Android架构分析之硬件抽象层(HAL)
作者：刘昊昱
博客：http://blog.csdn.net/liuhaoyutz
Android版本：2.3.7_r1
Linux内核版本：android-goldfish-2.6.29
一、硬件抽象层核心数据结构
Android硬件抽象层有三个核心数据结构,分别是hw_module_t , hw_module_methods_t, hw_device_t。定义在hardware/libhardware/include/hardware/hardware.h文件中：
40/**
41 * Every hardware module must have a data structure named HAL_MODULE_INFO_SYM
42 * and the fields of this data structure must begin with hw_module_t
43 * followed by module specific information.
44 */
45typedef struct hw_module_t {
46 /** tag must be initialized to HARDWARE_MODULE_TAG */
47 uint32_t tag;
48
49 /** major version number for the module */
50 uint16_t version_major;
51
52 /** minor version number of the module */
53 uint16_t version_minor;
54
55 /** Identifier of module */
56 const char *id;
57
58 /** Name of this module */
59 const char *name;
60
61 /** Author/owner/implementor of the module */
62 const char *author;
63
64 /** Modules methods */
65 struct hw_module_methods_t* methods;
66
67 /** module's dso */
68 void* dso;
69
70 /** padding to 128 bytes, reserved for future use */
71 uint32_t reserved[32-7];
72
73} hw_module_t;
74
75typedef struct hw_module_methods_t {
76 /** Open a specific device */
77 int (*open)(const struct hw_module_t* module, const char* id,
78 struct hw_device_t** device);
79
80} hw_module_methods_t;
81
82/**
83 * Every device data structure must begin with hw_device_t
84 * followed by module specific public methods and attributes.
85 */
86typedef struct hw_device_t {
87 /** tag must be initialized to HARDWARE_DEVICE_TAG */
88 uint32_t tag;
89
90 /** version number for hw_device_t */
91 uint32_t version;
92
93 /** reference to the module this device belongs to */
94 struct hw_module_t* module;
95
96 /** padding reserved for future use */
97 uint32_t reserved[12];
98
99 /** Close this device */
100 int (*close)(struct hw_device_t* device);
101
102} hw_device_t;
40-44行，注意这段说明文字，硬件抽象层HAL由一个一个的模块组成，Android规定，每一个模块都是一个命名为HAL_MODULE_INFO_SYM的自定义结构体，并且该结构体的第一个成员必须为hw_module_t类型的变量，其它成员变量根据需要由开发者设置。
82-85行，注意这段说明文字，每个设备对应一个自定义结构体，该结构体的第一个成员必须为hw_device_t，其它成员根据需要由开发者设置。
例如，sensor模块对应的结构体定义在hardware/libhardware/include/hardware/sensors.h文件中：
344/**
345 * Every hardware module must have a data structure named HAL_MODULE_INFO_SYM
346 * and the fields of this data structure must begin with hw_module_t
347 * followed by module specific information.
348 */
349struct sensors_module_t {
350 struct hw_module_t common;
351
352 /**
353 * Enumerate all available sensors. The list is returned in "list".
354 * @return number of sensors in the list
355 */
356 int (*get_sensors_list)(struct sensors_module_t* module,
357 struct sensor_t const** list);
358};
sensor设备对应的结构体如下：
392/**
393 * Every device data structure must begin with hw_device_t
394 * followed by module specific public methods and attributes.
395 */
396struct sensors_poll_device_t {
397 struct hw_device_t common;
398
399 /** Activate/deactivate one sensor.
400 *
401 * @param handle is the handle of the sensor to change.
402 * @param enabled set to 1 to enable, or 0 to disable the sensor.
403 *
404 * @return 0 on success, negative errno code otherwise
405 */
406 int (*activate)(struct sensors_poll_device_t *dev,
407 int handle, int enabled);
408
409 /**
410 * Set the delay between sensor events in nanoseconds for a given sensor.
411 * It is an error to set a delay inferior to the value defined by
412 * sensor_t::minDelay. If sensor_t::minDelay is zero, setDelay() is
413 * ignored and returns 0.
414 *
415 * @return 0 if successful, < 0 on error
416 */
417 int (*setDelay)(struct sensors_poll_device_t *dev,
418 int handle, int64_t ns);
419
420 /**
421 * Returns an array of sensor data.
422 * This function must block until events are available.
423 *
424 * @return the number of events read on success, or -errno in case of an error.
425 * This function should never return 0 (no event).
