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文章代码分析基于linux-5.19.13，架构基于aarch64（ARM64）。

1. 前言

复杂IC内部有很多具有独立功能的硬件模块，例如CPU cores、GPU cores、USB控制器、MMC控制器、等等，出于功耗、稳定性等方面的考虑，有些IC在内部为这些硬件模块设计了复位信号（reset signals），软件可通过寄存器（一般1个bit控制1个硬件）控制这些硬件模块的复位状态。
Linux kernel为了方便设备驱动的编写，抽象出一个简单的软件框架----reset framework，为reset的provider提供统一的reset资源管理手段，并为reset的consumer（各个硬件模块）提供便捷、统一的复位控制API。

2. 前言

reset子系统也分为了consumer和provider，结构体关系如下：

3. consumer

对于一个具体的硬件模块，它的要求很简单：复位我的硬件模块，而不必关注具体复位的手段（例如控制哪个寄存器的哪个bit位，等等）。

Linux kernel基于device tree提供了对应的reset framework：

首先，提供描述系统中reset资源的方法（参考provider的介绍），这样consumer可以基于这种描述，在自己的dts node中引用所需的reset信号。
然后，consumer设备在自己的dts node中使用“resets”、“reset-names”等关键字声明所需的reset的资源，例如("resets"字段的具体格式由reset provider决定)：

device {
  resets = <&rst 20>;
  reset-names = "reset"; 
};
This represents a device with a single reset signal named "reset".

bus { 
  resets = <&rst 10> <&rst 11> <&rst 12> <&rst 11>;
  reset-names = "i2s1", "i2s2", "dma", "mixer"; 
};
This represents a bus that controls the reset signal of each of four sub- ordinate devices. Consider for example a bus that fails to operate unless no child device has reset asserted.

最后，consumer driver在需要的时候，可以调用下面的API复位自己（具体可参考"include\linux\reset.h"）：

只有一个reset信号的话，可以使用最简单的device_reset API

static inline int __must_check device_reset(struct device *dev)

如果需要更为复杂的控制（例如有多个reset信号、需要控制处于reset状态的长度的等），可以使用稍微复杂的API

/* 通过reset_control_get或者devm_reset_control_get获得reset句柄 */
struct reset_control *reset_control_get(struct device *dev, const char *id);                             
struct reset_control *devm_reset_control_get(struct device *dev, const char *id);

/* 通过reset_control_put释放reset句柄 */
void reset_control_put(struct reset_control *rstc);  

/* 通过reset_control_reset进行复位，或者通过reset_control_assert使设备处于复位生效状态，通过reset_control_deassert使复位失效 */
int reset_control_reset(struct reset_control *rstc);   /先复位，延迟一会，然后解复位                        
int reset_control_assert(struct reset_control *rstc);  //复位                       
int reset_control_deassert(struct reset_control *rstc);//解复位

4. provider

kernel为reset provider提供的API位于"include/linux/reset-controller.h"中，很简单，无非就是：创建并填充reset controller设备（struct reset_controller_dev），并调用相应的接口:

reset_controller_register //注册reset_controller
reset_controller_unregister //注销reset_controller

reset controller的抽象也很简单：

/**
 * struct reset_controller_dev - reset controller entity that might
 *                               provide multiple reset controls
 * @ops: a pointer to device specific struct reset_control_ops
 * @owner: kernel module of the reset controller driver
 * @list: internal list of reset controller devices
 * @reset_control_head: head of internal list of requested reset controls
 * @dev: corresponding driver model device struct
 * @of_node: corresponding device tree node as phandle target
 * @of_reset_n_cells: number of cells in reset line specifiers
 * @of_xlate: translation function to translate from specifier as found in the
 *            device tree to id as given to the reset control ops, defaults
 *            to :c:func:`of_reset_simple_xlate`.
 * @nr_resets: number of reset controls in this reset controller device
 */
struct reset_controller_dev {
	const struct reset_control_ops *ops;//ops提供reset操作的实现，基本上是reset provider的所有工作量。
	struct module *owner;
	struct list_head list;////全局链表，复位控制器注册后挂载到全局链表
	struct list_head reset_control_head;////各个模块复位的链表头
	struct device *dev;
	struct device_node *of_node;
	int of_reset_n_cells;////用于解析consumer device dts node中的“resets = <>; ”节点,指示dts中引用时，需要几个参数
	int (*of_xlate)(struct reset_controller_dev *rcdev,
			const struct of_phandle_args *reset_spec);//用于解析consumer device dts node中的“resets = <>; ”节点
	unsigned int nr_resets;//该reset controller所控制的reset信号的个数
};

