上一篇介紹了GPIO的基礎，在這篇將詳細介紹 GPIOLIB
在以前，假設我們chip上面都有提供GPIO。假設我們今天要 call soc上面的GPIO我們可以使用gpio_direction_output()。但是如果要使用audio chip上面的GPIO以下圖為例，我們可能就得呼叫audio_gpio_direction_output() depends on chip 的driver實作。
為什麼呢？因為每個chip如何去控制他們的GPIO作法可能會不一樣，並且要填的memory位址也不一樣。這麼一來會造成什麼現象呢？如果chip一多，要分別控制上面的GPIO 我們可能就會看到一堆 audio_gpio_direction_output、 pmu_gpio_direction_output、 charger_gpio_direction_output。而且，以gpio_direction_output為例，可能每一個driver的實作都會有一些重複的code例如檢查是否valid等等。
因此，GPIOLIB的引入可以解決這個問題。每一個chip都使用gpio_chip結構來表示，每一個chip driver實作可以定義GPIO的個數、號碼及相關的callback function。
以gpio_direction_output為例。當我們呼叫gpio_direction_output(number)之後，gpio_direction_output實做在GPIOLIB中，它會去檢查這個number的GPIO是屬於哪個chip然後去call 該chip的driver的request實作。
註冊gpio_chip
我們從nVidia的git抓下來的kernel裡面隨便找driver/gpio/wm8994-gpio.c為例子trace一下。他的probe如下:
static struct gpio_chip template_chip = {
.label = "wm8994",
.owner = THIS_MODULE,
/**
*如果執行gpio_direction_input的GPIO number屬於這個chip
*則執行wm8994_gpio_direction_in這個callback function
*以下同義
*/
.direction_input = wm8994_gpio_direction_in,
.get = wm8994_gpio_get,
.direction_output = wm8994_gpio_direction_out,
.set = wm8994_gpio_set,
.dbg_show = wm8994_gpio_dbg_show,
.can_sleep = 1,
};
static int __devinit wm8994_gpio_probe(struct platform_device *pdev)
{
struct wm8994 *wm8994 = dev_get_drvdata(pdev->dev.parent);
//取得platform_device定義的platform_data
struct wm8994_pdata *pdata = wm8994->dev->platform_data;
struct wm8994_gpio *wm8994_gpio;
int ret;
wm8994_gpio = kzalloc(sizeof(*wm8994_gpio), GFP_KERNEL);
if (wm8994_gpio == NULL)
return -ENOMEM;
wm8994_gpio->wm8994 = wm8994;
/**
*我們剛剛有說每一個chip使用gpio_chip結構表示。
*上面宣告了template_chip的gpio_chip結構。
*/
wm8994_gpio->gpio_chip = template_chip;
wm8994_gpio->gpio_chip.ngpio = WM8994_GPIO_MAX;
wm8994_gpio->gpio_chip.dev = &pdev->dev;
//取得gpio_base。意思是這個chip的GPIO number從多少開始編號
if (pdata && pdata->gpio_base)
wm8994_gpio->gpio_chip.base = pdata->gpio_base;
else
wm8994_gpio->gpio_chip.base = -1;
//將這個gpio_chip 註冊進去。待會將分析gpiochip_add
ret = gpiochip_add(&wm8994_gpio->gpio_chip);
if (ret < 0) {
dev_err(&pdev->dev, "Could not register gpiochip, %d\n",
ret);
goto err;
}
platform_set_drvdata(pdev, wm8994_gpio);
return ret;
err:
kfree(wm8994_gpio);
return ret;
}
接著我們看一下gpiochip_add() 做了什麼事。
//擷取gpio_desc的定義。gpio_desc定義了GPIO是屬於哪個chip，他的flag及label。
struct gpio_desc {
struct gpio_chip *chip;
unsigned long flags;
#ifdef CONFIG_DEBUG_FS
const char *label;
#endif
};
//gpio_desc陣列存放著所有gpio的定義
static struct gpio_desc gpio_desc[ARCH_NR_GPIOS];
int gpiochip_add(struct gpio_chip *chip)
{
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);
if (base < 0) {
status = base;
goto unlock;
}
chip->base = base;
}
/**
*these GPIO numbers must not be managed by another gpio_chip
*換句話說就是沒有重複到其他gpio_chip定義的號碼
*/
for (id = base; id < base + chip->ngpio; id++) {
if (gpio_desc[id].chip != NULL) {
status = -EBUSY;
break;
}
}
if (status == 0) {
for (id = base; id < base + chip->ngpio; id++) {
//將這個chip上的所有GPIO的定義加入gpio_desc
gpio_desc[id].chip = chip;
/* 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;
}
}
of_gpiochip_add(chip);
unlock:
spin_unlock_irqrestore(&gpio_lock, flags);
if (status)
goto fail;
status = gpiochip_export(chip);
if (status)
goto fail;
return 0;
fail:
/* failures here can mean systems won't boot... */
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);
到這裡，我們的kernel已經多了wm8994這個chip的GPIO可以使用了。但是從我們呼叫gpio_direction_input是怎麼跑到driver定義的wm8994_gpio_direction_in呢？
GPIOLIB與CHIP DRIVER結合
我們看一下gpio_direction_input() 的code。
int gpio_direction_input(unsigned gpio)
{
unsigned long flags;
struct gpio_chip *chip;
struct gpio_desc *desc = &gpio_desc[gpio];
int status = -EINVAL;
spin_lock_irqsave(&gpio_lock, flags);
//先檢查這個gpio是否valid
if (!gpio_is_valid(gpio))
goto fail;
//找到這個gpio屬於哪個chip的
chip = desc->chip;
//如果chip不存在或這個gpio不能設為input則return fail
if (!chip || !chip->get || !chip->direction_input)
goto fail;
//下面兩行檢查該GPIO的編號是否在該chip的範圍內
gpio -= chip->base;
if (gpio >= chip->ngpio)
goto fail;
status = gpio_ensure_requested(desc, gpio);
if (status < 0)
goto fail;
/* now we know the gpio is valid and chip won't vanish */
spin_unlock_irqrestore(&gpio_lock, flags);
might_sleep_if(chip->can_sleep);
if (status) {
//呼叫chip定義的request檢查是否可用
status = chip->request(chip, gpio);
if (status < 0) {
pr_debug("GPIO-%d: chip request fail, %d\n",
chip->base + gpio, status);
/* and it's not available to anyone else ...
* gpio_request() is the fully clean solution.
*/
goto lose;
}
}
//如果是可用的，call chip的direction_input（）
status = chip->direction_input(chip, gpio);
if (status == 0)
clear_bit(FLAG_IS_OUT, &desc->flags);
trace_gpio_direction(chip->base + gpio, 1, status);
lose:
return status;
fail:
spin_unlock_irqrestore(&gpio_lock, flags);
if (status)
pr_debug("%s: gpio-%d status %d\n",
__func__, gpio, status);
return status;
}
EXPORT_SYMBOL_GPL(gpio_direction_input);
至此，應該就都串起來了吧！簡單來說就如下圖:
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