---- >8 ----
Indeed, it is an ugly workaround, but we need to pass an extra
information to regmap. Another approach I'm thinking about is to
introduce special macro for ->max_register initialization:

    #define REGMAP_MAX_REGISTER_IS_0 UINT_MAX
    #define REGMAP_MAX_REGISTER(_max) ((_max) ?: REGMAP_MAX_REGISTER_IS_0)

and initialize regmap_config in the following way:

    struct regmap_config config = {
        ...
        .max_register = REGMAP_MAX_REGISTER(...),
    };

This will eliminate the need for an extra flag in the config. But this
approach has another disadvantage, it imposes an extra requirement,
that is not satisfied by existing code.
---- >8 ----

When regmap consists of single register, 'regmap' subsystem is unable to
understand whether ->max_register is set or not, because in both cases it
is equal to zero. It leads to that the logic based on value of
->max_register doesn't work. For example using of REGCACHE_FLAT fails.

This patch introduces an extra parameter to regmap config, indicating
that zero value in ->max_register is authentic.

Signed-off-by: Jan Dakinevich <jan.dakinevich@salutedevices.com>
---
 drivers/base/regmap/internal.h      |  1 +
 drivers/base/regmap/regcache-flat.c |  2 +-
 drivers/base/regmap/regcache.c      |  4 +++-
 drivers/base/regmap/regmap.c        | 10 ++++++----
 include/linux/regmap.h              |  5 +++++
 5 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/drivers/base/regmap/internal.h b/drivers/base/regmap/internal.h
index 583dd5d7d46b..bcdb25bec77c 100644
--- a/drivers/base/regmap/internal.h
+++ b/drivers/base/regmap/internal.h
@@ -93,6 +93,7 @@ struct regmap {
 #endif
 
 	unsigned int max_register;
+	bool max_register_is_set;
 	bool (*writeable_reg)(struct device *dev, unsigned int reg);
 	bool (*readable_reg)(struct device *dev, unsigned int reg);
 	bool (*volatile_reg)(struct device *dev, unsigned int reg);
diff --git a/drivers/base/regmap/regcache-flat.c b/drivers/base/regmap/regcache-flat.c
index b7e4b2464102..9b17c77dec9d 100644
--- a/drivers/base/regmap/regcache-flat.c
+++ b/drivers/base/regmap/regcache-flat.c
@@ -23,7 +23,7 @@ static int regcache_flat_init(struct regmap *map)
 	int i;
 	unsigned int *cache;
 
-	if (!map || map->reg_stride_order < 0 || !map->max_register)
+	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)
diff --git a/drivers/base/regmap/regcache.c b/drivers/base/regmap/regcache.c
index ac63a73ccdaa..2e41cb12b8e2 100644
--- a/drivers/base/regmap/regcache.c
+++ b/drivers/base/regmap/regcache.c
@@ -187,8 +187,10 @@ int regcache_init(struct regmap *map, const struct regmap_config *config)
 			return 0;
 	}
 
-	if (!map->max_register && map->num_reg_defaults_raw)
+	if (!map->max_register_is_set && map->num_reg_defaults_raw) {
 		map->max_register = (map->num_reg_defaults_raw  - 1) * map->reg_stride;
+		map->max_register_is_set = true;
+	}
 
 	if (map->cache_ops->init) {
 		dev_dbg(map->dev, "Initializing %s cache\n",
diff --git a/drivers/base/regmap/regmap.c b/drivers/base/regmap/regmap.c
index 6db77d8e45f9..5cb425f6f02d 100644
--- a/drivers/base/regmap/regmap.c
+++ b/drivers/base/regmap/regmap.c
@@ -89,7 +89,7 @@ EXPORT_SYMBOL_GPL(regmap_check_range_table);
 
 bool regmap_writeable(struct regmap *map, unsigned int reg)
 {
-	if (map->max_register && reg > map->max_register)
+	if (map->max_register_is_set && reg > map->max_register)
 		return false;
 
 	if (map->writeable_reg)
@@ -112,7 +112,7 @@ bool regmap_cached(struct regmap *map, unsigned int reg)
 	if (!map->cache_ops)
 		return false;
 
