SMBUS_READ
smbus_cmd = SMBUS_ENCODE_RW(cmd, SMBUS_READ);
smbus_cmd = SMBUS_ENCODE_RW(smbus_cmd, SMBUS_READ);
cmd = SMBUS_ENCODE_RW(cmd, SMBUS_READ);
cmd, 0, SMBUS_READ, res)
if (operation == SMBUS_READ)