Linux下GPIO驱动（三） ----gpio_desc()的分析

上篇最后提出的疑问是结构体gpio_chip中的成员函数set等是怎么实现的，在回答之前先介绍下gpio_desc这个结构体。 

     如上图所示，右上方部分为GPIO驱动对其它驱动提供的GPIO操作接口，其对应的右下方部分为GPIO硬件操作接口，也就是说对外提供的接口最终会一一对应的对硬件GPIO进行操作。

     再来看左边部分，左上方部分为一全局数组，记录各个GPIO的描述符，即对应左下方的gpio_desc结构体，其中gpio_chip指向硬件层的GPIO，flags为一标志位，用来指示当前GPIO是否已经占用，当用gpio_request申请GPIO资源时，flags位就会置位，当调用gpio_free释放GPIO资源时，flags就会清零。label是一个字符串指针，用来作说明。

     在软件上，我们首先通过函数gpiochip_add注册一个gpio_chip对应的gpio_desc到全局数组gpio描述符中。其中，一个描述符对应一个GPIO，所以如果我们要使用多个GPIO，那么就在gpio_chip结构体的ngpio指定个数，base为起始的GPIO号。

//每个引脚分配一个gpio_desc数据结构

struct gpio_desc {

    struct gpio_chip    *chip;

    unsigned long        flags;

};

/**

 * struct gpio_chip - abstract a GPIO controller

 * @label: for diagnostics

 * @dev: optional device providing the GPIOs

 * @owner: helps prevent removal of modules exporting active GPIOs

 * @request: optional hook for chip-specific activation, such as

 *    enabling module power and clock; may sleep

 * @free: optional hook for chip-specific deactivation, such as

 *    disabling module power and clock; may sleep

 * @direction_input: configures signal 'offset' as input, or returns error

 * @get: returns value for signal 'offset'; for output signals this

 *    returns either the value actually sensed, or zero

 * @direction_output: configures signal 'offset' as output, or returns error

 * @set: assigns output value for signal 'offset'

 * @to_irq: optional hook supporting non-static gpio_to_irq() mappings;

 *    implementation may not sleep

 * @dbg_show: optional routine to show contents in debugfs; default code

 *    will be used when this is omitted, but custom code can show extra

 *    state (such as pullup/pulldown configuration).

 * @base: identifies the first GPIO number handled by this chip; or, if

 *    negative during registration, requests dynamic ID allocation.

 * @ngpio: the number of GPIOs handled by this controller; the last GPIO

 *    handled is (base + ngpio - 1).

 * @can_sleep: flag must be set iff get()/set() methods sleep, as they

 *    must while accessing GPIO expander chips over I2C or SPI

 * @names: if set, must be an array of strings to use as alternative

 *      names for the GPIOs in this chip. Any entry in the array

 *      may be NULL if there is no alias for the GPIO, however the

 *      array must be @ngpio entries long.  A name can include a single printk

 *      format specifier for an unsigned int.  It is substituted by the actual

 *      number of the gpio.

 *

 * A gpio_chip can help platforms abstract various sources of GPIOs so

 * they can all be accessed through a common programing interface.

 * Example sources would be SOC controllers, FPGAs, multifunction

 * chips, dedicated GPIO expanders, and so on.

 *

 * Each chip controls a number of signals, identified in method calls

 * by 'offset' values in the range 0..(@ngpio - 1).  When those signals

 * are referenced through calls like gpio_get_value(gpio), the offset

 * is calculated by subtracting @base from the gpio number.

 */

struct gpio_chip {

//这些函数实现在archarmmach-s5pv210gpiolib.c    

    const char        *label;

    struct device        *dev;

    struct module        *owner;

    int            (*request)(struct gpio_chip *chip,

                        unsigned offset);

    void            (*free)(struct gpio_chip *chip,

                        unsigned offset);

    int            (*direction_input)(struct gpio_chip *chip,

                        unsigned offset);

    int            (*get)(struct gpio_chip *chip,

                        unsigned offset);

    int            (*direction_output)(struct gpio_chip *chip,

                        unsigned offset, int value);

    int            (*set_debounce)(struct gpio_chip *chip,

                        unsigned offset, unsigned debounce);

    void            (*set)(struct gpio_chip *chip,

                        unsigned offset, int value);

    int            (*to_irq)(struct gpio_chip *chip,

                        unsigned offset);

    void            (*dbg_show)(struct seq_file *s,

                        struct gpio_chip *chip);

    int            base;

    u16            ngpio;

    const char        *const *names;

    unsigned        can_sleep:1;

    unsigned        exported:1;

