[2/3] at24: Support SMBus byte writes to 16-bit devices

Introduce at24_smbus_write_byte_data() to allow very slow (e.g.
248 B/s) write access to 16-bit EEPROM devices attached to SMBus
controllers like the Intel SCH.

Signed-off-by: Nate Case <ncase@xes-inc.com>
Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
---
 drivers/misc/eeprom/at24.c | 35 ++++++++++++++++++++++++++++++++---
 1 file changed, 32 insertions(+), 3 deletions(-)

On Thu, 3 Sep 2015 14:53:18 -0500 (CDT), Aaron Sierra wrote:
> Introduce at24_smbus_write_byte_data() to allow very slow (e.g.
> 248 B/s) write access to 16-bit EEPROM devices attached to SMBus
> controllers like the Intel SCH.
> 
> Signed-off-by: Nate Case <ncase@xes-inc.com>
> Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
> ---
>  drivers/misc/eeprom/at24.c | 35 ++++++++++++++++++++++++++++++++---
>  1 file changed, 32 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/misc/eeprom/at24.c b/drivers/misc/eeprom/at24.c
> index 4cf53a0..2ee1301 100644
> --- a/drivers/misc/eeprom/at24.c
> +++ b/drivers/misc/eeprom/at24.c
> @@ -134,6 +134,25 @@ MODULE_DEVICE_TABLE(i2c, at24_ids);
>  /*-------------------------------------------------------------------------*/
>  
>  /*
> + * Write a byte to an AT24 device using SMBus cycles.
> + */
> +static inline s32 at24_smbus_write_byte_data(struct at24_data *at24,
> +	struct i2c_client *client, u16 offset, u8 value)
> +{
> +	if (!(at24->chip.flags & AT24_FLAG_ADDR16))
> +		return i2c_smbus_write_byte_data(client, offset, value);
> +
> +	/*
> +	 * Emulate I2C multi-byte write by using SMBus "write word"
> +	 * cycle.  We split up the 16-bit offset among the "command"
> +	 * byte and the first data byte.
> +	 */
> +	return i2c_smbus_write_word_data(client,
> +		((offset >> 8) & 0xff),

Useless masking.

> +		(value << 8) | (offset & 0xff));
> +}
> +
> +/*
>   * Write block data to an AT24 device using SMBus cycles.
>   */
>  static inline s32 at24_smbus_write_i2c_block_data(struct at24_data *at24,
> @@ -401,8 +420,8 @@ static ssize_t at24_eeprom_write(struct at24_data *at24, const char *buf,
>  						client, offset, count, buf);
>  				break;
>  			case I2C_SMBUS_BYTE_DATA:
> -				status = i2c_smbus_write_byte_data(client,
> -						offset, buf[0]);
> +				status = at24_smbus_write_byte_data(at24,
> +						client, offset, buf[0]);
>  				break;
>  			}
>  
> @@ -590,7 +609,17 @@ static int at24_probe(struct i2c_client *client, const struct i2c_device_id *id)
>  		if (i2c_check_functionality(client->adapter,
>  				I2C_FUNC_SMBUS_WRITE_I2C_BLOCK)) {
>  			use_smbus_write = I2C_SMBUS_I2C_BLOCK_DATA;
> -		} else if (i2c_check_functionality(client->adapter,
> +		} else if (chip.flags & AT24_FLAG_ADDR16 &&

Parentheses around (x & y) please.

> +		    i2c_check_functionality(client->adapter,
> +					I2C_FUNC_SMBUS_WRITE_WORD_DATA)) {

Would be good to align this the same way the other two are aligned.

> +			/*
> +			 * We need SMBUS_WRITE_WORD_DATA to implement
> +			 * byte writes for 16-bit address devices.
> +			 */
> +			use_smbus_write = I2C_SMBUS_BYTE_DATA;
> +			chip.page_size = 1;
> +		} else if (!(chip.flags & AT24_FLAG_ADDR16) &&
> +			i2c_check_functionality(client->adapter,
>  				I2C_FUNC_SMBUS_WRITE_BYTE_DATA)) {
>  			use_smbus_write = I2C_SMBUS_BYTE_DATA;
>  			chip.page_size = 1;

Like patch 1/3, the code you are adding in this patch can't be tested.