regmap: add flat cache with sparse validity

The flat regcache will always assume the data in the cache is valid.
Since the cache is preferred over hardware access, this may shadow the
actual state of the device.

Add a new containing cache structure with the flat data table and a
bitmap indicating cache validity. REGCACHE_FLAT will still behave as
before, as the validity is ignored.

Define new cache type REGCACHE_FLAT_S: a flat cache with sparse
validity. The sparse validity is used to determine if a hardware access
should occur to initialize the cache on the fly, vs. at regmap init for
REGCACHE_FLAT. Contrary to REGCACHE_FLAT, this allows us to implement
regcache_ops.drop.

Signed-off-by: Sander Vanheule <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/base/regmap/internal.h

···
 					 const struct regmap_bus *bus,
 					 const struct regmap_config *config);
 
+extern struct regcache_ops regcache_flat_sparse_ops;
 extern struct regcache_ops regcache_rbtree_ops;
 extern struct regcache_ops regcache_maple_ops;
 extern struct regcache_ops regcache_flat_ops;

drivers/base/regmap/regcache-flat.c

···
 //
 // Author: Mark Brown <[e-mail]>
 
+#include <linux/bitmap.h>
+#include <linux/bitops.h>
 #include <linux/device.h>
+#include <linux/limits.h>
+#include <linux/overflow.h>
 #include <linux/seq_file.h>
 #include <linux/slab.h>
 
···
 	return regcache_get_index_by_order(map, reg);
 }
 
+struct regcache_flat_data {
+	unsigned long *valid;
+	unsigned int data[];
+};
+
 static int regcache_flat_init(struct regmap *map)
 {
 	int i;
-	unsigned int *cache;
+	size_t cache_data_size;
+	unsigned int cache_size;
+	struct regcache_flat_data *cache;
 
 	if (!map || map->reg_stride_order < 0 || !map->max_register_is_set)
 		return -EINVAL;
 
-	map->cache = kcalloc(regcache_flat_get_index(map, map->max_register)
-			     + 1, sizeof(unsigned int), map->alloc_flags);
-	if (!map->cache)
+	cache_size = regcache_flat_get_index(map, map->max_register) + 1;
+	cache_data_size = struct_size(cache, data, cache_size);
+
+	if (cache_data_size == SIZE_MAX) {
+		dev_err(map->dev, "cannot allocate regmap cache");
+		return -ENOMEM;
+	}
+
+	cache = kzalloc(cache_data_size, map->alloc_flags);
+	if (!cache)
 		return -ENOMEM;
 
-	cache = map->cache;
+	cache->valid = bitmap_zalloc(cache_size, map->alloc_flags);
+	if (!cache->valid)
+		goto err_free;
+
+	map->cache = cache;
 
 	for (i = 0; i < map->num_reg_defaults; i++) {
 		unsigned int reg = map->reg_defaults[i].reg;
 		unsigned int index = regcache_flat_get_index(map, reg);
 
-		cache[index] = map->reg_defaults[i].def;
+		cache->data[index] = map->reg_defaults[i].def;
+		__set_bit(index, cache->valid);
 	}
 
 	return 0;
+
+err_free:
+	kfree(cache);
+	return -ENOMEM;
 }
 
 static int regcache_flat_exit(struct regmap *map)
 {
-	kfree(map->cache);
+	struct regcache_flat_data *cache = map->cache;
+
+	if (cache)
+		bitmap_free(cache->valid);
+
+	kfree(cache);
 	map->cache = NULL;
 
 	return 0;
···
 static int regcache_flat_read(struct regmap *map,
 			      unsigned int reg, unsigned int *value)
 {
-	unsigned int *cache = map->cache;
+	struct regcache_flat_data *cache = map->cache;
 	unsigned int index = regcache_flat_get_index(map, reg);
 
-	*value = cache[index];
+	*value = cache->data[index];
+
+	return 0;
+}
+
+static int regcache_flat_sparse_read(struct regmap *map,
+				     unsigned int reg, unsigned int *value)
+{
+	struct regcache_flat_data *cache = map->cache;
+	unsigned int index = regcache_flat_get_index(map, reg);
+
+	if (unlikely(!test_bit(index, cache->valid)))
+		return -ENOENT;
+
+	*value = cache->data[index];
 
 	return 0;
 }
···
 static int regcache_flat_write(struct regmap *map, unsigned int reg,
 			       unsigned int value)
 {
-	unsigned int *cache = map->cache;
+	struct regcache_flat_data *cache = map->cache;
 	unsigned int index = regcache_flat_get_index(map, reg);
 
