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

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

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):

>From 6e9a115f7a9f70fede21e63630e0d7b01661cf7c Mon Sep 17 00:00:00 2001
From: Linus Walleij <linus.walleij@xxxxxxxxxx>
Date: Thu, 10 Dec 2015 15:55:29 +0100
Subject: [PATCH] gpio: generic: clamp values from bgpio_get_set()

The bgpio_get_set() call should return a value clamped to [0,1],
the current code will return a negative value if reading
bit 31, which gets interpreted as an error by the gpiolib core.

Reported-by: Clemens Gruber <clemens.gruber@xxxxxxxxxxxx>
Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
---
 drivers/gpio/gpio-generic.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpio/gpio-generic.c b/drivers/gpio/gpio-generic.c
index 72088028d7a9..ea581dc23d44 100644
--- a/drivers/gpio/gpio-generic.c
+++ b/drivers/gpio/gpio-generic.c
@@ -141,9 +141,9 @@ static int bgpio_get_set(struct gpio_chip *gc,
unsigned int gpio)
     unsigned long pinmask = bgc->pin2mask(bgc, gpio);

     if (bgc->dir & pinmask)
-        return bgc->read_reg(bgc->reg_set) & pinmask;
+        return !!(bgc->read_reg(bgc->reg_set) & pinmask);
     else
-        return bgc->read_reg(bgc->reg_dat) & pinmask;
+        return !!(bgc->read_reg(bgc->reg_dat) & pinmask);
 }

 static int bgpio_get(struct gpio_chip *gc, unsigned int gpio)
-- 
2.4.3

Yours,
Linus Walleij
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