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

On 01/21/2012 10:08 AM, Simon Glass wrote:

Hi Stephen,

On Wed, Jan 18, 2012 at 2:17 PM, Stephen Warren [off-list ref] wrote:

quoted

On 01/11/2012 09:32 PM, Simon Glass wrote:

quoted

This adds some support into fdtdec for reading GPIO definitions from
the fdt. ...

...

quoted

quoted

diff --git a/include/fdtdec.h b/include/fdtdec.h

...

quoted

+/* 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