CODEWITHSUNDEEP
linux-kernelraspberry-pilinux-device-driverkernel-modulegpio
leoll2
leoll2

Reputation: 163

How to use kernel GPIO descriptor interface

I'm trying to develop a simple Linux kernel module that manages a bunch of sensors/actuators pinned on the GPIO of a Raspberry Pi.
The GPIO functionalities I need are quite simple: get/set pin values, receive IRQs, ...

In my code, I have a misc_device which implements the usual open, read, write and open operations. In my read operation, for instance, I'd like to get the value (high/low) of a specific GPIO pin.

Luckily, the kernel provides an interface for such GPIO operations. Actually, there are two interfaces, according to the official GPIO doc: the legacy one, which is extremely simple yet deprecated, and the new descriptor-based one.
I'd like to use the latter for my project, and I understand how to implement all I need except for one thing: the device tree stuff.
With reference to board.txt, before I can call gpiod_get_index() and later gpiod_get_value(), first I need to setup the device tree somehow like this:

foo_device {
    compatible = "acme,foo";
    ...
    led-gpios = <&gpio 15 GPIO_ACTIVE_HIGH>, /* red */
            <&gpio 16 GPIO_ACTIVE_HIGH>, /* green */
            <&gpio 17 GPIO_ACTIVE_HIGH>; /* blue */

    power-gpios = <&gpio 1 GPIO_ACTIVE_LOW>;
};

However, I've absolutely no clue where to put that chunk of code, nor if I really need it. Mind that I have a misc device which looks like this, where aaa_fops contains the read operation:

static struct miscdevice aaa = {
    MISC_DYNAMIC_MINOR, "aaa", &aaa_fops
};

Using the old deprecated interface, my problem would be solved because it doesn't require to mess with the device tree, but I'd still like to use the new one if not too complex.

I've read a bunch of documentation, both official and unofficial, but couldn't find a straight and simple answer to my issue. I tried to find an answer in the kernel source code, especially in the drivers section, but only got lost in a valley of complex and messy stuff. The lack of working, minimal examples (WME) about kernel is significantly slowing down my learning process, just my opinion about it.
Could you please give me a WME of a simple device (preferably a misc) whose read() operation gets the value of a pin, using the new GPIO interface?

If you need more details about my code, just ask. Thanks in advance!

Note 1: I'm aware that most of my work could be done in userspace rather than kernelspace; my project is for educational purposes only, to learn the kernel.

Note 2: I choose a misc device because it's simple, but I can switch to a char device if needed.

Upvotes: 0

Views: 1150

Answers (1)

sawdust
sawdust

Reputation: 17067

... first I need to setup the device tree somehow like this:
...
However, I've absolutely no clue where to put that chunk of code

Device Tree nodes and properties should not be called "code".
Most devices are connected to a peripheral bus, so device nodes typically are child nodes of the peripheral bus node.

Could you please give me a WME of a simple device

You can find numerous examples of descriptor-based GPIO usage in the kernel source.
Since the documentation specifies the GPIO descriptor as a property named <function>-gpios, a grep of the directory arch/arm/boot/dts for the string "\-gpios" reports many possible examples.
In particular there's 

./bcm2835-rpi-b.dts:    hpd-gpios = <&gpio 46 GPIO_ACTIVE_HIGH>;

This hpd-gpios property belongs to the hdmi base-node defined in bcm283x.dtsi, and is used by the gpu/drm/vc4/vc4_hdmi.c driver.

/* General HDMI hardware state. */
struct vc4_hdmi {
        ... 
        int hpd_gpio;
        ...
};


static int vc4_hdmi_bind(struct device *dev, struct device *master, void *data)
{
    ...
        /* Only use the GPIO HPD pin if present in the DT, otherwise
         * we'll use the HDMI core's register.
         */
        if (of_find_property(dev->of_node, "hpd-gpios", &value)) {
                ...

                hdmi->hpd_gpio = of_get_named_gpio_flags(dev->of_node,
                                                         "hpd-gpios", 0,
                                                         &hpd_gpio_flags);
                if (hdmi->hpd_gpio < 0) {
                        ret = hdmi->hpd_gpio;
                        goto err_unprepare_hsm;
                }
                ...
        }

If the hpd-gpios property is defined/found and successfully retrieved from the board's DeviceTree, then the driver's structure member hpd_gpio holds the GPIO pin number.
Since this driver does not call devm_gpio_request(), the framework apparently allocates the GPIO pin for the driver.

The driver can then access the GPIO pin. 

static enum drm_connector_status
vc4_hdmi_connector_detect(struct drm_connector *connector, bool force)
{
    ...
        if (vc4->hdmi->hpd_gpio) {
                if (gpio_get_value_cansleep(vc4->hdmi->hpd_gpio) ^
                    vc4->hdmi->hpd_active_low)
                        return connector_status_connected;
                else
                        return connector_status_disconnected;
        }

Upvotes: 0

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