[U-Boot] [PATCH v4 03/20] fdt: Add basic support for decoding GPIO definitions

[Stephen Warren]

On 01/21/2012 10:08 AM, Simon Glass wrote:
> Hi Stephen,
> 
> On Wed, Jan 18, 2012 at 2:17 PM, Stephen Warren <swarren at nvidia.com> wrote:
>> On 01/11/2012 09:32 PM, Simon Glass wrote:
>>> This adds some support into fdtdec for reading GPIO definitions from
>>> the fdt. ...
...
>>> diff --git a/include/fdtdec.h b/include/fdtdec.h
>> ...
>>> +/* GPIOs are numbered from 0 */
>>> +enum {
>>> +     FDT_GPIO_NONE = -1U,    /* an invalid GPIO used to end our list */
>>
>> Is this due to the way U-Boot works right now, or something defined by
>> this patch? It's been pointed out that the kernel's choice to use -1 as
>> "invalid GPIO" rather than 0 was a mistake, since that prevents GPIO
>> fields being easily added to platform data structures, since you then
>> have to go and initialize every new instance to -1, rather than relying
>> on BSS initializing it to 0. I assume this is just the way U-Boot works,
>> so solving this is outside the scope of this patch.
> 
> It is nothing to do with U-Boot itself - we can choose any number.

Surely the value you choose for DT parsing has to align with the value
that U-Boot's GPIO API chooses, so you can't just choose any number.

> What is Linux using now / planning to use?

Linux uses -1 right now, but should really have used 0. I don't think
there's an actual plan to change this now, since the numbering scheme is
entrenched.

What Linux does isn't relevant; the numbering scheme I'm talking about
is the internal numbering scheme within U-Boot or Linux, which are
independent from each-other and the values in the device tree.

So that said, I was wondering if U-Boot's GPIO support (covering both
non-DT and DT cases) was new enough that it could use 0 to represent
"invalid GPIO" rather than -1, and hence FDT_GPIO_NONE be 0 not -1. It
probably isn't though; I guess U-Boot's GPIO numbering scheme is also
already entrenched?

-- 
nvpublic