十二、使用PWM调整LCD背光亮度

 

和手机一样,开发板中也带有调整背光亮度的功能。

调整背光亮度依赖于PWM,它通过调节脉冲宽度来控制背光亮度,此方式需要使用PWM驱动。本章将对其进行讲解。

 

 

一、用户空间调整背光亮度 

一般应用程序可以通过/sys/class/目录下的节点间接调整各个外设的参数。如下图,可通过命令行来控制背光亮度。设备节点不同开发板的目录不一定相同,读者需自行测试。

十二、使用PWM调整LCD背光亮度

如果读者确定自己的开发板有PWM控制背光的功能,但是在LCD、背光和PWM等相关目录没有找到调整亮度操作,可能的原因有PWM没有被编译进内核。

 

我们可以在配置中执行搜索操作,如:

$ make menuconfig  进入内核配置页面

按 / 键搜索,确认背光PWM是否被编译进内核。

如下图,我的背光PWM被编译进内核中了。需要注意,不同内核可能选项位置不同。

十二、使用PWM调整LCD背光亮度

 

 

二、PWM子系统

在Linux3.5版本中并没有引入PWM子系统,而是使用总线设备驱动模型。我在此以Linux-4.4为例谈论PWM子系统。

PWM也分为设备属性和行为。属性使用struct pwm_device表示,行为使用struct pwm_ops表示,两结构体定义如下:

struct pwm_device {
    const char *label;
    unsigned long flags;
    unsigned int hwpwm;
    unsigned int pwm;
    struct pwm_chip *chip;
    void *chip_data;
    struct mutex lock;

    unsigned int period;        /* PWM周期 */
    unsigned int duty_cycle;    /* 占空比 */
    enum pwm_polarity polarity; /* 极性反转 */
};
enum pwm_polarity {
    PWM_POLARITY_NORMAL,
    PWM_POLARITY_INVERSED,
};

struct pwm_ops {
    int (*request)(struct pwm_chip *chip, struct pwm_device *pwm);
    void (*free)(struct pwm_chip *chip, struct pwm_device *pwm);
    int (*config)(struct pwm_chip *chip, struct pwm_device *pwm,
              int duty_ns, int period_ns);
    int (*set_polarity)(struct pwm_chip *chip, struct pwm_device *pwm,
                enum pwm_polarity polarity);
    int (*enable)(struct pwm_chip *chip, struct pwm_device *pwm);
    void (*disable)(struct pwm_chip *chip, struct pwm_device *pwm);
#ifdef CONFIG_DEBUG_FS
    void (*dbg_show)(struct pwm_chip *chip, struct seq_file *s);
#endif
    struct module *owner;
};

pwm_ops的上层函数有:

/* 下列函数与pwm_ops位置一一对应 */
struct pwm_device *pwm_request(int pwm, const char *label);
void pwm_free(struct pwm_device *pwm);
int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);
int pwm_set_polarity(struct pwm_device *pwm, enum pwm_polarity polarity);
int pwm_enable(struct pwm_device *pwm);
void pwm_disable(struct pwm_device *pwm);

 

PWM虽然分层和LCD一致,但这两个结构体并不像LCD中可以互相访问。如LCD中fb_info定义有:

struct fb_info {
    ...
    struct fb_ops *fbops;    /* 可通过fb_info访问fb操作函数 */
    ...
};

既然struct pwm_device和struct pwm_ops不能互相访问,那么一定会有其他结构体连接两者。它就是struct pwm_chip:

struct pwm_chip {
    struct device *dev;
    struct list_head list;
    const struct pwm_ops *ops;  /* PWM操作函数 */
    int base;
    unsigned int npwm;          /* 这个chip中的PWM个数 */

    struct pwm_device *pwms;    /* PWM属性 */

    struct pwm_device * (*of_xlate)(struct pwm_chip *pc,
                    const struct of_phandle_args *args);
    unsigned int of_pwm_n_cells;
    bool can_sleep;
};

 

