What endianness is word in i2c_smbus_data?

What endianness is word in i2c_smbus_data?

[Andy Lutomirski]

I'm rather confused here.  In SMBUS, the "read word" operation returns
two bytes.  Just to be confusing, the SMBUS spec calls the first byte
"Data Byte Low" and the second byte "Data Byte High".  But they really
are the first and second bytes -- Read Word will return whatever Read
Byte would have as its first byte.  Let's call these bytes B1 and B2
for first and second.

The eeprom and at24 drivers expect data->word to be (B2 << 8) | B1.
That is, data->word is the cpu representation of the value on the bus
if that value is treated as little-endian.  Is that indeed the correct
interpretation?  If so, should it be documented somewhere?

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC

Re: What endianness is word in i2c_smbus_data?

[Paul Carpenter]

Andy Lutomirski wrote:
> I'm rather confused here.  In SMBUS, the "read word" operation returns
> two bytes.  Just to be confusing, the SMBUS spec calls the first byte
> "Data Byte Low" and the second byte "Data Byte High".  But they really
> are the first and second bytes -- Read Word will return whatever Read
> Byte would have as its first byte.  Let's call these bytes B1 and B2
> for first and second.

No that tells you WHATEVER the HOST endianness the bus transfer is
always treated the same, so when two bytes are combined (into a word,
the first will be the low byte for the HOST and the second will be the
high byte for the HOST. These may actually be different for the SLAVE
device.

> The eeprom and at24 drivers expect data->word to be (B2 << 8) | B1.
> That is, data->word is the cpu representation of the value on the bus
> if that value is treated as little-endian.  Is that indeed the correct
> interpretation?  If so, should it be documented somewhere?

Several I2C devices eg PCF8575 and MCP23017 often give 16 bit results as
2 byte transfers, knowing which byte order from device to bus to host
needs matching.

So knowing that a word is made up of bytes transmitted or received in a
particular order is important as some devices do transmit high byte
first.

> --Andy
> 


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