426 *
427 */
428 int (*poll)(struct sensors_poll_device_t *dev,
429 sensors_event_t* data, int count);
430};
对于三星公司的crespo(Nexus S的开发代号)，其sensor模块的真正实现代码定义在device/samsung/crespo/libsensors/sensors.cpp文件中：
108static struct hw_module_methods_t sensors_module_methods = {
109 open: open_sensors
110};
111
112struct sensors_module_t HAL_MODULE_INFO_SYM = {
113 common: {
114 tag: HARDWARE_MODULE_TAG,
115 version_major: 1,
116 version_minor: 0,
117 id: SENSORS_HARDWARE_MODULE_ID,
118 name: "Samsung Sensor module",
119 author: "Samsung Electronic Company",
120 methods: &sensors_module_methods,
121 },
122 get_sensors_list: sensors__get_sensors_list,
123};而在open_sensors函数中，对相应设备对应的sensors_poll_device_t结构进行了赋值：
305/** Open a new instance of a sensor device using name */
306static int open_sensors(const struct hw_module_t* module, const char* id,
307 struct hw_device_t** device)
308{
309 int status = -EINVAL;
310 sensors_poll_context_t *dev = new sensors_poll_context_t();
311
312 memset(&dev->device, 0, sizeof(sensors_poll_device_t));
313
314 dev->device.common.tag = HARDWARE_DEVICE_TAG;
315 dev->device.common.version = 0;
316 dev->device.common.module = const_cast(module);
317 dev->device.common.close = poll__close;
318 dev->device.activate = poll__activate;
319 dev->device.setDelay = poll__setDelay;
320 dev->device.poll = poll__poll;
321
322 *device = &dev->device.common;
323 status = 0;
324
325 return status;
326}
poll__close、poll__activate、poll__setDelay、poll__poll等函数也是在该文件中实现。
二、Android如何使用硬件抽象层
硬件抽象层的作用是对上层Application Framework屏蔽Linux底层驱动程序，那么Application Framework与硬件抽象层通信的接口是谁呢？答案是hw_get_module函数，该函数定义在hardware/libhardware/hardware.c文件中：
120int hw_get_module(const char *id, const struct hw_module_t **module)
121{
122 int status;
123 int i;
124 const struct hw_module_t *hmi = NULL;
125 char prop[PATH_MAX];
126 char path[PATH_MAX];
127
128 /*
129 * Here we rely on the fact that calling dlopen multiple times on
130 * the same .so will simply increment a refcount (and not load
131 * a new copy of the library).
132 * We also assume that dlopen() is thread-safe.
133 */
134
135 /* Loop through the configuration variants looking for a module */
136 for (i=0 ; ihw_get_module函数的作用是由第一个参数id指定的模块ID，找到模块对应的hw_module_t结构体，保存在第二个参数module中。
136-153行，这个for循环是为了获取模块名及路径，保存在path中。循环次数为HAL_VARIANT_KEYS_COUNT次，HAL_VARIANT_KEYS_COUNT是下面要用到的variant_keys数组的数组元素个数。
为了说明这个for循环是如何获得模块名及其路径，我们要先来看一下variant_keys数组的定义，这个数组也是定义在hardware/libhardware/hardware.c文件中：
34/**
35 * There are a set of variant filename for modules. The form of the filename
36 * is ".variant.so" so for the led module the Dream variants
37 * of base "ro.product.board", "ro.board.platform" and "ro.arch" would be:
38 *
39 * led.trout.so
40 * led.msm7k.so
41 * led.ARMV6.so
42 * led.default.so
43 */
44
45static const char *variant_keys[] = {
46 "ro.hardware", /* This goes first so that it can pick up a different
47 file on the emulator. */
48 "ro.product.board",
49 "ro.board.platform",
50 "ro.arch"
51};
52
53static const int HAL_VARIANT_KEYS_COUNT =
54 (sizeof(variant_keys)/sizeof(variant_keys[0]));
34-43行，这段注释说明了模块对应的动态库的命名规范。模块对应的动态库文件名格式为
