linux-kernel - Re: [PATCH 3/4] devicetree: bindings: Add defeature-repeated-start property for Cadence I2C

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Message-ID: <547DD6F0.6060006@metafoo.de>
Date:	Tue, 02 Dec 2014 16:12:48 +0100
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Wolfram Sang <wsa@...-dreams.de>,
	Harini Katakam <harinikatakamlinux@...il.com>
CC:	Mark Rutland <mark.rutland@....com>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	"ijc+devicetree@...lion.org.uk" <ijc+devicetree@...lion.org.uk>,
	"galak@...eaurora.org" <galak@...eaurora.org>,
	"michal.simek@...inx.com" <michal.simek@...inx.com>,
	"soren.brinkmann@...inx.com" <soren.brinkmann@...inx.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"vishnum@...inx.com" <vishnum@...inx.com>
Subject: Re: [PATCH 3/4] devicetree: bindings: Add defeature-repeated-start
 property for Cadence I2C

On 12/02/2014 03:15 PM, Wolfram Sang wrote:
>>> What do you do when disable repeated start? Sending STOP and START? If
>>> so, this is really something different than repeated start. By using
>>> I2C_FUNC_I2C a user expects repeated start, so if the HW does not
>>> support it, we should say so and don't try to emulate it with something
>>> different.
>>
>> Yes, we send stop.
>
> As said before, this is wrong. Another master could interfere between
> the messages when using stop+start. This is no replacement for repeated
> start.

More importantly a lot of I2C slaves also reset their internal state machine 
on a stop. So e.g. if reading a register is implemented by doing 
start,write,repeated start,read,stop and you replace that with 
start,write,stop,start,read,stop you'll always read register zero instead of 
the register you wanted to read.

- Lars

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