Linux cpuidle framework(3)_ARM64 generic CPU idle driver
1. 前言
本文以ARM64平台下的cpuidle driver为例，说明怎样在cpuidle framework的框架下，编写cpuidle driver。另外，本文在描述cpuidle driver的同时，会涉及到CPU hotplug的概念，因此也可作为CPU hotplug的引子。
2. arm64_idle_init
ARM64 generic CPU idle driver的代码位于“drivers/cpuidle/cpuidle-arm64.c”中，它的入口函数是arm64_idle_init，如下：
1: static int __init arm64_idle_init(void) 2: { 3: int cpu, ret; 4: struct cpuidle_driver *drv = &arm64_idle_driver; 5:
6: /*
7: * Initialize idle states data, starting at index 1. 8: * This driver is DT only, if no DT idle states are detected (ret == 0) 9: * let the driver initialization fail accordingly since there is no 10: * reason to initialize the idle driver if only wfi is supported. 11: */ 12: ret = dt_init_idle_driver(drv, arm64_idle_state_match, 1); 13: if (ret <= 0) { 14: if (ret) 15: pr_err("failed to initialize idle states\n"); 16: return ret ? : -ENODEV; 17: } 18:
19: /*
20: * Call arch CPU operations in order to initialize 21: * idle states suspend back-end specific data 22: */ 23: for_each_possible_cpu(cpu) { 24: ret = cpu_init_idle(cpu); 25: if (ret) { 26: pr_err("CPU %d failed to init idle CPU ops\n", cpu); 27: return ret; 28: } 29: } 30:
31: ret = cpuidle_register(drv, NULL); 32: if (ret) { 33: pr_err("failed to register cpuidle driver\n"); 34: return ret; 35: } 36:
37: return 0; 38: } 39: device_initcall(arm64_idle_init);由该函数的执行过程，可以看出cpuidle driver的实现过程，包括：
1）静态定义一个struct cpuidle_driver变量（这里为arm64_idle_driver）并填充必要的字段，如下
1: static struct cpuidle_driver arm64_idle_driver = { 2: .name = "arm64_idle", 3: .owner = THIS_MODULE, 4: /*
5: * State at index 0 is standby wfi and considered standard 6: * on all ARM platforms. If in some platforms simple wfi 7: * can't be used as "state 0", DT bindings must be implemented 8: * to work around this issue and allow installing a special 9: * handler for idle state index 0. 10: */ 11: .states[0] = { 12: .enter = arm64_enter_idle_state, 13: .exit_latency = 1, 14: .target_residency = 1, 15: .power_usage = UINT_MAX, 16: .flags = CPUIDLE_FLAG_TIME_VALID, 17: .name = "WFI", 18: .desc = "ARM64 WFI", 19: } 20: };该driver的名称为“arm64_idle”，会体现在sysfs（如/sys/devices/system/cpu/cpuidle/current_driver）中；
稍微留意一下上面的注释，所有的ARM平台，都应提供默认的WFI standby状态，作为idle state 0，如果有例外，则需要在DTS中另行处理；
对于state0，driver将其初始化为：exit latency和target residency均为1（最小值），power usage为整数中的最大值。由此可以看出，这些信息不是实际信息（因为driver不可能知道所有ARM平台的WFI相关的信息），而是相对信息，其中的含义是：所有其它的state，exit latency和target residency都会比state0大，power usage都会比state0小，够用了！多么巧妙地设计！
2）初始化其它的idle states（从state1开始），必须通过DTS操作，否则返回失败。具体可参考后续的描述。
3）对每一个cpu，调用cpu_init_idle接口，初始化用于支持cpuidle的、和cpu suspend有关的后端数据。后面会详细介绍。
4）调用cpuidle_register，将cpuidle driver注册到cpuidle core中（具体可参考“Linux cpuidle framework(2)_cpuidle core”）。
3. dt_init_idle_driver
dt_init_idle_driver函数用于从DTS中解析出cpuidle states的信息，并初始化arm64_idle_driver中的states数组。在分析这个函数之前，我们先来看一下cpuidle相关的DTS源文件是怎么写的：
注：写这篇文章所参考的kernel（3.18-rc4）中，没有ARM64平台使用了cpuidle功能，因此这里给不出ARM64平台下的参考文件。好在ARM和ARM64在cpuidle dts解析上面的流程是一样的，我们可以借用ARM中的例子。
