Re: [PATCH] pinctrl: qcom: add get_direction function

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

On Mon, Mar 6, 2017 at 10:52 PM, Timur Tabi <timur@xxxxxxxxxxxxxx> wrote:

> On ACPI platforms, the kernel has no control over the muxing (aka function)
> of the various pins.  Firmware configures the TLMM controller for all pins,
> and configures them for whatever functions they're supposed to be.

I think it would be better if pin control and ACPI play along, and I bet that
will happen in the future. This is I guess a question for ACPI standardization
work.

> Therefore, on ACPI, the driver should never change the function of any pin.
> If it's set to something other than 0, then it should never touch that pin.
> Don't write to it, don't change the direction, and definitely don't change
> the function.

Does that mean that pins with 0 are free to play around with?

> So would it be acceptable, for example, to change msm_gpio_set() such that
> if the function of that pin is non-zero, just return an error?

I would ask the driver maintainer about his opinion, and also whoever
is an authority on ACPI for the TLMM-chips, I am no expert
in ACPI. Hell I'm not even good at device tree. Not to mention SFI.
Oh well...

> After all, if pin #17 is set to UART and not GPIO, then there's no point in
> setting that value to 1 or 0, because it's not muxed for GPIO and therefore,
> that 1/0 is not actually going anywhere.  It won't be written to the pin.
>
> I hope I'm making sense.

In a way I guess, I'm just ignorant about how current ACPI implementations
work with hardware in this case so it is likely that you know this
better than me.

Yours,
Linus Walleij
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