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Date:	Fri, 4 Apr 2014 10:09:47 +0200
From:	Geert Uytterhoeven <>
To:	Harini Katakam <>
Cc:	Mark Brown <>,
	Grant Likely <>,
	Rob Herring <>,
	Pawel Moll <>,
	Mark Rutland <>,
	"" <>,
	Kumar Gala <>,
	linux-spi <>,
	"" <>,
	"" <>,
	"" <>
Subject: Re: [PATCH v2 2/2] devicetree: Add devicetree bindings documentation
 for Cadence SPI

Hi Harini,

On Fri, Apr 4, 2014 at 5:00 AM, Harini Katakam
<> wrote:
>>> +             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:
>>>>> +               num-chip-select = /bits/ 16 <4>;
>>> I was expecting you will comment this a little bit. :-)
>>> Because all just reading this num-cs as 32bit and then
>>> assigning this value to master->num_chipselect which is 16bit.
>> Well, everyone else has that problem then. Obviously it takes a bit
>> more care than just reading into a u32, but that is a kernel problem
>> and not a problem of the binding.
> They are not reading it directly with read_u32 but they are using
> intermediate u32 value which is assigned to u16 which is fine.
> This pattern is in most drivers(maybe all).
> The point is if binding should or can't simplify driver code.
> And from your reaction above I expect that it is up to driver
> owner and binding doc how you want to do it.

IMHO this "/bits/ 16" doesn't simplify the binding.

As "num-cs" is a generic spi subsystem binding, it should not be
restricted to 16 bits for the sake of a driver. As your hardware can drive 4
chip selects, you could represent it in 3 bits (don't!).

Simple integers are 32 bit in DT, so use a temporary.



Geert Uytterhoeven -- There's lots of Linux beyond ia32 --

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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