Re: [PATCH 6/6] gpio: tegra: Parameterize the number of banks

[Rob Herring <robherring2-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 01/13/2012 02:55 PM, Stephen Warren wrote:
> Rob Herring wrote at Thursday, January 05, 2012 10:25 AM:
>> On 01/04/2012 04:00 PM, Grant Likely wrote:
>>> On Wed, Jan 4, 2012 at 1:00 PM, Stephen Warren <swarren-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org> wrote:
>>>> Rob Herring wrote at Wednesday, January 04, 2012 12:54 PM:
>>>>> On 01/04/2012 12:39 PM, Stephen Warren wrote:
>>>>>> Tegra20's GPIO controller has 7 banks, and Tegra30's controller has 8
>>>>>> banks. Allow the number of banks to be configured at run-time by the
>>>>>> device tree.
>>>> ...
>>>>>> diff --git a/Documentation/devicetree/bindings/gpio/gpio_nvidia.txt
>>>> ...
>>>>>>  Required properties:
>>>>>>  - compatible : "nvidia,tegra20-gpio"
>>>>>>  - reg : Physical base address and length of the controller's registers.
>>>>>> +- nvidia,num-banks : The number of GPIO banks. This should be 7 for
>>>>>> +  Tegra20 and 8 for Tegra30. This must match the number of interrupt
>>>>>> +  specifiers in the interrupts property.
>>>>>
>>>>> You can determine the number of banks based on the compatible property
>>>>> rather than needing an additional property.
>>>>
>>>> That's certainly possible.
>>>>
>>>> However, if say nvidia,tegraNNN-gpio has 9 banks, we then have to
>>>> explicitly edit the driver to know that, whereas by using a property,
>>>> we wouldn't have to change the driver at all to support a future GPIO
>>>> controller. So, isn't it better to explicitly represent this in DT?
>>>>
>>>> Note that I have no idea how many GPIO banks our future chips will have,
>>>> so this might not turn out to save any work at all, but perhaps.
>>>
>>> It's an engineering/design decision that requires taste and instinct.
>>> Either approach is fine, you decide which one will be the best in the
>>> long term.
>>
>> Agreed. I'm really fine with it either way.
>>
>> Trying to predict future h/w is a bit pointless IMO. H/w designers
>> always find new ways to do things differently. i.MX family has gpio
>> interrupts hooked up 3 different ways for example. How would you handle
>> the case that the banks are sparsely implemented?
>>
>> Is adding support for a different number of banks every couple of years
>> really an issue? It's much more important to have properties for which
>> change with every board.
> 
> Thinking about this some more, I'm tempted to rework this patch to remove
> the extra DT property and just "detect" the number of banks based on the
> length of the interrupts property, (well, actually the number of IRQ
> resources that the platform device has) since each bank has its own
> interrupt. Does anyone disagree with doing that?

Sounds fine too me.

Rob