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

[Peter Crosthwaite]

On Fri, Apr 4, 2014 at 10:14 PM, Harini Katakam
<harinikatakamlinux@xxxxxxxxx> wrote:
> Hi Mark,
>
> On Fri, Apr 4, 2014 at 3:39 PM, Mark Brown <broonie@xxxxxxxxxx> 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@xxxxxxxxxx> 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?

Regards,
Peter

>>
>>> >> +             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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