platform驱动编写 基于gpio-keys

加入of_ （device tree操作api） 的platform驱动编写 基于gpio-keys

Linux在启动后，到C入口时，会执行以下操作，加载系统平台上的总线和设备：

start_kernel() --> setup_arch() --> unflatten_device_tree()  

在执行完unflatten_device_tree()后，DTS节点信息被解析出来，保存到allnodes链表中，allnodes会在后面被用到。随后，当系统启动到board文件时，会调用.init_machine，高通8974平台对应的是msm8974_init()。接着调用of_platform_populate(....)接口，加载平台总线和平台设备。

Device Tree 中的 I2C client 会透过 I2C host 驱动的 probe()函数中调用
of_i2c_register_devices(&i2c_dev->adapter);被自动展开

SPI host 驱动的 probe 函数透过
spi_register_master()注册 master 的时候，会自动展开依附于它的 slave。

整体platform驱动架构是这样的。platform bus一旦把platform driver里的of_device_id 

static struct of_device_id gpio_keys_of_match[] = {

{ .compatible = "gpio-keys", },

{ },

};

与dts(platform device)

gpio-keys {
compatible = "gpio-keys";
power {
label = "Power Button";
gpios = <&gpio3 29 1>;
linux,code = <116>; /* KEY_POWER */
gpio-key,wakeup;
};

volume-up {
label = "Volume Up";
gpios = <&gpio1 4 1>;
linux,code = <115>; /* KEY_VOLUMEUP */
};

volume-down {
label = "Volume Down";
gpios = <&gpio1 5 1>;
linux,code = <114>; /* KEY_VOLUMEDOWN */
};
};

里的compatible匹配到。则会调用platform里的probe成员函数

static int gpio_keys_probe(struct platform_device *pdev)
{
struct device *dev = &pdev->dev;
const struct gpio_keys_platform_data *pdata = dev_get_platdata(dev);

。。。。。。。。。。。。。。。。。。。。。。。

。。。。。。。。。。。。。。。。。。。

}

struct platform_device *pdev指向匹配成功的platform device.通过他我们可以找到对应于dts文件中的设备节点（定位到它就可以获取设备参数列表了）【因为新版内核struct device中包含了成员  struct device_node *of_node; /* associated device tree node */】

代码如下

gpio_keys_get_devtree_pdata(struct device *dev)
{
struct device_node *node, *pp;
struct gpio_keys_platform_data *pdata;
struct gpio_keys_button *button;
int error;
int nbuttons;
int i;

node = dev->of_node;
if (!node) {
error = -ENODEV;
goto err_out;
}

nbuttons = of_get_child_count(node);
if (nbuttons == 0) {
error = -ENODEV;
goto err_out;
}

pdata = kzalloc(sizeof(*pdata) + nbuttons * (sizeof *button),
GFP_KERNEL);
if (!pdata) {
error = -ENOMEM;
goto err_out;
}

pdata->buttons = (struct gpio_keys_button *)(pdata + 1);
pdata->nbuttons = nbuttons;

pdata->rep = !!of_get_property(node, "autorepeat", NULL);

i = 0;
for_each_child_of_node(node, pp) {
int gpio;
enum of_gpio_flags flags;

if (!of_find_property(pp, "gpios", NULL)) {
pdata->nbuttons--;
dev_warn(dev, "Found button without gpios\n");
continue;
}

gpio = of_get_gpio_flags(pp, 0, &flags);
if (gpio < 0) {
error = gpio;
if (error != -EPROBE_DEFER)
dev_err(dev,
"Failed to get gpio flags, error: %d\n",
error);
goto err_free_pdata;
}

button = &pdata->buttons[i++];

button->gpio = gpio;
button->active_low = flags & OF_GPIO_ACTIVE_LOW;

if (of_property_read_u32(pp, "linux,code", &button->code)) {
dev_err(dev, "Button without keycode: 0x%x\n",
button->gpio);
error = -EINVAL;
goto err_free_pdata;
}

button->desc = of_get_property(pp, "label", NULL);

if (of_property_read_u32(pp, "linux,input-type", &button->type))
button->type = EV_KEY;

button->wakeup = !!of_get_property(pp, "gpio-key,wakeup", NULL);

if (of_property_read_u32(pp, "debounce-interval",
&button->debounce_interval))
button->debounce_interval = 5;
}

if (pdata->nbuttons == 0) {
error = -EINVAL;
goto err_free_pdata;
}