45-51行，定义了variant_keys数组，这个数组有4个成员，即指向“ro.hardware”、“ ro.product.board”、“ ro.board.platform”、“ ro.arch”四个字符串的指针。我们可以将“ro.hardware”、“ ro.product.board”、“ ro.board.platform”、“ ro.arch”理解为属性，系统会通过适当的方法，根据平台、架构等给这些属性赋值。
例如，“ro.hardware”属性的属性值是在系统启动时由init进程负责设置的。它首先会读取/proc/cmdline文件，检查里面有没有一个名为androidboot.hardware的属性，如果有，就把它的值赋值给“ro.hardware”，否则，就将/proc/cpuinfo文件的内容读取出来，并解析出Haredware字段的内容赋值给“ro.hardware”。例如在Android模拟器中，从/proc/cpuinfo文件中读取出来的Hardware字段内容为goldfish，于是，init进程就会将 “ro.hardware” 属性设置为goldfish。
“ ro.product.board”、“ ro.board.platform”、“ ro.arch”属性是从/system/build.prop文件读取出来的。/system/build.prop文件是由编译系统中的编译脚本build/core/Makefile和shell脚本build/tools/buildinfo.sh生成的，这里不再详细分析。
53-54行，定义了HAL_VARIANT_KEYS_COUNT变量，它是variant_keys数组的大小。
从上面我们已经知道了variant_keys数组的内容，也知道了模块对应的动态库的命名规范。现在我们的问题是模块动态库命名规范格式
hw_get_module函数第138行，调用property_get(variant_keys[i], prop, NULL)函数，其作用是取得variant_keys[i]对应的属性值，保存在prop中。也就是说，在第1次循环时，是取得variant_keys[0]即“ro.hardware”对应的属性值，保存在prop中，如果没有取得到，property_get函数会返回0，则进入下一次循环，依次尝试取得“ ro.product.board”、“ ro.board.platform”、“ ro.arch”对应的属性值，保存在prop中。如果取得了某个variant_keys[i]对应的属性值，则在hw_get_module函数第141-142行，按
hw_get_module函数第148-153行，如果没有找到variant_keys[i]对应的属性，则使用
hw_get_module函数第156-160行，调用load(id, path, module)导入模块动态库，将模块对应的hw_module_t结构体，保存在module变量中。load函数也定义在hardware/libhardware/hardware.c文件中：
56/**
57 * Load the file defined by the variant and if successful
58 * return the dlopen handle and the hmi.
59 * @return 0 = success, !0 = failure.
60 */
61static int load(const char *id,
62 const char *path,
63 const struct hw_module_t **pHmi)
64{
65 int status;
66 void *handle;
67 struct hw_module_t *hmi;
68
69 /*
70 * load the symbols resolving undefined symbols before
71 * dlopen returns. Since RTLD_GLOBAL is not or'd in with
72 * RTLD_NOW the external symbols will not be global
73 */
74 handle = dlopen(path, RTLD_NOW);
75 if (handle == NULL) {
76 char const *err_str = dlerror();
77 LOGE("load: module=%s\n%s", path, err_str?err_str:"unknown");
78 status = -EINVAL;
79 goto done;
80 }
81
82 /* Get the address of the struct hal_module_info. */
83 const char *sym = HAL_MODULE_INFO_SYM_AS_STR;
84 hmi = (struct hw_module_t *)dlsym(handle, sym);
85 if (hmi == NULL) {
86 LOGE("load: couldn't find symbol %s", sym);
87 status = -EINVAL;
88 goto done;
89 }
90
91 /* Check that the id matches */
92 if (strcmp(id, hmi->id) != 0) {
93 LOGE("load: id=%s != hmi->id=%s", id, hmi->id);
94 status = -EINVAL;
95 goto done;
96 }
97
98 hmi->dso = handle;
99
100 /* success */
101 status = 0;
102
103 done:
104 if (status != 0) {
105 hmi = NULL;
106 if (handle != NULL) {
107 dlclose(handle);
108 handle = NULL;
109 }
110 } else {
111 LOGV("loaded HAL id=%s path=%s hmi=%p handle=%p",
112 id, path, *pHmi, handle);
113 }
114
115 *pHmi = hmi;
116
117 return status;
118}
第74行，调用dlopen(path, RTLD_NOW)导入path指定的模块动态库。
第83-84行，通过dlsym函数取得HAL_MODULE_INFO_SYM_AS_STR指定的变量的地址，这个地址就是模块对应的自定义结构体地址。
第115行，将hw_module_t结构赋值给传递进来的参数pHmi，即返回给上层调用函数。
分析到这里，我们可以看出，通过hw_get_module函数，Application Framework代码可以通过指定的模块ID找到模块hw_module_t结构体。有了hw_module_t结构体，就可以调用hw_module_t-> methods->open函数，在open函数中，完成对设备对应的hw_device_t结构体的初始化，并指定设备相关的自定义函数。