linux-kernel - Re: [PATCH] devicetree - document using aliases to set spi bus number.

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Date:	Tue, 24 May 2016 13:41:26 -0700
From:	Frank Rowand <frowand.list@...il.com>
To:	Mark Rutland <mark.rutland@....com>
CC:	Christer Weinigel <christer@...nigel.se>,
	linux-kernel@...r.kernel.org, linux-spi@...r.kernel.org,
	devicetree@...r.kernel.org, Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH] devicetree - document using aliases to set spi bus number.

On 5/24/2016 10:41 AM, Mark Rutland wrote:
> On Tue, May 24, 2016 at 06:39:20PM +0200, Christer Weinigel wrote:
>> Document how to use devicetree aliases to assign a stable
>> bus number to a spi bus.
>>
>> Signed-off-by: Christer Weinigel <christer@...nigel.se>
>>
>> ---
>>
>> Trivial documentation change.
>>
>> Not having used devicetree that much it was surprisingly hard to
>> figure out how to assign a stable bus number to a spi bus.  Add a
>> simple example that shows how to do that.
>>
>> Mark Cced as the SPI maintainer.  Or should trivial documentation
>> fixes like this be addressed to someone else?
>>
>>   /Christer
>>
>>  Documentation/devicetree/bindings/spi/spi-bus.txt | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/spi/spi-bus.txt b/Documentation/devicetree/bindings/spi/spi-bus.txt
>> index 42d5954..c35c4c2 100644
>> --- a/Documentation/devicetree/bindings/spi/spi-bus.txt
>> +++ b/Documentation/devicetree/bindings/spi/spi-bus.txt
>> @@ -94,3 +94,13 @@ SPI example for an MPC5200 SPI bus:
>>  			reg = <1>;
>>  		};
>>  	};
>> +
>> +Normally SPI buses are assigned dynamic bus numbers starting at 32766
>> +and counting downwards.  It is possible to assign the bus number
>> +statically using devicetee aliases.  For example, on the MPC5200 the
>> +"spi@f00" device above is connected to the "soc" bus.  To set its
>> +bus_num to 1 add an aliases entry like this:
> 
> As Mark Brown pointed out, this is very Linux-specific (at least in the
> wording of the above).

Yes, Linux-specific.  So the Linux documentation of bindings is the
correct place for it.

> 
> Generally, aliases are there to match _physical_ identifiers (e.g. to
> match physical labels for UART0, UART1, and on).
> 
> I'm not sure whether that applies here.

The code and behavior is in the Linux kernel.  It should be visible in
the documentation instead of being a big mystery of how it works.

> 
> Thanks,
> Mark.
> --
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