1: /* arch/arm/boot/dts/vexpress-v2p-ca15_a7.dts */ 2: cpus { 3: #address-cells = <1>; 4: #size-cells = <0>; 5:
6: cpu0: cpu@0 { 7: device_type = "cpu"; 8: compatible = "arm,cortex-a15"; 9: reg = <0>; 10: cci-control-port = <&cci_control1>; 11: cpu-idle-states = <&CLUSTER_SLEEP_BIG>; 12: }; 13:
14: cpu1: cpu@1 { 15: device_type = "cpu"; 16: compatible = "arm,cortex-a15"; 17: reg = <1>; 18: cci-control-port = <&cci_control1>; 19: cpu-idle-states = <&CLUSTER_SLEEP_BIG>; 20: }; 21:
22: cpu2: cpu@2 { 23: device_type = "cpu"; 24: compatible = "arm,cortex-a7"; 25: reg = <0x100>; 26: cci-control-port = <&cci_control2>; 27: cpu-idle-states = <&CLUSTER_SLEEP_LITTLE>; 28: }; 29:
30: cpu3: cpu@3 { 31: device_type = "cpu"; 32: compatible = "arm,cortex-a7"; 33: reg = <0x101>; 34: cci-control-port = <&cci_control2>; 35: cpu-idle-states = <&CLUSTER_SLEEP_LITTLE>; 36: }; 37:
38: cpu4: cpu@4 { 39: device_type = "cpu"; 40: compatible = "arm,cortex-a7"; 41: reg = <0x102>; 42: cci-control-port = <&cci_control2>; 43: cpu-idle-states = <&CLUSTER_SLEEP_LITTLE>; 44: }; 45: idle-states { 46: CLUSTER_SLEEP_BIG: cluster-sleep-big { 47: compatible = "arm,idle-state"; 48: local-timer-stop; 49: entry-latency-us = <1000>; 50: exit-latency-us = <700>; 51: min-residency-us = <2000>; 52: }; 53:
54: CLUSTER_SLEEP_LITTLE: cluster-sleep-little { 55: compatible = "arm,idle-state"; 56: local-timer-stop; 57: entry-latency-us = <1000>; 58: exit-latency-us = <500>; 59: min-residency-us = <2500>; 60: }; 61: };cpuidle有关的DTS信息，从属于cpus node中，先看最后面的idle-states node，它负责定义该ARM平台支持的所有的idle states，每个子node就是一个state：
各个state定义都以“arm,idle-state”标识；
entry-latency-us、exit-latency-us、min-residency-us分别定义了改idle state的几个重要参数，local-timer-stop对应CPUIDLE_FLAG_TIMER_STOP flag（具体含义可参考“Linux cpuidle framework(2)_cpuidle core”中的描述）；
这些信息会被dt_init_idle_driver解析出来，并保存在arm64_idle_driver的state数组中。
在每个cpu的node中，通过cpu-idle-states字段，指明该CPU支持的idle states，可以有多个。
结合上面的DTS信息，dt_init_idle_driver函数的执行过程如下。
1: /* drivers/cpuidle/dt_idle_states.c */ 2: int dt_init_idle_driver(struct cpuidle_driver *drv, 3: const struct of_device_id *matches, 4: unsigned int start_idx) 5: { 6: struct cpuidle_state *idle_state; 7: struct device_node *state_node, *cpu_node; 8: int i, err = 0; 9: const cpumask_t *cpumask; 10: unsigned int state_idx = start_idx; 11:
12: if (state_idx >= CPUIDLE_STATE_MAX) 13: return -EINVAL; 14: /*
15: * We get the idle states for the first logical cpu in the 16: * driver mask (or cpu_possible_mask if the driver cpumask is not set) 17: * and we check through idle_state_valid() if they are uniform 18: * across CPUs, otherwise we hit a firmware misconfiguration. 19: */ 20: cpumask = drv->cpumask ? : cpu_possible_mask; 21: cpu_node = of_cpu_device_node_get(cpumask_first(cpumask)); 22:
23: for (i = 0; ; i++) { 24: state_node = of_parse_phandle(cpu_node, "cpu-idle-states", i); 25: if (!state_node) 26: break; 27:
28: if (!idle_state_valid(state_node, i, cpumask)) { 29: pr_warn("%s idle state not valid, bailing out\n", 30: state_node->full_name); 31: err = -EINVAL; 32: break; 33: } 34:
