linux-kernel - Re: [PATCH 1/3] hwmon: Driver for SMM665 Six-Channel Active DC Output Controller/Monitor

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Date:	Sat, 19 Jun 2010 10:23:12 +0200
From:	Jean Delvare <khali@...ux-fr.org>
To:	guenter.roeck@...csson.com
Cc:	Jonathan Cameron <kernel@...23.retrosnub.co.uk>,
	Hans de Goede <hdegoede@...hat.com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"lm-sensors@...sensors.org" <lm-sensors@...sensors.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] hwmon: Driver for SMM665 Six-Channel Active DC 
 Output Controller/Monitor

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
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