regmap: Add reg_default_cb callback for flat cache defaults

Commit e062bdfdd6 ("regmap: warn users about uninitialized flat cache")
warns when REGCACHE_FLAT is used without full defaults. This causes
false positives on hardware where many registers reset to zero but are
not listed in reg_defaults, forcing drivers to maintain large tables
just to silence the warning.

Add a reg_default_cb() hook so drivers can supply defaults for registers
not present in reg_defaults when populating REGCACHE_FLAT. This keeps
the warning quiet for known zero-reset registers without bloating
tables. Provide a generic regmap_default_zero_cb() helper for drivers
that need zero defaults.

The hook is only used for REGCACHE_FLAT; the core does not
check readable/writeable access, so drivers must provide readable_reg/
writeable_reg callbacks and handle holes in the register map.

Signed-off-by: Sheetal <sheetal@nvidia.com>
Link: https://patch.msgid.link/20260123095346.1258556-3-sheetal@nvidia.com
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/base/regmap/internal.h

@@ -117,6 +117,9 @@ struct regmap {
 		    void *val_buf, size_t val_size);
 	int (*write)(void *context, const void *data, size_t count);
 
+	int (*reg_default_cb)(struct device *dev, unsigned int reg,
+			      unsigned int *val);
+
 	unsigned long read_flag_mask;
 	unsigned long write_flag_mask;
 

drivers/base/regmap/regcache-flat.c

@@ -79,6 +79,25 @@ static int regcache_flat_populate(struct regmap *map)
 		__set_bit(index, cache->valid);
 	}
 
+	if (map->reg_default_cb) {
+		dev_dbg(map->dev,
+			"Populating regcache_flat using reg_default_cb callback\n");
+
+		for (i = 0; i <= map->max_register; i += map->reg_stride) {
+			unsigned int index = regcache_flat_get_index(map, i);
+			unsigned int value;
+
+			if (test_bit(index, cache->valid))
+				continue;
+
+			if (map->reg_default_cb(map->dev, i, &value))
+				continue;
+
+			cache->data[index] = value;
+			__set_bit(index, cache->valid);
+		}
+	}
+
 	return 0;
 }
 

drivers/base/regmap/regcache.c

@@ -223,7 +223,8 @@ int regcache_init(struct regmap *map, const struct regmap_config *config)
 			goto err_free;
 	}
 
-	if (map->num_reg_defaults && map->cache_ops->populate) {
+	if (map->cache_ops->populate &&
+	    (map->num_reg_defaults || map->reg_default_cb)) {
 		dev_dbg(map->dev, "Populating %s cache\n", map->cache_ops->name);
 		map->lock(map->lock_arg);
 		ret = map->cache_ops->populate(map);

drivers/base/regmap/regmap.c

@@ -813,6 +813,7 @@ struct regmap *__regmap_init(struct device *dev,
 	map->precious_reg = config->precious_reg;
 	map->writeable_noinc_reg = config->writeable_noinc_reg;
 	map->readable_noinc_reg = config->readable_noinc_reg;
+	map->reg_default_cb = config->reg_default_cb;
 	map->cache_type = config->cache_type;
 
 	spin_lock_init(&map->async_lock);
@@ -1435,6 +1436,7 @@ int regmap_reinit_cache(struct regmap *map, const struct regmap_config *config)
 	map->precious_reg = config->precious_reg;
 	map->writeable_noinc_reg = config->writeable_noinc_reg;
 	map->readable_noinc_reg = config->readable_noinc_reg;
+	map->reg_default_cb = config->reg_default_cb;
 	map->cache_type = config->cache_type;
 
 	ret = regmap_set_name(map, config);

include/linux/regmap.h

@@ -359,6 +359,10 @@ typedef void (*regmap_unlock)(void *);
  * @reg_defaults: Power on reset values for registers (for use with
  *                register cache support).
  * @num_reg_defaults: Number of elements in reg_defaults.
+ * @reg_default_cb: Optional callback to return default values for registers
+ *                  not listed in reg_defaults. This is only used for
+ *                  REGCACHE_FLAT population; drivers must ensure the readable_reg/
+ *                  writeable_reg callbacks are defined to handle holes.
  *
  * @read_flag_mask: Mask to be set in the top bytes of the register when doing
  *                  a read.
@@ -449,6 +453,8 @@ struct regmap_config {
 	const struct regmap_access_table *rd_noinc_table;
 	const struct reg_default *reg_defaults;
 	unsigned int num_reg_defaults;
+	int (*reg_default_cb)(struct device *dev, unsigned int reg,
+			      unsigned int *def);
 	enum regcache_type cache_type;
 	const void *reg_defaults_raw;
 	unsigned int num_reg_defaults_raw;
@@ -1349,6 +1355,14 @@ static inline int regmap_write_bits(struct regmap *map, unsigned int reg,
 	return regmap_update_bits_base(map, reg, mask, val, NULL, false, true);
 }
 
+static inline int regmap_default_zero_cb(struct device *dev,
+					 unsigned int reg,
+					 unsigned int *def)
+{
+	*def = 0;
+	return 0;
+}
+
 int regmap_get_val_bytes(struct regmap *map);
 int regmap_get_max_register(struct regmap *map);
 int regmap_get_reg_stride(struct regmap *map);