35: if (state_idx == CPUIDLE_STATE_MAX) { 36: pr_warn("State index reached static CPU idle driver states array size\n"); 37: break; 38: } 39:
40: idle_state = &drv->states[state_idx++]; 41: err = init_state_node(idle_state, matches, state_node); 42: if (err) { 43: pr_err("Parsing idle state node %s failed with err %d\n", 44: state_node->full_name, err); 45: err = -EINVAL; 46: break; 47: } 48: of_node_put(state_node); 49: } 50:
51: of_node_put(state_node); 52: of_node_put(cpu_node); 53: if (err) 54: return err; 55: /*
56: * Update the driver state count only if some valid DT idle states 57: * were detected 58: */ 59: if (i) 60: drv->state_count = state_idx; 61:
62: /*
63: * Return the number of present and valid DT idle states, which can 64: * also be 0 on platforms with missing DT idle states or legacy DT 65: * configuration predating the DT idle states bindings. 66: */ 67: return i; 68: } 69: EXPORT_SYMBOL_GPL(dt_init_idle_driver);1）1421、2833行，获取第一个cpu的node，通过其中的“cpu-idle-states”字段，解析出该cpu支持的cpuidle states。同时通过idle_state_valid接口，检查其它CPU是否同样支持这些states，如果不支持，返回错误。也就是说，ARM64 generic CPU idle driver，只支持那些所有cpuidle state都相同的ARM64平台。
2）40~47行，针对每一个支持的state，调用init_state_node接口，解析cpuidle state相关的信息，并保存在drv->state数组的指定index中。解析的过程中，会为每个state指定enter回调函数，对ARM64而言，统一使用arm64_enter_idle_state接口。其它的解析过程，比较简单，不再详细说明了。
4. cpu_init_idle
cpuidle功能的支持，需要依赖CPU的、和电源管理有关的底层代码实现。对ARM64来说，kernel将这些底层代码抽象为一系列的操作函数集（struct cpu_operations，具体可参考arch/arm64/kernel/cpu_ops.c）。对同一个ARM平台，可能有多种类型的操作函数集，设计者可以根据需要选择一种使用（具体可参考本站后续的文档）。
以ARM64为例，ARM document规定了一种PSCI（Power State Coordination Interface）接口，它由firmware实现，用于电源管理有关的操作，如IDLE相关的、SMP相关的、Hotplug相关的、等等。
对cpuidle来说，需要在cpuidle driver注册之前，调用cpu_init_idle，该函数会根据当前使用的操作函数集，调用其中的cpu_init_idle回调函数，进行idle相关的初始化操作。具体的内容，蜗蜗会在其它文章中说明，这里就暂停了。
5. arm64_enter_idle_state
idle state的enter函数用于使CPU进入指定的idle state，如下：
1: static int arm64_enter_idle_state(struct cpuidle_device *dev, 2: struct cpuidle_driver *drv, int idx) 3: { 4: int ret; 5:
6: if (!idx) { 7: cpu_do_idle(); 8: return idx; 9: } 10:
11: ret = cpu_pm_enter(); 12: if (!ret) { 13: /*
14: * Pass idle state index to cpu_suspend which in turn will 15: * call the CPU ops suspend protocol with idle index as a 16: * parameter. 17: */ 18: ret = cpu_suspend(idx); 19:
20: cpu_pm_exit(); 21: } 22:
23: return ret ? -1 : idx; 24: }如果是idle state0（即WFI），调用传统cpu_do_idle接口，该接口的实现，可参照“Linux cpuidle framework(1)_概述和软件架构”中第2章ARM9的例子，在kernel source code中跟踪；
对于其它的state，首先调用cpu_pm_enter，发出CPU即将进入low power state的通知。如果成功，则调用cpu_suspend接口，让cpu进入指定的idle状态。最后，从idle返回时，发送退出low power state的通知；
cpu_pm_enter/cpu_pm_exit位于kernel/cpu_pm.c中，会在其它文章介绍；
cpu_suspend位于arch/arm64/kernel/suspend.c中，直接调用操作函数集（struct cpu_operations，如PSCI）中的cpu_suspend回调函数。具体会在其它文章中描述。
原创文章，转发请注明出处。蜗窝科技，www.wowotech.net。
继续讨论
新讨论请用 GitHub 账号在下方发言——评论会保存在本仓库的 Discussions 里，与站点同寿； 再下方的「历史评论」是原站的只读存档。
评论（13 条，只读存档）
以下为原站的历史评论（只读存档）；继续讨论请用上方的评论区。
王语双 主页
Layman
wowo
jiffy
hello-world
jiffy
wowo
fengmiao
fengmiao
wowo
bsp
bsp
伊斯科明