Introduction

Welcome to Part - 8 of Our path to libmraa with 96Boards series. In this part I will provide a quick summary of the new GPIO API added to MRAA library.

GPIO in MRAA

Last month we had a pretty interesting discussion in the Linux GPIO Mailing List regarding MRAA Thread. Most of them were criticism against the way MRAA is doing GPIO access on Chardev capable linux based systems. I already explained the significance of GPIO Chardev support in Previous blog. kernel developers shared their frustration regarding the MRAA dependent pin mapping as it is prone to go wrong when an external gpiochip like GPIO expander got probed before the SoC’s internal gpiochip.

To understand this scenario, let’s consider the below MRAA GPIO pin mapping for Dragonboard410c:

int db410c_chardev_map[MRAA_96BOARDS_LS_GPIO_COUNT][2] = {
    { 0, 36 }, { 0, 12 }, { 0, 13 }, { 0, 69 }, { 0, 115 }, { 2, 3 },
    { 0, 24 }, { 0, 25 }, { 0, 35 }, { 0, 34 }, { 0, 28 },  { 0, 33 },
};

The above lookup table has the entries of gpiochip and its internal pin number. This gets mapped to 96Boards specific GPIO numbers on Low speed expansion header as below in 96Boards platform code:

    // GPIOs are labelled "GPIO-A" through "GPIO-L"
    for (i = 0; i < MRAA_96BOARDS_LS_GPIO_COUNT; i++) {
        mraa_96boards_pininfo(b, 23 + i, ls_gpio_pins ? ls_gpio_pins[i] : -1, 1, "GPIO-%c", 'A' + i,
                              chardev_map ? (*chardev_map)[i][0] : -1,
                              chardev_map ? (*chardev_map)[i][1] : -1);
    }

The above mapping should work ideally on Dragonboard410c in most cases. But sometimes, when an external gpiochip gets probed before the SoC specific internal gpiochip, above mapping will go wrong since the gpiochip’s id gets allocated based on the probing order during system boot. At the same time, the chip specific internal pin number will always be the same since it is obtained from platform declaration mechanism like devicetree.

For overcoming this issue, we need a way to access a GPIO without specifying the gpiochip number. How can we do that? Well, the kernel already provides a nice feature for accessing a GPIO using its line name and I decided to make use of that! I went and created a Pull request which adds a new MRAA GPIO init API which takes the line name and returns the MRAA specific GPIO descriptor. And the user can then make use of the returned descriptor for doing all GPIO related access in MRAA as before.

Pull Request: https://github.com/intel-iot-devkit/mraa/pull/965

Below code snippet can be used to access a GPIO with the help of new API:

    mraa_gpio_context gpio;
    mraa_result_t res = MRAA_SUCCESS;

    /* initialize mraa for the platform. not needed most of the times */
    mraa_init();

    /* initialize GPIO-A */
    gpio = mraa_gpio_init_by_name("GPIO-A");
    if (gpio == NULL) {
        fprintf(stderr, "Failed to initialize pin %d\n", GPIO_1);
    }

    /* set GPIO-A to output */
    res = mraa_gpio_dir(gpio, MRAA_GPIO_OUT);
    if (res != MRAA_SUCCESS)
        goto err;

    /* write 1 to GPIO-A */
    res = mraa_gpio_write(gpio, 1);
    if (res != MRAA_SUCCESS)
        goto err;

        res = mraa_gpio_close(gpio);
        if (res != MRAA_SUCCESS) {
                mraa_result_print(res);
        }

    return res;
err:
    mraa_result_print(res);
    return res;

Sounds pretty simple, right? But there exists a dependency… Your board specific platform declaration mechanism like devicetree should declare GPIO line name for making use of this new API. For most of the 96Boards CE boards, we had added line names in the mainline kernel and it will work without any issues. For older devicetree, the user has to manually add line names by taking the reference from below patch:

https://www.spinics.net/lists/devicetree/msg276218.html

Conclusion

This concludes the summary of Part - 8 of Our path to libmraa with 96Boards blog. Since the initial MRAA Pull Request has been merged, I’m planning to add few examples to MRAA along with bindings for supported languages. Stay tuned!

This article is Part 1 in a 2-Part Series.