-	cache[index] = value;
+	cache->data[index] = value;
+
+	return 0;
+}
+
+static int regcache_flat_sparse_write(struct regmap *map, unsigned int reg,
+				      unsigned int value)
+{
+	struct regcache_flat_data *cache = map->cache;
+	unsigned int index = regcache_flat_get_index(map, reg);
+
+	cache->data[index] = value;
+	__set_bit(index, cache->valid);
+
+	return 0;
+}
+
+static int regcache_flat_drop(struct regmap *map, unsigned int min,
+			      unsigned int max)
+{
+	struct regcache_flat_data *cache = map->cache;
+	unsigned int bitmap_min = regcache_flat_get_index(map, min);
+	unsigned int bitmap_max = regcache_flat_get_index(map, max);
+
+	bitmap_clear(cache->valid, bitmap_min, bitmap_max + 1 - bitmap_min);
 
 	return 0;
 }
···
 	.exit = regcache_flat_exit,
 	.read = regcache_flat_read,
 	.write = regcache_flat_write,
+};
+
+struct regcache_ops regcache_flat_sparse_ops = {
+	.type = REGCACHE_FLAT_S,
+	.name = "flat-sparse",
+	.init = regcache_flat_init,
+	.exit = regcache_flat_exit,
+	.read = regcache_flat_sparse_read,
+	.write = regcache_flat_sparse_write,
+	.drop = regcache_flat_drop,
 };

drivers/base/regmap/regcache.c

···
 #include "internal.h"
 
 static const struct regcache_ops *cache_types[] = {
+	&regcache_flat_sparse_ops,
 	&regcache_rbtree_ops,
 	&regcache_maple_ops,
 	&regcache_flat_ops,

drivers/base/regmap/regmap-kunit.c

···
 		return "none";
 	case REGCACHE_FLAT:
 		return "flat";
+	case REGCACHE_FLAT_S:
+		return "flat-sparse";
 	case REGCACHE_RBTREE:
 		return "rbtree";
 	case REGCACHE_MAPLE:
···
 	{ .cache = REGCACHE_NONE, .fast_io = true },
 	{ .cache = REGCACHE_FLAT },
 	{ .cache = REGCACHE_FLAT, .fast_io = true },
+	{ .cache = REGCACHE_FLAT_S },
+	{ .cache = REGCACHE_FLAT_S, .fast_io = true },
 	{ .cache = REGCACHE_RBTREE },
 	{ .cache = REGCACHE_RBTREE, .fast_io = true },
 	{ .cache = REGCACHE_MAPLE },
···
 static const struct regmap_test_param real_cache_types_only_list[] = {
 	{ .cache = REGCACHE_FLAT },
 	{ .cache = REGCACHE_FLAT, .fast_io = true },
+	{ .cache = REGCACHE_FLAT_S },
+	{ .cache = REGCACHE_FLAT_S, .fast_io = true },
 	{ .cache = REGCACHE_RBTREE },
 	{ .cache = REGCACHE_RBTREE, .fast_io = true },
 	{ .cache = REGCACHE_MAPLE },
···
 	{ .cache = REGCACHE_FLAT,   .from_reg = 0x2002 },
 	{ .cache = REGCACHE_FLAT,   .from_reg = 0x2003 },
 	{ .cache = REGCACHE_FLAT,   .from_reg = 0x2004 },
+	{ .cache = REGCACHE_FLAT_S, .from_reg = 0 },
+	{ .cache = REGCACHE_FLAT_S, .from_reg = 0, .fast_io = true },
+	{ .cache = REGCACHE_FLAT_S, .from_reg = 0x2001 },
+	{ .cache = REGCACHE_FLAT_S, .from_reg = 0x2002 },
+	{ .cache = REGCACHE_FLAT_S, .from_reg = 0x2003 },
+	{ .cache = REGCACHE_FLAT_S, .from_reg = 0x2004 },
 	{ .cache = REGCACHE_RBTREE, .from_reg = 0 },
 	{ .cache = REGCACHE_RBTREE, .from_reg = 0, .fast_io = true },
 	{ .cache = REGCACHE_RBTREE, .from_reg = 0x2001 },
···
 KUNIT_ARRAY_PARAM(real_cache_types, real_cache_types_list, param_to_desc);
 