-	if (map->max_register && reg > map->max_register)
+	if (map->max_register_is_set && reg > map->max_register)
 		return false;
 
 	map->lock(map->lock_arg);
@@ -129,7 +129,7 @@ bool regmap_readable(struct regmap *map, unsigned int reg)
 	if (!map->reg_read)
 		return false;
 
-	if (map->max_register && reg > map->max_register)
+	if (map->max_register_is_set && reg > map->max_register)
 		return false;
 
 	if (map->format.format_write)
@@ -787,6 +787,7 @@ struct regmap *__regmap_init(struct device *dev,
 	map->bus = bus;
 	map->bus_context = bus_context;
 	map->max_register = config->max_register;
+	map->max_register_is_set = map->max_register ?: config->max_register_is_0;
 	map->wr_table = config->wr_table;
 	map->rd_table = config->rd_table;
 	map->volatile_table = config->volatile_table;
@@ -1412,6 +1413,7 @@ int regmap_reinit_cache(struct regmap *map, const struct regmap_config *config)
 	regmap_debugfs_exit(map);
 
 	map->max_register = config->max_register;
+	map->max_register_is_set = map->max_register ?: config->max_register_is_0;
 	map->writeable_reg = config->writeable_reg;
 	map->readable_reg = config->readable_reg;
 	map->volatile_reg = config->volatile_reg;
@@ -3383,7 +3385,7 @@ EXPORT_SYMBOL_GPL(regmap_get_val_bytes);
  */
 int regmap_get_max_register(struct regmap *map)
 {
-	return map->max_register ? map->max_register : -EINVAL;
+	return map->max_register_is_set ? map->max_register : -EINVAL;
 }
 EXPORT_SYMBOL_GPL(regmap_get_max_register);
 
diff --git a/include/linux/regmap.h b/include/linux/regmap.h
index c9182a47736e..b743241cfb7c 100644
--- a/include/linux/regmap.h
+++ b/include/linux/regmap.h
@@ -332,6 +332,10 @@ typedef void (*regmap_unlock)(void *);
  * @io_port:	  Support IO port accessors. Makes sense only when MMIO vs. IO port
  *		  access can be distinguished.
  * @max_register: Optional, specifies the maximum valid register address.
+ * @max_register_is_0: Optional, specifies that zero value in @max_register
+ *                     should be taken into account. This is a workaround to
+ *                     apply handling of @max_register for regmap that contains
+ *                     only one register.
  * @wr_table:     Optional, points to a struct regmap_access_table specifying
  *                valid ranges for write access.
  * @rd_table:     As above, for read access.
@@ -422,6 +426,7 @@ struct regmap_config {
 	bool io_port;
 
 	unsigned int max_register;
+	bool max_register_is_0;
 	const struct regmap_access_table *wr_table;
 	const struct regmap_access_table *rd_table;
 	const struct regmap_access_table *volatile_table;
-- 
2.34.1

On Fri, 26 Jan 2024 23:08:36 +0300, Jan Dakinevich wrote:
> When regmap consists of single register, 'regmap' subsystem is unable to
> understand whether ->max_register is set or not, because in both cases it
> is equal to zero. It leads to that the logic based on value of
> ->max_register doesn't work. For example using of REGCACHE_FLAT fails.
> 
> This patch introduces an extra parameter to regmap config, indicating
> that zero value in ->max_register is authentic.
> 
> [...]

Applied to

   https://git.kernel.org/pub/scm/linux/kernel/git/broonie/regmap.git for-next

Thanks!

[1/1] regmap: rework ->max_register handling
      commit: 0ec74ad3c157bd4bcbcc8b294777733687e8cd2a

All being well this means that it will be integrated into the linux-next
tree (usually sometime in the next 24 hours) and sent to Linus during
the next merge window (or sooner if it is a bug fix), however if
problems are discovered then the patch may be dropped or reverted.

You may get further e-mails resulting from automated or manual testing
and review of the tree, please engage with people reporting problems and
send followup patches addressing any issues that are reported if needed.

If any updates are required or you are submitting further changes they
should be sent as incremental updates against current git, existing
patches will not be replaced.

Please add any relevant lists and maintainers to the CCs when replying
to this mail.

Thanks,
Mark