/*
   * Copyright (C) 2012 Red Hat. All rights reserved.
   *
   * This file is released under the GPL.
   */
  
  #ifndef DM_CACHE_POLICY_H
  #define DM_CACHE_POLICY_H
  
  #include "dm-cache-block-types.h"
  
  #include <linux/device-mapper.h>
  
  /*----------------------------------------------------------------*/
  
  /* FIXME: make it clear which methods are optional.  Get debug policy to
   * double check this at start.
   */
  
  /*
   * The cache policy makes the important decisions about which blocks get to
   * live on the faster cache device.
   *
   * When the core target has to remap a bio it calls the 'map' method of the
   * policy.  This returns an instruction telling the core target what to do.
   *
   * POLICY_HIT:
   *   That block is in the cache.  Remap to the cache and carry on.
   *
   * POLICY_MISS:
   *   This block is on the origin device.  Remap and carry on.
   *
   * POLICY_NEW:
   *   This block is currently on the origin device, but the policy wants to
   *   move it.  The core should:
   *
   *   - hold any further io to this origin block
   *   - copy the origin to the given cache block
   *   - release all the held blocks
   *   - remap the original block to the cache
   *
   * POLICY_REPLACE:
   *   This block is currently on the origin device.  The policy wants to
   *   move it to the cache, with the added complication that the destination
   *   cache block needs a writeback first.  The core should:
   *
   *   - hold any further io to this origin block
   *   - hold any further io to the origin block that's being written back
   *   - writeback
   *   - copy new block to cache
   *   - release held blocks
   *   - remap bio to cache and reissue.
   *
   * Should the core run into trouble while processing a POLICY_NEW or
   * POLICY_REPLACE instruction it will roll back the policies mapping using
   * remove_mapping() or force_mapping().  These methods must not fail.  This
   * approach avoids having transactional semantics in the policy (ie, the
   * core informing the policy when a migration is complete), and hence makes
   * it easier to write new policies.
   *
   * In general policy methods should never block, except in the case of the
   * map function when can_migrate is set.  So be careful to implement using
   * bounded, preallocated memory.
   */
  enum policy_operation {
  	POLICY_HIT,
  	POLICY_MISS,
  	POLICY_NEW,
  	POLICY_REPLACE
  };
  
  /*
   * This is the instruction passed back to the core target.
   */
  struct policy_result {
  	enum policy_operation op;
  	dm_oblock_t old_oblock;	/* POLICY_REPLACE */
  	dm_cblock_t cblock;	/* POLICY_HIT, POLICY_NEW, POLICY_REPLACE */
  };
  
  typedef int (*policy_walk_fn)(void *context, dm_cblock_t cblock,
  			      dm_oblock_t oblock, uint32_t hint);
  
  /*
   * The cache policy object.  Just a bunch of methods.  It is envisaged that
   * this structure will be embedded in a bigger, policy specific structure
   * (ie. use container_of()).
   */
  struct dm_cache_policy {
  
  	/*
  	 * FIXME: make it clear which methods are optional, and which may
  	 * block.
  	 */
  
  	/*
  	 * Destroys this object.
  	 */
  	void (*destroy)(struct dm_cache_policy *p);
  
  	/*
  	 * See large comment above.
  	 *
  	 * oblock      - the origin block we're interested in.
  	 *
  	 * can_block - indicates whether the current thread is allowed to
  	 *             block.  -EWOULDBLOCK returned if it can't and would.
  	 *
  	 * can_migrate - gives permission for POLICY_NEW or POLICY_REPLACE
  	 *               instructions.  If denied and the policy would have
  	 *               returned one of these instructions it should
  	 *               return -EWOULDBLOCK.
  	 *
  	 * discarded_oblock - indicates whether the whole origin block is
  	 *               in a discarded state (FIXME: better to tell the
  	 *               policy about this sooner, so it can recycle that
  	 *               cache block if it wants.)
  	 * bio         - the bio that triggered this call.
  	 * result      - gets filled in with the instruction.
  	 *
  	 * May only return 0, or -EWOULDBLOCK (if !can_migrate)
  	 */
  	int (*map)(struct dm_cache_policy *p, dm_oblock_t oblock,
  		   bool can_block, bool can_migrate, bool discarded_oblock,
  		   struct bio *bio, struct policy_result *result);
  
