On July 15, 2014 9:56:17 AM GMT+01:00, Antonio Borneo <borneo.antonio@xxxxxxxxx> wrote: >On Tue, Jul 15, 2014 at 4:21 AM, Jonathan Cameron <jic23@xxxxxxxxxx> >wrote: >> On 14/07/14 18:32, Philippe Reynes wrote: > >Hi Jonathan, > >regarding your comment below > ><snip> >>> +static int max5821_get_value(struct iio_dev *indio_dev, >>> + int *val, int channel) >>> +{ >>> + struct max5821_data *data = iio_priv(indio_dev); >>> + struct i2c_client *client = data->client; >>> + u8 outbuf[1]; >>> + u8 inbuf[2]; >>> + int ret; >>> + >>> + switch (channel) { >>> + case 0: >>> + outbuf[0] = MAX5821_READ_DAC_A_COMMAND; >>> + break; >>> + case 1: >>> + outbuf[0] = MAX5821_READ_DAC_B_COMMAND; >>> + break; >>> + default: >>> + return -EINVAL; >>> + } >>> + >>> + ret = i2c_master_send(client, outbuf, 1); >>> + if (ret < 0) >>> + return ret; >>> + else if (ret != 1) >>> + return -EIO; >>> + >>> + ret = i2c_master_recv(client, inbuf, 2); >>> + if (ret < 0) >>> + return ret; >>> + else if (ret != 2) >>> + return -EIO; >> >> It somehow always feels like this error handling should be in the >> i2c core. Just how often does it make sense to receive too little >> from and i2c transaction? Anyhow, such is life ;) > >You wrote: > >> You could set this up to use i2c_transfer instead of separating it >like >> this. > >Accordingly to: >- Documentation/i2c/i2c-protocol >- Documentation/i2c/writing-clients >a sequence of i2c_master_send() and i2c_master_recv() is not fully >equivalent to a single i2c_transfer(); in latter case the transactions >would be combined and the stop bit in between would be removed. > >I checked the datasheet of max5821 and it reports that >"Each transmit sequence is framed by a START (S) or REPEATED START >(Sr) condition and a STOP (P) condition." >So combined transaction should work with this device. > >But we have few I2C controllers that cannot send combined transactions >and would return error. >E.g. in drivers/i2c/busses/i2c-powermac.c >i2c_powermac_master_xfer() returns -EOPNOTSUPP when num!=1. > >What is the proper way to address this: >- use combine transactions, since supported by majority of (but not >all) controllers? >or >- keep individual transactions, if not strictly required by the >protocol of the I2C device? I would go with working on the vast majority unless we have a user actually using such an i2c controller. > >Thanks, >Antonio -- Sent from my Android phone with K-9 Mail. Please excuse my brevity. -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html