linux-next regression caused by "gpiolib: request the gpio before querying its direction"

[Geert Uytterhoeven]

Hi Russell,

On Thu, Aug 31, 2017 at 11:22 AM, Russell King - ARM Linux
<linux at armlinux.org.uk> wrote:
> On Thu, Aug 31, 2017 at 09:30:54AM +0200, Geert Uytterhoeven wrote:
>> > Note that on my side, I've however not been convinced by this semantic:
>> > I find it weird that when you request a GPIO, it gets automatically
>> > muxed as such, without an explicit pinctrl configuration in the DT.
>>
>> On lots of hardware, you first have to select between GPIO and "other
>> function".
>> For "other function", you have to select the actual function later.
>> In that case, switching to a GPIO can be done by flipping a single bit.
>
> What about hardware which you can configure for some alternate function
> but still monitor the pin via GPIO, even though it's not mux'd as GPIO.
>
> For instance, you may have a timer block which can capture on both
> edges of an external event signal, which needs the pin to be muxed for
> that function.  However, you need to read the state of the pin, and
> that is only available through GPIO.  Muxing the pin to be a GPIO just
> because someone requests the GPIO is, imho, ill thought-out and breaks
> some use cases.

Yes, reading from the GPIO can work if the pin is muxed to another function.

> IMHO, the pinmux settings should always be specified in DT, and that's
> what we should be using everywhere, not doing broken backdoor games like
> "the gpio is being requested, it's obvious that we want this pin to be
> a gpio" - that really doesn't follow.

Do we need to specify pinmux setting for pins used by e.g. the SDRAM
controller, which doesn't have a Linux driver?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds