linux-kernel - Re: [PATCH v5 1/3] i2c: mux: Add i2c-arb-gpio-challenge 'mux' driver

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Date:	Tue, 16 Apr 2013 09:42:14 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Wolfram Sang <wsa@...-dreams.de>
CC:	Doug Anderson <dianders@...omium.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Simon Glass <sjg@...omium.org>,
	Naveen Krishna Chatradhi <ch.naveen@...sung.com>,
	grant.likely@...retlab.ca, yuvaraj.cd@...il.com,
	ben.dooks@...ethink.co.uk, u.kleine-koenig@...gutronix.de,
	broonie@...nsource.wolfsonmicro.com,
	girish.shivananjappa@...aro.org, bhushan.r@...sung.com,
	sreekumar.c@...sung.com, prashanth.g@...sung.com, olof@...om.net,
	djkurtz@...omium.org, linux@...ck-us.net,
	Rob Herring <rob.herring@...xeda.com>,
	Rob Landley <rob@...dley.net>,
	"Ben Dooks (embedded platforms)" <ben-linux@...ff.org>,
	Stephen Warren <swarren@...dia.com>,
	Jean Delvare <khali@...ux-fr.org>,
	Peter Korsgaard <peter.korsgaard@...co.com>,
	Guenter Roeck <guenter.roeck@...csson.com>,
	devicetree-discuss@...ts.ozlabs.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org
Subject: Re: [PATCH v5 1/3] i2c: mux: Add i2c-arb-gpio-challenge 'mux' driver

On 04/16/2013 03:36 AM, Wolfram Sang wrote:
> Doug,
> 
> On Tue, Apr 09, 2013 at 02:34:28PM -0700, Doug Anderson wrote:
>> The i2c-arb-gpio-challenge driver implements an I2C arbitration scheme
>> where masters need to claim the bus with a GPIO before they can start
>> a transcation.  This should generally only be used when standard I2C
>> multimaster isn't appropriate for some reason (errata/bugs).
>>
>> This driver is based on code that Simon Glass added to the i2c-s3c2410
>> driver in the Chrome OS kernel 3.4 tree.  The current incarnation as a
>> mux driver is as suggested by Grant Likely.  See
>> <https://patchwork.kernel.org/patch/1877311/> for some history.

>> diff --git a/Documentation/devicetree/bindings/i2c/i2c-arb-gpio-challenge.txt b/Documentation/devicetree/bindings/i2c/i2c-arb-gpio-challenge.txt

>> +GPIO-based I2C Arbitration
>> +==========================
>> +This uses GPIO lines to arbitrate who is the master of an I2C bus in a
>> +multimaster situation.
> 
> "uses GPIO lines and a challange & response mechanism" or something like
> that. There might be other GPIO based arbitrations in the future (though
> I hope there won't).

The existing text appears clearer to me; this document should spell out
the exact details of the protocol in later paragraphs, so there's no
need to try and spell it out here.

>> +- their-claim-gpios: The GPIOs that the other sides use the claim the bus.
>> +  Note that some implementations may only support a single other master.
> 
> Stronger? "Currently, only one other master is supported"?

The DT binding documentation, which should be OS-/driver-agnostic,
should describe the binding, not the implementation. The limitation that
Linux imposes is OS-specific and hence should not be mentioned here as
an absolute, or perhaps even at all.
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