regmap-i2c: add SMBus byte/word reg16 bus for adapters lacking I2C_FUNC_I2C

patches-sonic/cisco-regmap-i2c-smbus-byte-word-reg16.patch

@@ -0,0 +1,112 @@
+From: Nishanth Sampath Kumar <nissampa@cisco.com>
+Date: Mon, 07 Apr 2026 12:00:00 -0700
+Subject: [PATCH v2] regmap-i2c: add SMBus byte/word reg16 bus for adapters
+ lacking I2C_FUNC_I2C
+
+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))