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

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Date:	Fri, 4 Apr 2014 20:00:19 +0530
From:	Harini Katakam <harinikatakamlinux@...il.com>
To:	Mark Brown <broonie@...nel.org>
Cc:	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 7:38 PM, Harini Katakam
<harinikatakamlinux@...il.com> wrote:
> Hi Mark,
>
> On Fri, Apr 4, 2014 at 6:16 PM, Mark Brown <broonie@...nel.org> wrote:
>> On Fri, Apr 04, 2014 at 05:44:23PM +0530, Harini Katakam wrote:
>>> 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:
>>
>>> >> 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.
>>
>> Why does that need to be configured?  Surely the presence of slaves is
>> enough information.
>
> OK. I understand.
> Can you comment on the case where a decoder is used?
>
> 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.)
>
> How should the driver know whether it is 4 or 16 select lines for example?
> This has to be written to master->num_chipselect.
>
> Regards,
> Harini

Just adding to my above comment.
Alternately, I could not use the "num-cs" property and add another
optional property to be used only when required.
The driver sets a default value already.

Regards,
Harini
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