question about comment in i2c.h

[h-iwamoto at kit.hi-ho.ne.jp (Hideki IWAMOTO)]

I want to ask about the following comment in i2c.h.
What does "one more for read length in block process call" mean?
In i2c_smbus_block_process_call and i2c_smbus_xfer_emulated,
block[0] is used for the read length.

 union i2c_smbus_data {
         __u8 byte;
         __u16 word;
         __u8 block[I2C_SMBUS_BLOCK_MAX + 3]; /* block[0] is used for length */
                           /* one more for read length in block process call */
                                                     /* and one more for PEC */
 };


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Hideki IWAMOTO  h-iwamoto at kit.hi-ho.ne.jp