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

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Date:	Fri, 18 Jun 2010 22:37:38 +0100
From:	Jonathan Cameron <kernel@...23.retrosnub.co.uk>
To:	guenter.roeck@...csson.com
CC:	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lm-sensors@...sensors.org" <lm-sensors@...sensors.org>,
	Hans de Goede <hdegoede@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [lm-sensors] [PATCH 1/3] hwmon: Driver for SMM665 Six-Channel
 Active DC Output Controller/Monitor

On 06/18/10 21:56, 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.
>>
>>> +             rv = i2c_smbus_read_byte_data(client, adc << 3);
>>> +             if (rv >= 0) {
>>> +                     /* 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.
> 
> Guenter
To my mind this looks like a case for adding another 'mangling' flag
to the core, but I guess that might require bus driver implementation
which would obviously be a pain.  Perhaps the approach you have taken
is the best plan.  My issue with it at the moment is that you are
detecting any error rather than specifically an unexpected NACK.


Jonathan
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