android hardware.c 源码分析

android的jni通过ID来找hal模块，使用了hw_get_module()函数，本文就通过这个函数的来分析一下hal层的模块是如何匹配的。

首先要了解三个结构体hw_module_t,hw_module_methods_t,hw_device_t。

结构体定义的源码位于：hardware/libhardware/include/hardware/hardware.h

typedef struct hw_module_t {
    /** tag must be initialized to HARDWARE_MODULE_TAG */
    uint32_t tag;		//必须初始化为 HARDWARE_MODULE_TAG

    /**
     * The API version of the implemented module. The module owner is
     * responsible for updating the version when a module interface has
     * changed.
     *
     * The derived modules such as gralloc and audio own and manage this field.
     * The module user must interpret the version field to decide whether or
     * not to inter-operate with the supplied module implementation.
     * For example, SurfaceFlinger is responsible for making sure that
     * it knows how to manage different versions of the gralloc-module API,
     * and AudioFlinger must know how to do the same for audio-module API.
     *
     * The module API version should include a major and a minor component.
     * For example, version 1.0 could be represented as 0x0100. This format
     * implies that versions 0x0100-0x01ff are all API-compatible.
     *
     * In the future, libhardware will expose a hw_get_module_version()
     * (or equivalent) function that will take minimum/maximum supported
     * versions as arguments and would be able to reject modules with
     * versions outside of the supplied range.
     */
    uint16_t module_api_version;		// 模块的api版本
#define version_major module_api_version

    /** Identifier of module */
    const char *id;						// 模块ID，hw_get_module()中要用到

    /** Name of this module */
    const char *name;					// 模块名称

    /** Author/owner/implementor of the module */
    const char *author;

    /** Modules methods */
    struct hw_module_methods_t* methods;	// 模块方法结构体,后面会介绍hw_module_methods_t

    /** module's dso */
    void* dso;					// 用于保存动态链接库.so文件打开之后返回的句柄。
								// 使用dlopen()函数打开动态链接库，返回值就是句柄，操作动态链接库(dlsym())都要使用到这个句柄。

    /** padding to 128 bytes, reserved for future use */
    uint32_t reserved[32-7];

} hw_module_t;

/*
 * 模块方法结构体，只定义了一个open方法
 */
typedef struct hw_module_methods_t {
    /** Open a specific device */
    int (*open)(const struct hw_module_t* module, const char* id,
            struct hw_device_t** device);

} hw_module_methods_t;

typedef struct hw_device_t {
    /** tag must be initialized to HARDWARE_DEVICE_TAG */
    uint32_t tag;

    /**
     * Version of the module-specific device API. This value is used by
     * the derived-module user to manage different device implementations.
     *
     * The module user is responsible for checking the module_api_version
     * and device version fields to ensure that the user is capable of
     * communicating with the specific module implementation.
     *
     * One module can support multiple devices with different versions. This
     * can be useful when a device interface changes in an incompatible way
     * but it is still necessary to support older implementations at the same
     * time. One such example is the Camera 2.0 API.
     *
     * This field is interpreted by the module user and is ignored by the
     * HAL interface itself.
     */
    uint32_t version;

    /** reference to the module this device belongs to */
    struct hw_module_t* module;

    /** padding reserved for future use */
    uint32_t reserved[12];

    /** Close this device */
    int (*close)(struct hw_device_t* device);

} hw_device_t;

hw_get_module()函数源码位置

hardware/libhardware/hardware.c

/*
 * Copyright (C) 2008 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include <hardware/hardware.h>

#include <cutils/properties.h>

#include <dlfcn.h>
#include <string.h>
#include <pthread.h>
#include <errno.h>
#include <limits.h>

#define LOG_TAG "HAL"
#include <utils/Log.h>

/** Base path of the hal modules */
#define HAL_LIBRARY_PATH1 "/system/lib/hw"
#define HAL_LIBRARY_PATH2 "/vendor/lib/hw"

/**
 * There are a set of variant filename for modules. The form of the filename
 * is "<MODULE_ID>.variant.so" so for the led module the Dream variants
 * of base "ro.product.board", "ro.board.platform" and "ro.arch" would be:
 *
 * led.trout.so
 * led.msm7k.so
 * led.ARMV6.so
 * led.default.so
 */

static const char *variant_keys[] = {
    "ro.hardware",  /* This goes first so that it can pick up a different
                       file on the emulator. */
    "ro.product.board",
    "ro.board.platform",
    "ro.arch"
};

static const int HAL_VARIANT_KEYS_COUNT =
    (sizeof(variant_keys)/sizeof(variant_keys[0]));

