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

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Date:	Fri, 4 Apr 2014 11:09:19 +0100
From:	Mark Brown <broonie@...nel.org>
To:	Harini Katakam <harinikatakamlinux@...il.com>
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

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.

> >> +             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.

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