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
%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