[RFC PATCH v2 1/3] regmap: Add flat_cache_default_is_zero flag for flat cache

Sheetal .
2026-01-12T04:30:58Z

From: Sheetal <sheetal@nvidia.com>

Commit e062bdfdd6ad ("regmap: warn users about uninitialized flat
cache") added a warning for drivers using REGCACHE_FLAT when reading
registers not present in reg_defaults.

For hardware where registers have a power-on-reset value of zero
or drivers that wish to treat zero as a valid cache default, adding
all such registers to reg_defaults has drawbacks:

1. Maintenance burden: Drivers must list every readable register
regardless of its reset value.

2. No functional benefit: Entries like { REG, 0x0 } only set the
validity bit; the flat cache value is already zero.

3. Code bloat: Large reg_defaults arrays increase driver size.

Add a flat_cache_default_is_zero flag to struct regmap_config. When
set, the flat cache marks registers as valid on first read instead
of warning.

The valid bit is set on first read rather than marking all registers
valid at init time for the following reasons:
- Avoids writes to register holes or unused addresses during sync.
- Safer for drivers that don't have writeable_reg callback defined.

Signed-off-by: Sheetal <sheetal@nvidia.com>
---
drivers/base/regmap/internal.h | 2 ++
drivers/base/regmap/regcache-flat.c | 12 ++++++++----
drivers/base/regmap/regcache.c | 1 +
include/linux/regmap.h | 1 +
4 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/base/regmap/internal.h b/drivers/base/regmap/internal.h
index 1477329410ec..xxxxxxxxxxxx 100644
--- a/drivers/base/regmap/internal.h
+++ b/drivers/base/regmap/internal.h
@@ -157,6 +157,8 @@ struct regmap {
bool cache_dirty;
/* if set, the HW registers are known to match map->reg_defaults */
bool no_sync_defaults;
+ /* if set, zero is a valid default for REGCACHE_FLAT cache type registers not in reg_defaults */
+ bool flat_cache_default_is_zero;

struct reg_sequence *patch;
int patch_regs;
diff --git a/drivers/base/regmap/regcache-flat.c b/drivers/base/regmap/regcache-flat.c
index 53cc59c84e2f..xxxxxxxxxxxx 100644
--- a/drivers/base/regmap/regcache-flat.c
+++ b/drivers/base/regmap/regcache-flat.c
@@ -88,10 +88,14 @@ static int regcache_flat_read(struct regmap *map,
struct regcache_flat_data *cache = map->cache;
unsigned int index = regcache_flat_get_index(map, reg);

- /* legacy behavior: ignore validity, but warn the user */
- if (unlikely(!test_bit(index, cache->valid)))
- dev_warn_once(map->dev,
- "using zero-initialized flat cache, this may cause unexpected behavior");
+ /* legacy behavior: ignore validity, but warn if zero is not a valid default */
+ if (unlikely(!test_bit(index, cache->valid))) {
+ if (map->flat_cache_default_is_zero)
+ set_bit(index, cache->valid);
+ else
+ dev_warn_once(map->dev,
+ "using zero-initialized flat cache, this may cause unexpected behavior");
+ }

*value = cache->data[index];

diff --git a/drivers/base/regmap/regcache.c b/drivers/base/regmap/regcache.c
index 319c342bf5a0..xxxxxxxxxxxx 100644
--- a/drivers/base/regmap/regcache.c
+++ b/drivers/base/regmap/regcache.c
@@ -177,6 +177,7 @@ int regcache_init(struct regmap *map, const struct regmap_config *config)
map->reg_defaults_raw = config->reg_defaults_raw;
map->cache_word_size = BITS_TO_BYTES(config->val_bits);
map->cache_size_raw = map->cache_word_size * config->num_reg_defaults_raw;
+ map->flat_cache_default_is_zero = config->flat_cache_default_is_zero;

map->cache = NULL;
map->cache_ops = cache_types[i];
diff --git a/include/linux/regmap.h b/include/linux/regmap.h
index b0b9be750d93..xxxxxxxxxxxx 100644
--- a/include/linux/regmap.h
+++ b/include/linux/regmap.h
@@ -452,6 +452,7 @@ struct regmap_config {
enum regcache_type cache_type;
const void *reg_defaults_raw;
unsigned int num_reg_defaults_raw;
+ bool flat_cache_default_is_zero;

unsigned long read_flag_mask;
unsigned long write_flag_mask;
--
2.34.1