Re: gpio-mxc: gpiod_get_value returns INT_MIN when the GPIO is off

[Linus Walleij]

On Sun, Nov 29, 2015 at 12:02 AM, Clemens Gruber
<clemens.gruber@xxxxxxxxxxxx> wrote:

> I am writing a module which reads several GPIO input states from a
> Freescale i.MX6Q. I can control the input states through hardware
> buttons attached to my development board. They are all connected exactly
> the same, with external pull up resistors.
> I am using the current torvalds/linux master branch.
>
> The list in my DT looks like this:
> my-gpios = <&gpio1 7 GPIO_ACTIVE_LOW>,
>         <&gpio1 8 GPIO_ACTIVE_LOW>,
>         /* .... */
>         <&gpio6 31 GPIO_ACTIVE_LOW>,
>         <&gpio7 11 GPIO_ACTIVE_LOW>;

So IIUC this us using drivers/gpio/gpio-mxc.c, correct?

> In my platform driver module, I first call gpiod_get_array(dev, "my",
> GPIOD_IN);
>
> Then I can successfully read in the input states of my GPIOs and get
> "correct" values for all but one of them: GPIO6_31
> Meaning: When no button is pressed, every call to
> gpiod_get_value(my_gpios->desc[i]) returns 0 and if one of the buttons
> is pressed, it returns 1.
> Except for gpio6 31: It returns -2^31 (INT_MIN) when no button is
> pressed and 1 if a button is pressed.
>
> I double checked the pinmuxing and the hardware, the input state should
> be off / 0 and not -2^31. The internal pull up is also enabled.
> Muxing: fsl,pins = <MX6QDL_PAD_EIM_BCLK__GPIO6_IO31 0x1b099>;
>
> I read through the source of gpio-generic.c and came across
> bgpio_get_set. If I interpret the code correctly, it is set as the get
> function of the gpio chip. (because BGPIOF_READ_OUTPUT_REG_SET is passed
> as flags argument when bgpio_init is called from gpio-mxc.c)
> In the case of gpio6_31, the pin2mask function would return 1 << 31 as
> unsigned long, meaning 2^31 and then the unsigned long return value of
> read_reg is bitmasked with 2^31 which would result in 2^31 if it is on
> and 0 if it is off.
> But because gpiod_get_value returns an int, the 2^31 unsigned long is
> casted to a signed int, which means it returns -2^31 if it is active.
> But in this case, it returns -2^31 if it is inactive and 1 if it is
> active...
>
> I can workaround by checking gpiod_get_value(desc) == 1 but I'd really
> like to know what's causing this (odd?) behavior.
>
> Am I doing something wrong here?

No it looks like a bug to me, sadly I'm not really using generic GPIO
with any hardware enough. I cooked up the following patch, can you test
it (tell me if you want me to send it separately):