Re: [PATCH 2/2] dt/bindings: control CS via standard GPIO operations instead of SPI-HW

[Martin Sperl <kernel@xxxxxxxxxxxxxxxx>]

> On 17.03.2015, at 04:18, Stephen Warren <swarren@xxxxxxxxxxxxx> wrote:
>> But then it means in principle that this is a more general issue
>> that just became apparent now.
> 
> Yes.
> 
...
>> So this is maybe the way forward for the whole default-dt?
>> 
>> For SPI it would look like this:
>> &gpio {
>>        spi0_pins: spi0_pins {
>>                brcm,pins = <7 8 9 10 11>;
>>                brcm,function = <4>; /* alt0 */
>>        };
>> 	...
>> }
>> 
>> &spi0 {
>> 	...
>>        pinctrl-0 = <&spi0_pins>;
>> 	...
>> }
>> 
>> And if you keep spi0 disabled in the dtsi files then the ALT
>> modes should not be set.
> 
> Yes, so long as it's disabled by default that would be OK. However, I
> wonder why we don't just rely on the firmware to set up the pinmux,
> since as you mentioned it does it now?
...
>> Obviously we could also split the gpio-block into 
>> "normal SPI" and "CS" pins, which would allow changing the
>> "defaults" also in the dts that gets build.
>> 
>> So how should we proceed?
> 
> If we do put any default CS GPIO setup in the kernel DT, we should
> indeed put it into a separate node (pinctrl state) so that the user can
> override it easily without any interactions with any other pins/...
So I will create a patch to separate the spi portions out as 
mentioned above.

I will actually create 2 gpio-sections:
one for the SPI transfer pins (GPIO 9, 10, 11) 
and one for the chipselects (8, 7), which can get overridden
either as output or as ALT0 in a customized device tree.

SPI by default shall be disabled.

Martin
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