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arch\arm\mach-s5pv210\gpiolib.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:;
    unsigned        exported:;
};

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 = ;

    for (i = ; 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 ;
}

void __init samsung_gpiolib_add_4bit_chips(struct s3c_gpio_chip *chip,
                       int nr_chips)
{
    for (; nr_chips > ; 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 functions\n",
                   gc->label);
    } else
        printk(KERN_ERR "gpio: %s has no PM function\n", gc->label);
#endif

    /* gpiochip_add() prints own failure message on error. */
    ret = gpiochip_add(gc);
    if (ret >= )
        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 = ;
    unsigned    id;
    int        base = chip->base;

    if ((!gpio_is_valid(base) || !gpio_is_valid(base + chip->ngpio - ))
            && base >= ) {
        status = -EINVAL;
        goto fail;
    }

    spin_lock_irqsave(&gpio_lock, flags);

    if (base < ) {
        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
       if (base < ) {
            status = base;
            goto unlock;
        }
        chip->base = base;
    }

    /* these GPIO numbers must not be managed by another gpio_chip */
    for (id = base; id < base + chip->ngpio; id++) {
        if (gpio_desc[id].chip != NULL) {
            status = -EBUSY;
            break;
        }
    }
    if (status == ) {// 分配到空间，正常情况下
      for (id = base; id < base + chip->ngpio; id++) {
            gpio_desc[id].chip = chip;// 这里将gpio_desc与s3c_gpio_chip联系起来，他们的chip成员指向的是同一个数据结构 

/* REVISIT:  most hardware initializes GPIOs as
             * inputs (often with pullups enabled) so power
             * usage is minimized.  Linux code should set the
             * gpio direction first thing; but until it does,
             * we may expose the wrong direction in sysfs.
             */
            gpio_desc[id].flags = !chip->direction_input
                ? ( << FLAG_IS_OUT)
                : ;
        }
    }

unlock:
    spin_unlock_irqrestore(&gpio_lock, flags);
    if (status == )
        status = gpiochip_export(chip);
fail:
    /* failures here can mean systems won't boot... */
    if (status)
        pr_err("gpiochip_add: gpios %d..%d (%s) failed to register\n",
            chip->base, chip->base + chip->ngpio - ,
            chip->label ? : "generic");
    return status;
}
EXPORT_SYMBOL_GPL(gpiochip_add);