 static const struct regmap_test_param sparse_cache_types_list[] = {
+	{ .cache = REGCACHE_FLAT_S, .from_reg = 0 },
+	{ .cache = REGCACHE_FLAT_S, .from_reg = 0, .fast_io = true },
+	{ .cache = REGCACHE_FLAT_S, .from_reg = 0x2001 },
+	{ .cache = REGCACHE_FLAT_S, .from_reg = 0x2002 },
+	{ .cache = REGCACHE_FLAT_S, .from_reg = 0x2003 },
+	{ .cache = REGCACHE_FLAT_S, .from_reg = 0x2004 },
 	{ .cache = REGCACHE_RBTREE, .from_reg = 0 },
 	{ .cache = REGCACHE_RBTREE, .from_reg = 0, .fast_io = true },
 	{ .cache = REGCACHE_RBTREE, .from_reg = 0x2001 },
···
 	{ .cache = REGCACHE_NONE,   .val_endian = REGMAP_ENDIAN_BIG },
 	{ .cache = REGCACHE_FLAT,   .val_endian = REGMAP_ENDIAN_LITTLE },
 	{ .cache = REGCACHE_FLAT,   .val_endian = REGMAP_ENDIAN_BIG },
+	{ .cache = REGCACHE_FLAT_S, .val_endian = REGMAP_ENDIAN_LITTLE },
+	{ .cache = REGCACHE_FLAT_S, .val_endian = REGMAP_ENDIAN_BIG },
 	{ .cache = REGCACHE_RBTREE, .val_endian = REGMAP_ENDIAN_LITTLE },
 	{ .cache = REGCACHE_RBTREE, .val_endian = REGMAP_ENDIAN_BIG },
 	{ .cache = REGCACHE_MAPLE,  .val_endian = REGMAP_ENDIAN_LITTLE },
···
 static const struct regmap_test_param raw_cache_types_list[] = {
 	{ .cache = REGCACHE_FLAT,   .val_endian = REGMAP_ENDIAN_LITTLE },
 	{ .cache = REGCACHE_FLAT,   .val_endian = REGMAP_ENDIAN_BIG },
+	{ .cache = REGCACHE_FLAT_S, .val_endian = REGMAP_ENDIAN_LITTLE },
+	{ .cache = REGCACHE_FLAT_S, .val_endian = REGMAP_ENDIAN_BIG },
 	{ .cache = REGCACHE_RBTREE, .val_endian = REGMAP_ENDIAN_LITTLE },
 	{ .cache = REGCACHE_RBTREE, .val_endian = REGMAP_ENDIAN_BIG },
 	{ .cache = REGCACHE_MAPLE,  .val_endian = REGMAP_ENDIAN_LITTLE },

include/linux/regmap.h

···
 #define REGMAP_DOWNSHIFT(s)	(s)
 
 /*
- * The supported cache types, the default is no cache.  Any new caches
- * should usually use the maple tree cache unless they specifically
- * require that there are never any allocations at runtime and can't
- * provide defaults in which case they should use the flat cache.  The
- * rbtree cache *may* have some performance advantage for very low end
- * systems that make heavy use of cache syncs but is mainly legacy.
+ * The supported cache types, the default is no cache.  Any new caches should
+ * usually use the maple tree cache unless they specifically require that there
+ * are never any allocations at runtime in which case they should use the sparse
+ * flat cache.  The rbtree cache *may* have some performance advantage for very
+ * low end systems that make heavy use of cache syncs but is mainly legacy.
+ * These caches are sparse and entries will be initialized from hardware if no
+ * default has been provided.
+ * The non-sparse flat cache is provided for compatibility with existing users
+ * and will zero-initialize cache entries for which no defaults are provided.
+ * New users should use the sparse flat cache.
  */
 enum regcache_type {
 	REGCACHE_NONE,
 	REGCACHE_RBTREE,
 	REGCACHE_FLAT,
 	REGCACHE_MAPLE,
+	REGCACHE_FLAT_S,
 };
 
 /**