[U-Boot] [PATCH 09/14] fdt: Add polarity-aware gpio functions to fdtdec

[Simon Glass]

Hi Stephen,

On Wed, Oct 31, 2012 at 9:50 PM, Stephen Warren <swarren at wwwdotorg.org> wrote:
> On 10/31/2012 05:59 PM, Simon Glass wrote:
>> Hi,
>>
>> On Fri, Oct 26, 2012 at 12:17 AM, Lucas Stach <dev at lynxeye.de> wrote:
>>> Am Donnerstag, den 25.10.2012, 19:31 -0700 schrieb Simon Glass:
>>>> From: Sean Paul <seanpaul at chromium.org>
>>>>
>>>> Add get and set gpio functions to fdtdec that take into account the
>>>> polarity field in fdtdec_gpio_state.flags.
>>>>
>>> In another thread Stephen Warren and I came to the conclusion that we
>>> most likely should remove this polarity flag from the GPIO bindings.
>>>
>>> Currently it is only for the USB VBUS GPIO which should move over to
>>> regulators once they land in U-Boot. Do you have any other applications
>>> for this flag, so we might reconsider removing it?
>>>
>>
>> Well, any time you have a flag which is inverted in meaning, it can be
>> useful. We have several switches on the board which can be active high
>> or low, and polarity is used for that.
>>
>> In fact, it would be nice IMO to be able to specify input/output as
>> well. I know the exynos bindings do this. There is a noddy function
>> called fdtdec_setup_gpio() in U-Boot which really needs to be sorted
>> out. I discussed with Stephen some time ago how GPIOs should be
>> SOC-specific and it should be possible to set up a GPIO with a single
>> call, as Linux does. The more information there is in the binding, the
>> more it can do automatically.
>>
>> Does the Tegra Linux GPIO binding still have a polarity?
>
> Yes it does, although in practice it can't be used (and hence should
> really be removed), since not all GPIO bindings have such a flag, so
> there is always a need for a separate property to indicate the polarity
> (c.f. fixed-regulator with GPIO control bindings for example).
>

I've had a bit of time to look into this. I see that the regulator
framework in the kernel seems to be used for various control purposes,
and provides useful polarity stuff. I was rather hoping that GPIOs
could be a bit more high level in U-Boot, with information about:

- input/output
- drive strength
- polarity
- pull up/down

In fact most of these are actually supported in most kernel bindings,
but of course it is binding-specific. Would it be useful to ask for a
polarity setting in the GPIO. When it is not available, the polarity
would then always be normal.

This might avoid moving polarity and input/output selecting down into
each client of the gpio, which seems undesirable in general.

Should we consider a second level of indirection for GPIOs to support
these non-binding features? It seems a bit complicated though.

However, if it is too late to do this, or not desirable for some
reason, then we should just drop this patch.

Regards,
Simon