AMD PIIX4 SMBus adapters, present on AMD SP5/EPYC-based platforms
(including Cisco 8000 series routers), support SMBUS_BYTE_DATA and
SMBUS_WORD_DATA but lack I2C_FUNC_I2C and I2C_FUNC_SMBUS_I2C_BLOCK.

When at24 (or any driver) requests a regmap with reg_bits=16 and
val_bits=8 on such an adapter, regmap_get_i2c_bus() finds no matching
bus and returns -ENOTSUPP.  The existing regmap_i2c_smbus_i2c_block_reg16
bus type already implements 16-bit addressed reads using only
write_byte_data() + read_byte() primitives, but its selection is gated
on I2C_FUNC_SMBUS_I2C_BLOCK which these adapters lack.

Add a new regmap_smbus_byte_word_reg16 bus that:

  READ:  reuses regmap_i2c_smbus_i2c_read_reg16() -- sets the 16-bit
         address via write_byte_data(addr_lo, addr_hi), then reads
         bytes sequentially via read_byte() (EEPROM auto-increments).
         Requires only SMBUS_BYTE_DATA.

  WRITE: uses write_word_data(addr_hi, (data << 8) | addr_lo) to
         encode one data byte per SMBus WORD transaction.
         Requires only SMBUS_WORD_DATA.  Single-byte writes only.

The new bus is selected in regmap_get_i2c_bus() when reg_bits=16,
val_bits=8, and the adapter has SMBUS_BYTE_DATA | SMBUS_WORD_DATA but
not I2C_FUNC_I2C or SMBUS_I2C_BLOCK.  The branch is placed after the
existing I2C_BLOCK_reg16 check so adapters with full block support
continue to use the faster path.

This fixes at24 EEPROM probe failures on PIIX4:
  at24 3-0055: probe with driver at24 failed with error -524

No driver changes are required -- at24 already passes reg_bits=16 to
devm_regmap_init_i2c(), which now succeeds.

Signed-off-by: Nishanth Sampath Kumar <nissampa@cisco.com>
---
v1 -> v2: Moved fix from at24 driver into regmap-i2c core per review
          feedback from Bartosz Golaszewski and Mark Brown.

 drivers/base/regmap/regmap-i2c.c | 49 ++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

--- a/drivers/base/regmap/regmap-i2c.c
+++ b/drivers/base/regmap/regmap-i2c.c
@@ -303,6 +303,50 @@
 	.max_raw_write = I2C_SMBUS_BLOCK_MAX - 2,
 };
 
+/*
+ * SMBus byte/word reg16 support for adapters that have SMBUS_BYTE_DATA
+ * and SMBUS_WORD_DATA but lack I2C_FUNC_I2C and I2C_FUNC_SMBUS_I2C_BLOCK,
+ * such as the AMD PIIX4.
+ *
+ * READ:  set 16-bit EEPROM address via write_byte_data(addr_lo, addr_hi),
+ *        then sequentially read bytes via read_byte() (EEPROM auto-
+ *        increments the address pointer).  Same as the I2C-block reg16
+ *        read path above.
+ *
+ * WRITE: encode the low address byte and data into a word transaction:
+ *        write_word_data(addr_hi, (data_byte << 8) | addr_lo).
+ *        Only single-byte writes are supported (one value per transaction).
+ */
+static int regmap_smbus_word_write_reg16(void *context, const void *data,
+					 size_t count)
+{
+	struct device *dev = context;
+	struct i2c_client *i2c = to_i2c_client(dev);
+	u8 addr_hi, addr_lo, val;
+
+	/*
+	 * data layout: [addr_hi, addr_lo, val0, val1, ...].
+	 * Only single-byte value writes are supported; multi-byte would
+	 * require raw I2C (or repeated word writes with incrementing address).
+	 */
+	if (count != 3)
+		return -EINVAL;
+
+	addr_hi = ((u8 *)data)[0];
+	addr_lo = ((u8 *)data)[1];
+	val = ((u8 *)data)[2];
+
+	return i2c_smbus_write_word_data(i2c, addr_hi,
+					 cpu_to_le16(((u16)val << 8) | addr_lo));
+}
+
+static const struct regmap_bus regmap_smbus_byte_word_reg16 = {
+	.write = regmap_smbus_word_write_reg16,
+	.read = regmap_i2c_smbus_i2c_read_reg16,
+	.max_raw_read = I2C_SMBUS_BLOCK_MAX - 2,
+	.max_raw_write = 1,
+};
+
 static const struct regmap_bus *regmap_get_i2c_bus(struct i2c_client *i2c,
 					const struct regmap_config *config)
 {
@@ -321,6 +365,11 @@
 		i2c_check_functionality(i2c->adapter,
 					I2C_FUNC_SMBUS_I2C_BLOCK))
 		bus = &regmap_i2c_smbus_i2c_block_reg16;
+	else if (config->val_bits == 8 && config->reg_bits == 16 &&
+		 i2c_check_functionality(i2c->adapter,
+					I2C_FUNC_SMBUS_BYTE_DATA |
+					I2C_FUNC_SMBUS_WORD_DATA))
+		bus = &regmap_smbus_byte_word_reg16;
 	else if (config->val_bits == 16 && config->reg_bits == 8 &&
 		 i2c_check_functionality(i2c->adapter,
 					 I2C_FUNC_SMBUS_WORD_DATA))

On Tue, 07 Apr 2026 16:39:27 -0700, Nishanth Sampath Kumar wrote:
> regmap-i2c: add SMBus byte/word reg16 bus for adapters lacking I2C_FUNC_I2C

Applied to

   https://git.kernel.org/pub/scm/linux/kernel/git/broonie/regmap.git for-7.1

Thanks!

[1/1] regmap-i2c: add SMBus byte/word reg16 bus for adapters lacking I2C_FUNC_I2C
      https://git.kernel.org/broonie/sound/c/f7b242428c6c

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

On Tue, Apr 07, 2026 at 04:39:27PM -0700, Nishanth Sampath Kumar wrote:

> +static const struct regmap_bus regmap_smbus_byte_word_reg16 = {
> +	.write = regmap_smbus_word_write_reg16,
> +	.read = regmap_i2c_smbus_i2c_read_reg16,

Not an issue for this patch but the existing function is broken on big
endian systems, it needs to peer into reg with u8 pointers since the
core already put it into wire format.