More GPIO madness on iMX6 - and the crappy ARM port of Linux

[swarren@wwwdotorg.org (Stephen Warren)]

On 01/17/2014 01:24 PM, Russell King - ARM Linux wrote:
> On Fri, Jan 17, 2014 at 12:40:02PM -0700, Stephen Warren wrote:
>> On 01/17/2014 11:47 AM, Russell King - ARM Linux wrote:
>>> So, we have this wonderful GPIO layer which abstracts GPIO stuff and
>>> hides stuff.  It's really wonderful, because you don't have to care
>>> about how the GPIOs are actually accessed in drivers anymore.
>> ...
>>> 1. What should gpio_get_value() return for an output?
>>
>> Some HW can't ever read back the value of an output pin, so isn't
>> calling gpio_get_value() undefined for output pins?
> 
> As has been pointed out, that's not how gpio_get_valie() is documented.
> It's documented to return the value of the pin where possible.  In my
> case, it _is_ possible to read back the value of the pin - it just
> needs the appropriate chip configuration to make it happen.
> 
> Now to the crunch point of my email: where subsystems differ completely
> _unnecessarily_ from what is expected from them - such as returning the
> current state of the output where it's possible to do so - then this
> kind of difference *reduces* the portability of that subsystem, and
> makes being able to move from one SoC to another _unnecessarily_ more
> difficult.

If the gpio_get_value() was guaranteed never to be valid for an output
pin, that would actually be *more* portable, not less; it would work the
same way everywhere.

I believe you want gpio_get_value() to return either the driven or
actual pin value where it can on the current HW, but just e.g. hard-code
0 on other HW. That would introduce a core feature that works some
places but not others, and hence make drivers that relied on the feature
less portable between HW with different actual features.