tracker.h

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
---
 drivers/md/dm-cache-background-tracker.h | 40 ++++++++++++++++++++++--
 1 file changed, 37 insertions(+), 3 deletions(-)

diff --git a/drivers/md/dm-cache-background-tracker.h b/drivers/md/dm-cache-background-tracker.h
index 27ab90dbc275..b5056e8275c1 100644
--- a/drivers/md/dm-cache-background-tracker.h
+++ b/drivers/md/dm-cache-background-tracker.h
@@ -12,19 +12,44 @@
 
 /*----------------------------------------------------------------*/
 
+/*
+ * The cache policy decides what background work should be performed,
+ * such as promotions, demotions and writebacks. The core cache target
+ * is in charge of performing the work, and does so when it sees fit.
+ *
+ * The background_tracker acts as a go between. Keeping track of future
+ * work that the policy has decided upon, and handing (issuing) it to
+ * the core target when requested.
+ *
+ * There is no locking in this, so calls will probably need to be
+ * protected with a spinlock.
+ */
+
 struct background_work;
 struct background_tracker;
 
 /*
- * FIXME: discuss lack of locking in all methods.
+ * Create a new tracker, it will not be able to queue more than
+ * 'max_work' entries.
  */
 struct background_tracker *btracker_create(unsigned max_work);
+
+/*
+ * Destroy the tracker. No issued, but not complete, work should
+ * exist when this is called. It is fine to have queued but unissued
+ * work.
+ */
 void btracker_destroy(struct background_tracker *b);
 
 unsigned btracker_nr_writebacks_queued(struct background_tracker *b);
 unsigned btracker_nr_demotions_queued(struct background_tracker *b);
 
 /*
+ * Queue some work within the tracker. 'work' should point to the work
+ * to queue, this will be copied (ownership doesn't pass).  If pwork
+ * is not NULL then it will be set to point to the tracker's internal
+ * copy of the work.
+ *
  * returns -EINVAL iff the work is already queued.  -ENOMEM if the work
  * couldn't be queued for another reason.
  */
@@ -33,11 +58,20 @@ int btracker_queue(struct background_tracker *b,
 		   struct policy_work **pwork);
 
 /*
+ * Hands out the next piece of work to be performed.
  * Returns -ENODATA if there's no work.
  */
 int btracker_issue(struct background_tracker *b, struct policy_work **work);
-void btracker_complete(struct background_tracker *b,
-		       struct policy_work *op);
+
+/*
+ * Informs the tracker that the work has been completed and it may forget
+ * about it.
+ */
+void btracker_complete(struct background_tracker *b, struct policy_work *op);
+
+/*
+ * Predicate to see if an origin block is already scheduled for promotion.
+ */
 bool btracker_promotion_already_present(struct background_tracker *b,
 					dm_oblock_t oblock);