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?
Thanks,
Antonio
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