};

下面分析gpio_desc中成员chip的成员函数的实现：

__init int s5pv210_gpiolib_init(void)//在Linux初始化期间，此函数就执行了

{

    struct s3c_gpio_chip *chip = s5pv210_gpio_4bit;

    int nr_chips = ARRAY_SIZE(s5pv210_gpio_4bit);

    int i = 0;

    for (i = 0; i < nr_chips; i++, chip++) {

        if (chip->config == NULL)

            chip->config = &gpio_cfg;

        if (chip->base == NULL)

            chip->base = S5PV210_BANK_BASE(i);

    }

    samsung_gpiolib_add_4bit_chips(s5pv210_gpio_4bit, nr_chips);

    return 0;

}

void __init samsung_gpiolib_add_4bit_chips(struct s3c_gpio_chip *chip,

                       int nr_chips)

{

    for (; nr_chips > 0; nr_chips--, chip++) {

        samsung_gpiolib_add_4bit(chip);

        s3c_gpiolib_add(chip);

    }

}

void __init samsung_gpiolib_add_4bit(struct s3c_gpio_chip *chip)

{

    chip->chip.direction_input = samsung_gpiolib_4bit_input;

    chip->chip.direction_output = samsung_gpiolib_4bit_output;

    chip->pm = __gpio_pm(&s3c_gpio_pm_4bit);

}

__init void s3c_gpiolib_add(struct s3c_gpio_chip *chip)

{

    struct gpio_chip *gc = &chip->chip;

    int ret;

    BUG_ON(!chip->base);

    BUG_ON(!gc->label);

    BUG_ON(!gc->ngpio);

    spin_lock_init(&chip->lock);// 初始化s3c_gpio_chip的自旋锁

if (!gc->direction_input)

        gc->direction_input = s3c_gpiolib_input;//chip->direction_input

    if (!gc->direction_output)

        gc->direction_output = s3c_gpiolib_output;//chip->direction_output

    if (!gc->set)

        gc->set = s3c_gpiolib_set;//chip->set此处就回答了上篇的疑问

    if (!gc->get)

        gc->get = s3c_gpiolib_get;//chip->get

#ifdef CONFIG_PM

    if (chip->pm != NULL) {

        if (!chip->pm->save || !chip->pm->resume)

            printk(KERN_ERR 'gpio: %s has missing PM functionsn',

                   gc->label);

    } else

        printk(KERN_ERR 'gpio: %s has no PM functionn', gc->label);

#endif

    /* gpiochip_add() prints own failure message on error. */

    ret = gpiochip_add(gc);

    if (ret >= 0)

        s3c_gpiolib_track(chip);

}

/**

 * gpiochip_add() - register a gpio_chip

 * @chip: the chip to register, with chip->base initialized

 * Context: potentially before irqs or kmalloc will work

 *

 * Returns a negative errno if the chip can't be registered, such as

 * because the chip->base is invalid or already associated with a

 * different chip.  Otherwise it returns zero as a success code.

 *

 * When gpiochip_add() is called very early during boot, so that GPIOs

 * can be freely used, the chip->dev device must be registered before

 * the gpio framework's arch_initcall().  Otherwise sysfs initialization

 * for GPIOs will fail rudely.

 *

 * If chip->base is negative, this requests dynamic assignment of

 * a range of valid GPIOs.

 */

int gpiochip_add(struct gpio_chip *chip) // 在gpio_desc[]中分配空间，并链接chip结构；注册一个gpio_chip对应的gpio_desc到全局数组gpio描述符中

{

    unsigned long    flags;

    int        status = 0;

    unsigned    id;

    int        base = chip->base;

    if ((!gpio_is_valid(base) || !gpio_is_valid(base + chip->ngpio - 1))

            && base >= 0) {

        status = -EINVAL;

        goto fail;

    }

    spin_lock_irqsave(&gpio_lock, flags);

    if (base < 0) {

        base = gpiochip_find_base(chip->ngpio);// 这个函数在gpiolib.c中，在gpio_desc[]中分配chip->ngpio个空间(从最后往前分配)，返回第一个index