linux-kernel - Re: [PATCH v2 2/2] devicetree: Add devicetree bindings documentation for Cadence SPI

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Date:	Fri, 4 Apr 2014 18:09:35 +0530
From:	Harini Katakam <harinikatakamlinux@...il.com>
To:	Peter Crosthwaite <peter.crosthwaite@...inx.com>
Cc:	Mark Brown <broonie@...nel.org>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	"ijc+devicetree@...lion.org.uk" <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>, linux-spi@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] devicetree: Add devicetree bindings documentation
 for Cadence SPI

Hi,

On Fri, Apr 4, 2014 at 5:54 PM, Peter Crosthwaite
<peter.crosthwaite@...inx.com> wrote:
> On Fri, Apr 4, 2014 at 10:14 PM, Harini Katakam
> <harinikatakamlinux@...il.com> wrote:
>> Hi Mark,
>>
>> On Fri, Apr 4, 2014 at 3:39 PM, Mark Brown <broonie@...nel.org> wrote:
>>> On Fri, Apr 04, 2014 at 08:30:38AM +0530, Harini Katakam wrote:
>>>> On Fri, Apr 4, 2014 at 3:04 AM, Mark Brown <broonie@...nel.org> wrote:
>>>> > On Thu, Apr 03, 2014 at 04:40:31PM +0530, Harini Katakam wrote:
>>>
>>>> >> +Optional properties:
>>>> >> +- num-cs             : Number of chip selects used.
>>>
>>>> > How does this translate to the hardware?
>>>
>>>> This IP can drive 4 slaves.
>>>> The CS line to be driven is selected in spi device structure and
>>>> that is driven by the software.
>>>
>>> So why specify this in the binding at all?  If the device always has the
>>> same number of chip selects it's redundant.
>>
>> I'm sorry, I should have explained that better.
>> The IP can support upto 4 chip selects.
>> The num-cs value here is the number of chip selects actually used on the board.
>>
>
> Just to clarify - do you mean SoC or board level?
>

I mean board level.
One more reason it will be useful to keep this property is because
there is support for adding a decoder where extended slaves can be selected
through the IP's control register.
(This is not currently implemented in the driver but will be in the future.)

Regards,
Harini
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