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

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Date:	Fri, 4 Apr 2014 19:38:14 +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 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
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