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

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Date:	Fri, 4 Apr 2014 17:44:23 +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 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.

>
>> >> +             num-cs = /bits/ 16 <4>;
>
>> > What's going on with the /bits/ - is this something that's required for
>> > the property?
>
>> The master->num-chipselect property is 16 bit but writing <4> here directly
>> leads to 0 being read in of_property_read (because it's big endian).
>> Instead using of property read u32 and then copying, we decided to do this.
>> This was discussed on v2 between Michal and Rob:
>
> No, don't do that.  You're contorting the binding to serve Linux's
> implementation needs and you've not documented this requirement either.
> If there *was* a need to have the number be 16 bits you'd need to
> document that reqirement in the binding, though like I say I don't think
> the number needs to be there at all.

OK. I'll remove the /bits/ 16 and handle it in the driver.

Regards,
Harini
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