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Date: Fri, 18 Jun 2010 14:54:08 -0700 From: Guenter Roeck <guenter.roeck@...csson.com> To: Jonathan Cameron <kernel@...23.retrosnub.co.uk> 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 Fri, 2010-06-18 at 17:37 -0400, Jonathan Cameron wrote: > 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. Yes, but looking through bus implementations, I don't think there is a consistent way to detect the exact error reason. How about if I weed out -EOPNOTSUPP, -ETIMEDOUT, and -EINVAL ? Guenter -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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