如果我们需要注册或注销PWM,使用的结构体应该也是struct pwm_chip。也就是PWM以chip注册。

注册函数pwmchip_add()实现如下:

 1 /* chip的属性 */
 2 static struct attribute *pwm_chip_attrs[] = {
 3     &dev_attr_export.attr,      /* 注册pwm,创建设备节点 */
 4     &dev_attr_unexport.attr,    /* 注销pwm,销毁设备节点 */
 5     &dev_attr_npwm.attr,        /* 当前chip中pwm个数 */
 6     NULL,
 7 };
 8 
 9 int pwmchip_add(struct pwm_chip *chip)
10 {
11     return pwmchip_add_with_polarity(chip, PWM_POLARITY_NORMAL);
12 }
13 EXPORT_SYMBOL_GPL(pwmchip_add);
14 
15 int pwmchip_add_with_polarity(struct pwm_chip *chip, enum pwm_polarity polarity)
16 {
17     struct pwm_device *pwm;
18     unsigned int i;
19     int ret;
20 ...
21     /* pwm子系统和input子系统类似,按顺序填充的,但pwm子系统没有设备号 */
22     ret = alloc_pwms(chip->base, chip->npwm);
23 ...
24     /* 申请空间 */
25     chip->pwms = kzalloc(chip->npwm * sizeof(*pwm), GFP_KERNEL);
26 ...
27     chip->base = ret;
28     /* 设置pwm_device */
29     for (i = 0; i < chip->npwm; i++) {
30         pwm = &chip->pwms[i];
31         pwm->chip = chip;
32         pwm->pwm = chip->base + i;
33         pwm->hwpwm = i;
34         pwm->polarity = polarity;
35 ...
36     }
37 ...
38     /* 创建pwmchip%d设备 */
39     pwmchip_sysfs_export(chip);
40 ...
41 }
42 
43 void pwmchip_sysfs_export(struct pwm_chip *chip)
44 {
45 ...
46     parent = device_create(&pwm_class, chip->dev, MKDEV(0, 0), chip, "pwmchip%d", chip->base);
47 ...
48 }

 

注销函数pwmchip_remove()实现如下:

 1 int pwmchip_remove(struct pwm_chip *chip)
 2 {
 3     unsigned int i;
 4     int ret = 0;
 5     /* 此函数会卸载此chip下的所有PWM */
 6     pwmchip_sysfs_unexport_children(chip);
 7 ...
 8     free_pwms(chip);
 9     /* 注销设备 */
10     pwmchip_sysfs_unexport(chip);
11 ...
12 }
13 EXPORT_SYMBOL_GPL(pwmchip_remove);
14 
15 void pwmchip_sysfs_unexport(struct pwm_chip *chip)
16 {
17     struct device *parent;
18 
19     parent = class_find_device(&pwm_class, NULL, chip,
20                    pwmchip_sysfs_match);
21     if (parent) {
22         /* for class_find_device() */
23         put_device(parent);
24         device_unregister(parent);
25     }
26 }

 

在注册和注销pwmchip函数中,只是创建和删除和设备节点。但是在此之前,它并没有创建类,而是使用了类pwm_class。分析可知PWM子系统在初始化过程中一定创建了类pwm_class。

 1 static struct class pwm_class = {
 2     .name = "pwm",                    /* 类名为pwm */
 3     .owner = THIS_MODULE,
 4     .dev_groups = pwm_chip_groups,    /* 存放所有chip */
 5 };
 6 
 7 static int __init pwm_sysfs_init(void)
 8 {
 9     return class_register(&pwm_class);
10 }

 

现在,我们来整理一下整体框架。

十二、使用PWM调整LCD背光亮度

 

 

二、三星平台驱动分析

1. platform_device

 1 static struct resource samsung_pwm_resource[] = {
 2     DEFINE_RES_MEM(SAMSUNG_PA_TIMER, SZ_4K),
 3 };
 4 
 5 struct platform_device samsung_device_pwm = {
 6     .name        = "samsung-pwm",
 7     .id        = -1,
 8     .num_resources    = ARRAY_SIZE(samsung_pwm_resource),
 9     .resource    = samsung_pwm_resource,
10 };

