linux-kernel - Re: [PATCH RFC v2 3/5] spmi: add generic SPMI controller binding documentation

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Date:	Wed, 28 Aug 2013 12:32:54 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Josh Cartwright <joshc@...eaurora.org>
CC:	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <rob.herring@...xeda.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ian.campbell@...rix.com>,
	Kumar Gala <galak@...eaurora.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-arm-msm@...r.kernel.org,
	Sagar Dharia <sdharia@...eaurora.org>,
	Gilad Avidov <gavidov@...eaurora.org>,
	Michael Bohan <mbohan@...eaurora.org>,
	devicetree@...r.kernel.org, Wolfram Sang <wsa@...-dreams.de>
Subject: Re: [PATCH RFC v2 3/5] spmi: add generic SPMI controller binding
 documentation

On 08/28/2013 12:00 PM, Josh Cartwright wrote:
> On Tue, Aug 27, 2013 at 03:55:19PM -0600, Stephen Warren wrote:
>> On 08/27/2013 11:01 AM, Josh Cartwright wrote:
>> ...
>>> If we want to ensure for the generic bindings that we are fulling
>>> characterizing/describing the SPMI bus, then we'll additionally need to
>>> tackle an additional identified assumption:
>>>
>>>   4. One master per SPMI bus.  (The SPMI spec allows for up to 4
>>>      masters)
>>>
>>> On the Snapdragon 800 series, there exists only one software-controlled
>>> master, but it is conceivably possible to have a setup with two
>>> software-controlled masters on the same SPMI bus.
>>>
>>> This necessarily means that the description of the slaves and the
>>> masters will need to be decoupled; I'm imagining a generic binding
>>> supporting multiple masters would look something like this:
>>
>> Is there a need to represent the other masters in the DT? Sure they're
>> there in HW, but if there's no specific way for the
>> CPU-to-which-the-DT-applies to actually interact with those other
>> masters (except perhaps by experiencing some arbitration delays) then
>> presumably there's no need to represent the other masters in DT?
> 
> My example is contrived, but there is nothing in the SPMI spec
> preventing two masters from being controllable by the same
> CPU-to-which-the-DT-applies, sharing the same underlying bus.

That's true.

> I would also expect this configuration to be uncommon, I'm checking with
> some folks with more SPMI experience to make sure they agree.
> 
> Interestingly, i2c as far as I can tell has also made the same
> assumption.  There doesn't appear to be any way to express a
> multi-master i2c setup where both masters are controlled by the same OS.

Yes, I think it's a fair assumption that we don't need to represent
that; I immediately thought about the I2C counter-example after reading
your first paragraph.
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