struct reset_control_ops也比较单纯，如下：

/**
 * struct reset_control_ops - reset controller driver callbacks
 *
 * @reset: for self-deasserting resets, does all necessary
 *         things to reset the device
 * @assert: manually assert the reset line, if supported
 * @deassert: manually deassert the reset line, if supported
 * @status: return the status of the reset line, if supported
 */
struct reset_control_ops {
	int (*reset)(struct reset_controller_dev *rcdev, unsigned long id);   //控制设备完成一次完整的复位过程
	int (*assert)(struct reset_controller_dev *rcdev, unsigned long id);  //控制设备reset状态的生效
	int (*deassert)(struct reset_controller_dev *rcdev, unsigned long id);//控制设备reset状态的失效。
	int (*status)(struct reset_controller_dev *rcdev, unsigned long id);  //复位状态查询
};

5. reset驱动的设备树描述总结

5.1 对于provider

reset:reset-controller{
  compatible = "xx,xx-reset";
  reg = <0x0 0x30390000 0x0 0x10000>;
  #reset-cells = <1>;
};

上述是一个reset控制器的节点，0x30390000 是寄存器基址，0x1000是映射大小。"#reset-cells"代表引用该reset时需要的cells个数。

5.2 对于consumer

例如，#reset-cells = <1>; 则正确引用为：

mmc:mmc@0x12345678{
    ......
    resets = <&reset  0>;//0代表reset设备id，id是自定义的，但是不能超过reset驱动中指定的设备个数
    ......
};

6. 开源reset驱动实例

6.1 实例1(比较容易理解)

设备树： arch/arm/boot/dts/imx7d.dtsi

	pcie: pcie@0x33800000 {
		compatible = "fsl,imx7d-pcie", "snps,dw-pcie";

		....

		resets = <&src IMX7_RESET_PCIEPHY>,
			 <&src IMX7_RESET_PCIE_CTRL_APPS_EN>;
		reset-names = "pciephy", "apps";
		status = "disabled";
	};

驱动代码： drivers/reset/reset-imx7.c

...

struct imx7_src {
	struct reset_controller_dev rcdev;
	struct regmap *regmap;
};

enum imx7_src_registers {
	SRC_A7RCR0		= 0x0004,
	SRC_M4RCR		= 0x000c,
	SRC_ERCR		= 0x0014,
	SRC_HSICPHY_RCR		= 0x001c,
	SRC_USBOPHY1_RCR	= 0x0020,
	SRC_USBOPHY2_RCR	= 0x0024,
	SRC_MIPIPHY_RCR		= 0x0028,
	SRC_PCIEPHY_RCR		= 0x002c,
	SRC_DDRC_RCR		= 0x1000,
};

struct imx7_src_signal {
	unsigned int offset, bit;
};

static const struct imx7_src_signal imx7_src_signals[IMX7_RESET_NUM] = {
	[IMX7_RESET_A7_CORE_POR_RESET0] = { SRC_A7RCR0, BIT(0) },
	[IMX7_RESET_A7_CORE_POR_RESET1] = { SRC_A7RCR0, BIT(1) },
	[IMX7_RESET_A7_CORE_RESET0]     = { SRC_A7RCR0, BIT(4) },
	[IMX7_RESET_A7_CORE_RESET1]	= { SRC_A7RCR0, BIT(5) },
	[IMX7_RESET_A7_DBG_RESET0]	= { SRC_A7RCR0, BIT(8) },
	[IMX7_RESET_A7_DBG_RESET1]	= { SRC_A7RCR0, BIT(9) },
	...
};

static struct imx7_src *to_imx7_src(struct reset_controller_dev *rcdev)
{
	return container_of(rcdev, struct imx7_src, rcdev);
}

static int imx7_reset_set(struct reset_controller_dev *rcdev,
			  unsigned long id, bool assert)
{
	struct imx7_src *imx7src = to_imx7_src(rcdev);
	const struct imx7_src_signal *signal = &imx7_src_signals[id];
	unsigned int value = assert ? signal->bit : 0;

	switch (id) {
	case IMX7_RESET_PCIEPHY:
		/*
		 * wait for more than 10us to release phy g_rst and
		 * btnrst
		 */
		if (!assert)
			udelay(10);
		break;

	case IMX7_RESET_PCIE_CTRL_APPS_EN:
		value = (assert) ? 0 : signal->bit;
		break;
	}

	return regmap_update_bits(imx7src->regmap,
				  signal->offset, signal->bit, value);
}

static int imx7_reset_assert(struct reset_controller_dev *rcdev,
			     unsigned long id)
{
	return imx7_reset_set(rcdev, id, true);
}

static int imx7_reset_deassert(struct reset_controller_dev *rcdev,
			       unsigned long id)
{
	return imx7_reset_set(rcdev, id, false);
}

static const struct reset_control_ops imx7_reset_ops = {
	.assert		= imx7_reset_assert,
	.deassert	= imx7_reset_deassert,
};

static int imx7_reset_probe(struct platform_device *pdev)
{
	struct imx7_src *imx7src;
	struct device *dev = &pdev->dev;
	struct regmap_config config = { .name = "src" };

	imx7src = devm_kzalloc(dev, sizeof(*imx7src), GFP_KERNEL);
	if (!imx7src)
		return -ENOMEM;

	imx7src->regmap = syscon_node_to_regmap(dev->of_node);
	if (IS_ERR(imx7src->regmap)) {
		dev_err(dev, "Unable to get imx7-src regmap");
		return PTR_ERR(imx7src->regmap);
	}
	regmap_attach_dev(dev, imx7src->regmap, &config);

	imx7src->rcdev.owner     = THIS_MODULE;
	imx7src->rcdev.nr_resets = IMX7_RESET_NUM;
	imx7src->rcdev.ops       = &imx7_reset_ops;
	imx7src->rcdev.of_node   = dev->of_node;

	return devm_reset_controller_register(dev, &imx7src->rcdev);
}

static const struct of_device_id imx7_reset_dt_ids[] = {
	{ .compatible = "fsl,imx7d-src", },
	{ /* sentinel */ },
};

static struct platform_driver imx7_reset_driver = {
	.probe	= imx7_reset_probe,
	.driver = {
		.name		= KBUILD_MODNAME,
		.of_match_table	= imx7_reset_dt_ids,
	},
};
builtin_platform_driver(imx7_reset_driver);

6.2 实例2(在gpio子系统中嵌套reset子系统)

设备树： arc/arm64/boot/dts/myzr/myimx8mm.dts

&pcie0{
	pinctrl-names = "default";
	pinctrl-0 = <&pinctrl_i2c4_pcieclk>, <&pinctrl_gpio1_pciendis>, <&pinctrl_sd2_pciewake>, <&pinctrl_sai2_pcienrst>;
	disable-gpio = <&gpio1 5 GPIO_ACTIVE_LOW>;
	reset-gpio = <&gpio4 21 GPIO_ACTIVE_LOW>;
	ext_osc = <1>;
	status = "okay";
};

驱动代码： drivers/reset/gpio-reset.c

...

struct gpio_reset_data {
	struct reset_controller_dev rcdev;
	unsigned int gpio;
	bool active_low;
	s32 delay_us;
	s32 post_delay_ms;
};

static void gpio_reset_set(struct reset_controller_dev *rcdev, int asserted)
{
	struct gpio_reset_data *drvdata = container_of(rcdev,
			struct gpio_reset_data, rcdev);
	int value = asserted;

	if (drvdata->active_low)
		value = !value;

	gpio_set_value_cansleep(drvdata->gpio, value);
}

static int gpio_reset(struct reset_controller_dev *rcdev, unsigned long id)
{
	struct gpio_reset_data *drvdata = container_of(rcdev,
			struct gpio_reset_data, rcdev);

	if (drvdata->delay_us < 0)
		return -ENOSYS;

	gpio_reset_set(rcdev, 1);
	udelay(drvdata->delay_us);
	gpio_reset_set(rcdev, 0);

	if (drvdata->post_delay_ms < 0)
		return 0;

	msleep(drvdata->post_delay_ms);
	return 0;
}

static int gpio_reset_assert(struct reset_controller_dev *rcdev,
		unsigned long id)
{
	gpio_reset_set(rcdev, 1);

	return 0;
}

static int gpio_reset_deassert(struct reset_controller_dev *rcdev,
		unsigned long id)
{
	gpio_reset_set(rcdev, 0);

	return 0;
}

static struct reset_control_ops gpio_reset_ops = {
	.reset = gpio_reset,
	.assert = gpio_reset_assert,
	.deassert = gpio_reset_deassert,
};

static int of_gpio_reset_xlate(struct reset_controller_dev *rcdev,
			       const struct of_phandle_args *reset_spec)
{
	if (WARN_ON(reset_spec->args_count != 0))
		return -EINVAL;

	return 0;
}

static int gpio_reset_probe(struct platform_device *pdev)
{
	...

	drvdata = devm_kzalloc(&pdev->dev, sizeof(*drvdata), GFP_KERNEL);

	...

	drvdata->rcdev.of_node = np;
	drvdata->rcdev.owner = THIS_MODULE;
	drvdata->rcdev.nr_resets = 1;                  ////该reset controller所控制的reset信号的个数
	drvdata->rcdev.ops = &gpio_reset_ops;          //ops提供reset操作的实现。
	drvdata->rcdev.of_xlate = of_gpio_reset_xlate; 
	reset_controller_register(&drvdata->rcdev);    //注册reset controller

	return 0;
}

static int gpio_reset_remove(struct platform_device *pdev)
{
	struct gpio_reset_data *drvdata = platform_get_drvdata(pdev);

	reset_controller_unregister(&drvdata->rcdev);

	return 0;
}

static struct of_device_id gpio_reset_dt_ids[] = {
	{ .compatible = "gpio-reset" },
	{ }
};

#ifdef CONFIG_PM_SLEEP
static int gpio_reset_suspend(struct device *dev)
{
	pinctrl_pm_select_sleep_state(dev);

	return 0;
}
static int gpio_reset_resume(struct device *dev)
{
	pinctrl_pm_select_default_state(dev);

	return 0;
}
#endif

static const struct dev_pm_ops gpio_reset_pm_ops = {
	SET_LATE_SYSTEM_SLEEP_PM_OPS(gpio_reset_suspend, gpio_reset_resume)
};

static struct platform_driver gpio_reset_driver = {
	.probe = gpio_reset_probe,
	.remove = gpio_reset_remove,
	.driver = {
		.name = "gpio-reset",
		.owner = THIS_MODULE,
		.of_match_table = of_match_ptr(gpio_reset_dt_ids),
		.pm = &gpio_reset_pm_ops,
	},
};

static int __init gpio_reset_init(void)
{
	return platform_driver_register(&gpio_reset_driver);
}
arch_initcall(gpio_reset_init);

static void __exit gpio_reset_exit(void)
{
	platform_driver_unregister(&gpio_reset_driver);
}
...

7. reset驱动的实质

操作soc对应的reset寄存器，以实现内核IP的复位，或者操作gpio管脚的电平，间接复位接到该pin脚的从设备。

参考

[1] Documentation/devicetree/bindings/reset/reset.txt
[2] Linux reset framework
[2] Linux reset子系统及驱动实例文章来源地址https://www.toymoban.com/news/detail-579414.html

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