  	/*
  	 * Sometimes we want to see if a block is in the cache, without
  	 * triggering any update of stats.  (ie. it's not a real hit).
  	 *
  	 * Must not block.
  	 *
  	 * Returns 0 if in cache, -ENOENT if not, < 0 for other errors
  	 * (-EWOULDBLOCK would be typical).
  	 */
  	int (*lookup)(struct dm_cache_policy *p, dm_oblock_t oblock, dm_cblock_t *cblock);
  
  	void (*set_dirty)(struct dm_cache_policy *p, dm_oblock_t oblock);
  	void (*clear_dirty)(struct dm_cache_policy *p, dm_oblock_t oblock);
  
  	/*
  	 * Called when a cache target is first created.  Used to load a
  	 * mapping from the metadata device into the policy.
  	 */
  	int (*load_mapping)(struct dm_cache_policy *p, dm_oblock_t oblock,
  			    dm_cblock_t cblock, uint32_t hint, bool hint_valid);
  
  	int (*walk_mappings)(struct dm_cache_policy *p, policy_walk_fn fn,
  			     void *context);
  
  	/*
  	 * Override functions used on the error paths of the core target.
  	 * They must succeed.
  	 */
  	void (*remove_mapping)(struct dm_cache_policy *p, dm_oblock_t oblock);
  	void (*force_mapping)(struct dm_cache_policy *p, dm_oblock_t current_oblock,
  			      dm_oblock_t new_oblock);
  
  	/*
  	 * This is called via the invalidate_cblocks message.  It is
  	 * possible the particular cblock has already been removed due to a
  	 * write io in passthrough mode.  In which case this should return
  	 * -ENODATA.
  	 */
  	int (*remove_cblock)(struct dm_cache_policy *p, dm_cblock_t cblock);
  
  	/*
  	 * Provide a dirty block to be written back by the core target.
  	 *
  	 * Returns:
  	 *
  	 * 0 and @cblock,@oblock: block to write back provided
  	 *
  	 * -ENODATA: no dirty blocks available
  	 */
  	int (*writeback_work)(struct dm_cache_policy *p, dm_oblock_t *oblock, dm_cblock_t *cblock);
  
  	/*
  	 * How full is the cache?
  	 */
  	dm_cblock_t (*residency)(struct dm_cache_policy *p);
  
  	/*
  	 * Because of where we sit in the block layer, we can be asked to
  	 * map a lot of little bios that are all in the same block (no
  	 * queue merging has occurred).  To stop the policy being fooled by
  	 * these the core target sends regular tick() calls to the policy.
  	 * The policy should only count an entry as hit once per tick.
  	 */
  	void (*tick)(struct dm_cache_policy *p);
  
  	/*
  	 * Configuration.
  	 */
  	int (*emit_config_values)(struct dm_cache_policy *p,
  				  char *result, unsigned maxlen);
  	int (*set_config_value)(struct dm_cache_policy *p,
  				const char *key, const char *value);
  
  	/*
  	 * Book keeping ptr for the policy register, not for general use.
  	 */
  	void *private;
  };
  
  /*----------------------------------------------------------------*/
  
  /*
   * We maintain a little register of the different policy types.
   */
  #define CACHE_POLICY_NAME_SIZE 16
  #define CACHE_POLICY_VERSION_SIZE 3
  
  struct dm_cache_policy_type {
  	/* For use by the register code only. */
  	struct list_head list;
  
  	/*
  	 * Policy writers should fill in these fields.  The name field is
  	 * what gets passed on the target line to select your policy.
  	 */
  	char name[CACHE_POLICY_NAME_SIZE];
  	unsigned version[CACHE_POLICY_VERSION_SIZE];
  
  	/*
  	 * For use by an alias dm_cache_policy_type to point to the
  	 * real dm_cache_policy_type.
  	 */
  	struct dm_cache_policy_type *real;
  
  	/*
  	 * Policies may store a hint for each each cache block.
  	 * Currently the size of this hint must be 0 or 4 bytes but we
  	 * expect to relax this in future.
  	 */
  	size_t hint_size;
  
  	struct module *owner;
  	struct dm_cache_policy *(*create)(dm_cblock_t cache_size,
  					  sector_t origin_size,
  					  sector_t block_size);
  };
  
  int dm_cache_policy_register(struct dm_cache_policy_type *type);
  void dm_cache_policy_unregister(struct dm_cache_policy_type *type);
  
  /*----------------------------------------------------------------*/
  
  #endif	/* DM_CACHE_POLICY_H */