dm cache: Add some documentation to dm-cache-background-tracker.h

/*----------------------------------------------------------------*/

/*

 * 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.

 */

		   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);