/**
 * Load the file defined by the variant and if successful
 * return the dlopen handle and the hmi.
 * @return 0 = success, !0 = failure.
 */
static int load(const char *id,
        const char *path,
        const struct hw_module_t **pHmi)
{
    int status;
    void *handle;
    struct hw_module_t *hmi;

    /*
     * load the symbols resolving undefined symbols before
     * dlopen returns. Since RTLD_GLOBAL is not or'd in with
     * RTLD_NOW the external symbols will not be global
     */
	 // 打开生成的动态链接库
    handle = dlopen(path, RTLD_NOW);
    if (handle == NULL) {
		// 查看错误信息
        char const *err_str = dlerror();
        ALOGE("load: module=%s\n%s", path, err_str?err_str:"unknown");
        status = -EINVAL;
        goto done;
    }

	// 根据连接符的名称和句柄找到hw_modult_t的首地址
    /* Get the address of the struct hal_module_info. */
    const char *sym = HAL_MODULE_INFO_SYM_AS_STR;
    hmi = (struct hw_module_t *)dlsym(handle, sym);
    if (hmi == NULL) {
        ALOGE("load: couldn't find symbol %s", sym);
        status = -EINVAL;
        goto done;
    }

	// 匹配id
    /* Check that the id matches */
    if (strcmp(id, hmi->id) != 0) {
        ALOGE("load: id=%s != hmi->id=%s", id, hmi->id);
        status = -EINVAL;
        goto done;
    }
	// 保存动态链接库的句柄
    hmi->dso = handle;

    /* success */
    status = 0;

    done:
    if (status != 0) {
        hmi = NULL;
        if (handle != NULL) {
			// 关闭打开的库
            dlclose(handle);
            handle = NULL;
        }
    } else {
        ALOGV("loaded HAL id=%s path=%s hmi=%p handle=%p",
                id, path, *pHmi, handle);
    }
	// 保存地址,这样一来外部调用的函数就可以访问对应ID的模块中的数据
    *pHmi = hmi;

    return status;
}

//编写的hal的模块，在编译之后会生成对应的.so库文件。
int hw_get_module_by_class(const char *class_id, const char *inst,
                           const struct hw_module_t **module)
{
    int status;
    int i;
    const struct hw_module_t *hmi = NULL;
    char prop[PATH_MAX];
    char path[PATH_MAX];
    char name[PATH_MAX];
	// 这里的inst在调用的时候已经是NULL这里假设id为“led"
	// 那么name现在就是"led"
    if (inst)
        snprintf(name, PATH_MAX, "%s.%s", class_id, inst);
    else
        strlcpy(name, class_id, PATH_MAX);

    /*
     * Here we rely on the fact that calling dlopen multiple times on
     * the same .so will simply increment a refcount (and not load
     * a new copy of the library).
     * We also assume that dlopen() is thread-safe.
     */

    /* Loop through the configuration variants looking for a module */
    for (i=0 ; i<HAL_VARIANT_KEYS_COUNT+1 ; i++) {
		// 查看系统是否根据variant_keys定义了属性。
		// variant_keys[i] 的值可能是 trout、msm7k、ARMV6.
        if (i < HAL_VARIANT_KEYS_COUNT) {
            if (property_get(variant_keys[i], prop, NULL) == 0) {
                continue;
            }
			// 将属性也添加到path中。
			// path: /vendor/lib/hw/led.trout.so
			// path: /vendor/lib/hw/led.msm7k.so
			// path: /vendor/lib/hw/led.ARMV6.so
            snprintf(path, sizeof(path), "%s/%s.%s.so",
                     HAL_LIBRARY_PATH2, name, prop);

			// 根据前面得到的path，查看.so文件是否可读
            if (access(path, R_OK) == 0) break;

			// path: /system/lib/hw/led.trout.so
			// path: /system/lib/hw/led.msm7k.so
			// path: /system/lib/hw/led.ARMV6.so
            snprintf(path, sizeof(path), "%s/%s.%s.so",
                     HAL_LIBRARY_PATH1, name, prop);
            if (access(path, R_OK) == 0) break;
        } else {
			// 没有定义属性，采用默认的名称
			// path: /system/lib/hw/led.default.so
            snprintf(path, sizeof(path), "%s/%s.default.so",
                     HAL_LIBRARY_PATH1, name);
            if (access(path, R_OK) == 0) break;
        }
    }

    status = -ENOENT;
    if (i < HAL_VARIANT_KEYS_COUNT+1) {
        /* load the module, if this fails, we're doomed, and we should not try
         * to load a different variant. */
        status = load(class_id, path, module);	// 加载模块
    }

    return status;
}

int hw_get_module(const char *id, const struct hw_module_t **module)
{
    return hw_get_module_by_class(id, NULL, module);
}

Tony Liu

2017-3-1, Shenzhen