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

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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>
Link: https://patch.msgid.link/20260407233927.498932-1-nissampa@cisco.com
Signed-off-by: Mark Brown <broonie@kernel.org>
This commit is contained in:
Nishanth Sampath Kumar 2026-04-07 16:39:27 -07:00 committed by Mark Brown
parent 8ad7f3b265
commit bad4bd28ab
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GPG Key ID: 24D68B725D5487D0
1 changed files with 49 additions and 0 deletions
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49
drivers/base/regmap/regmap-i2c.c
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@ -303,6 +303,50 @@ static const struct regmap_bus regmap_i2c_smbus_i2c_block_reg16 = {
.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 @@ static const struct regmap_bus *regmap_get_i2c_bus(struct i2c_client *i2c,
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))