[PATCH] misc: i2c_eeprom: fix at24c32 offset_len

[Rasmus Villemoes]

On 28/04/2022 13.11, Eugen Hristev wrote:
> According to at24c32 datasheet:
> 
> RANDOM READ: A random read requires a “dummy” byte write sequence to load in
> the dataword address. Once the device address word and data word address are
> clocked in and acknowledged by the EEPROM, the microcontroller must generate
> another start condition.
> 
> BYTE WRITE: A write operation requires two 8-bit data word addresses following
> the device address word and acknowledgment. Upon receipt of this address, the
> EEPROM will again respond with a zero and then clock in the first 8-bit data
> word.
> 
> From this, my understanding is that dataword is 1 byte, and when reading the
> offset is just 1 byte.

Yes, you read data byte by byte, but that doesn't mean all those bytes
can be addressed using a single byte...

>  drivers/misc/i2c_eeprom.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/misc/i2c_eeprom.c b/drivers/misc/i2c_eeprom.c
> index 89a450d0f8..c8c67cf028 100644
> --- a/drivers/misc/i2c_eeprom.c
> +++ b/drivers/misc/i2c_eeprom.c
> @@ -230,7 +230,7 @@ static const struct i2c_eeprom_drv_data atmel24c32_data = {
>  	.size = 4096,
>  	.pagesize = 32,
>  	.addr_offset_mask = 0,
> -	.offset_len = 2,
> +	.offset_len = 1,
>  };

But this can't be correct, because how could one then possibly address
all 4096 bytes? Note that some eeproms larger than 256 bytes exist but
still use a 1-byte address; that's because they then respond to multiple
i2c addresses - that's what the "addr_offset_mask" is about.

Something else must be going on in your case, I think. Are you sure the
device tree is correct, i.e. that the eeprom is actually that one and
not one that does indeed use 1-byte addressing? I got curious and
downloaded the "SAM9X60-EK Board Design Files" from
https://www.microchip.com/en-us/development-tool/DT100126#Documentation,
inside which one finds mention of a "MCHP MEMORY SERIAL EEPROM 2Kb I2C
24AA025E48T-I/OT SOT-23-6", and 2Kb == 256 byte.

If you have the physical board handy, I'd try to locate the eeprom and
see what's printed on it.

Rasmus