 

2. platform_driver

 1 static struct platform_driver pwm_samsung_driver = {
 2     .driver        = {
 3         .name    = "samsung-pwm",
 4         .pm    = &pwm_samsung_pm_ops,
 5         .of_match_table = of_match_ptr(samsung_pwm_matches),
 6     },
 7     .probe        = pwm_samsung_probe,
 8     .remove        = pwm_samsung_remove,
 9 };
10 module_platform_driver(pwm_samsung_driver);

 

我们来分析probe()函数,它设置并注册了pwmchip:

 1 struct samsung_pwm_chip {
 2     struct pwm_chip chip;    /* pwm_chip */
 3     struct samsung_pwm_variant variant;
 4     u8 inverter_mask;
 5 
 6     void __iomem *base;
 7     struct clk *base_clk;
 8     struct clk *tclk0;
 9     struct clk *tclk1;
10 };
11 
12 static int pwm_samsung_probe(struct platform_device *pdev)
13 {
14     struct device *dev = &pdev->dev;
15     struct samsung_pwm_chip *chip;
16     struct resource *res;
17     unsigned int chan;
18     int ret;
19 
20     /* 分配内存 */
21     chip = devm_kzalloc(&pdev->dev, sizeof(*chip), GFP_KERNEL);
22     /* 设置chip */
23     chip->chip.dev = &pdev->dev;
24     chip->chip.ops = &pwm_samsung_ops;
25     chip->chip.base = -1;
26     chip->chip.npwm = SAMSUNG_PWM_NUM;
27     chip->inverter_mask = BIT(SAMSUNG_PWM_NUM) - 1;
28 ...
29         memcpy(&chip->variant, pdev->dev.platform_data, sizeof(chip->variant));
30 
31     res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
32     chip->base = devm_ioremap_resource(&pdev->dev, res);
33 ...
34     chip->base_clk = devm_clk_get(&pdev->dev, "timers");
35 ...
36     ret = clk_prepare_enable(chip->base_clk);
37 ...
38     for (chan = 0; chan < SAMSUNG_PWM_NUM; ++chan)
39         if (chip->variant.output_mask & BIT(chan))
40             pwm_samsung_set_invert(chip, chan, true);
41 
42     /* Following clocks are optional. */
43     chip->tclk0 = devm_clk_get(&pdev->dev, "pwm-tclk0");
44     chip->tclk1 = devm_clk_get(&pdev->dev, "pwm-tclk1");
45 
46     platform_set_drvdata(pdev, chip);
47     /* 注册pwm */
48     ret = pwmchip_add(&chip->chip);
49 ...
50     return 0;
51 }

 

remove()应该注销pwmchip:

 1 static int pwm_samsung_remove(struct platform_device *pdev)
 2 {
 3     struct samsung_pwm_chip *chip = platform_get_drvdata(pdev);
 4     int ret;
 5 
 6     ret = pwmchip_remove(&chip->chip);
 7     if (ret < 0)
 8         return ret;
 9 
10     clk_disable_unprepare(chip->base_clk);
11 
12     return 0;
13 }

 

关于三星平台的struct pwm_ops pwm_samsung_ops在此不做分析,这些函数只是对寄存器进行读写操作。

 

最后总结一下:

1. PWM是通过注册chip来注册一个芯片中所有的PWM的

2. 对于具体的PWM,可以使用sysfs中的attributre属性来分辨

3. 注册完成某个PWM后,它的周期、占空比、极性和使能可以在sysfs中更改

4. 在sysfs中修改降低了驱动程序修改的次数

 

pwmchip在/sys/class/目录下生成,我们可以执行以下命令查看和修改pwmchip0的占空比:

# cd /sys/class/pwm/

# ls

# cd ./pwmchip0

# ls

# cd ./pwm0

# ls

# ls duty_cycle

# echo 50 > duty_cycle

# ls duty_cycle

 

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