hwmon: Driver for SMM665 Six-Channel Active DC Output Controller/Monitor
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From: Jean Delvare on 19 Jun 2010 04:30 On Fri, 18 Jun 2010 13:56:53 -0700, Guenter Roeck wrote: > [...] > > > + /* > > > + * Algorithm for reading ADC, per SMM665 datasheet > > > + * > > > + * {[S][addr][W][Ack]} {[offset][Ack]} {[S][addr][R][Nack]} > > > + * [wait 70 uS] > > > + * {[S][addr][R][Ack]} {[datahi][Ack]} {[datalo][Ack][P]} > > > + * > > > + * To implement the first part of this exchange, > > > + * do a full read transaction and expect a failure/Nack. > > > + * This sets up the address pointer on the SMM665 > > > + * and starts the ADC conversion. > > > + * Then do a two-byte read transaction. > > > + */ > > Is there no better way of handling this? There are protocol mangling hacks > > to tell the i2c core to ignore a NAKs under some circumstances. This is only available on raw I2C messages, not on the higher-level i2c_smbus_*() API. And not all bus drivers support it. And that's not what is needed here anyway: ignoring the nack means we would continue reading from the chip, while it really doesn't want to talk to us at that time. > > > + rv = i2c_smbus_read_byte_data(client, adc << 3); > > > + if (rv >= 0) { You should check for -ENXIO explicitly. According to Documentation/i2c/fault-codes, this is the value bus drivers should return on missing Ack. > > > + /* No error, something is wrong. Retry. */ > > > + rv = -1; > > > + continue; > > > + } > > I looked through the core i2c code, but did not find anything I can > use. > > Problem is that per smm665 specification, the first NACK is expected. So > we do not just want to ignore this NACK, we want to actively check if > the command "failed" as expected, and report an error if it did _not_ > fail. Do you really have to trigger the Nack for the ADC conversion to start? Can't you just use i2c_smbus_read_byte() (no _data) for the first part of the transaction? -- Jean Delvare -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |