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