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: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 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 >= 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
if (base < 0) { 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 == 0) {// 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 ? (1 << FLAG_IS_OUT) : 0; } } unlock: spin_unlock_irqrestore(&gpio_lock, flags); if (status == 0) 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 - 1, chip->label ? : "generic"); return status; } EXPORT_SYMBOL_GPL(gpiochip_add);