AndroidHAL硬件抽象层，从字面意思可以看出是对硬件设备的抽象和封装，为Android在不同硬件设备提供统一的访问接口。HAL处于Androidframework和Linuxkerneldriver之间，HAL存在的意义有以下2个方面：
先看下AndroidCameraHALModule是如何定义的:
static hw_module_methods_t v4l2_module_methods = {
.open = v4l2_camera_hal::open_dev};
camera_module_t HAL_MODULE_INFO_SYM __attribute__((visibility("default"))) = {
.common =
{
.tag = HARDWARE_MODULE_TAG,
.module_api_version = CAMERA_MODULE_API_VERSION_2_4,
.hal_api_version = HARDWARE_HAL_API_VERSION,
.id = CAMERA_HARDWARE_MODULE_ID,
.name = "V4L2 Camera HAL v3",
.author = "The Android Open Source Project",
.methods = &v4l2_module_methods,
.dso = nullptr,
.reserved = {0},
},
.get_number_of_cameras = v4l2_camera_hal::get_number_of_cameras,
.get_camera_info = v4l2_camera_hal::get_camera_info,
.set_callbacks = v4l2_camera_hal::set_callbacks,
.get_vendor_tag_ops = v4l2_camera_hal::get_vendor_tag_ops,
.open_legacy = v4l2_camera_hal::open_legacy,
.set_torch_mode = v4l2_camera_hal::set_torch_mode,
.init = nullptr,
.reserved = {nullptr, nullptr, nullptr, nullptr, nullptr}};
/**
* Every hardware module must have a data structure named HAL_MODULE_INFO_SYM
* and the fields of this data structure must begin with hw_module_t
* followed by module specific information.
*/
typedef struct hw_module_t {
/** tag must be initialized to HARDWARE_MODULE_TAG */
uint32_t 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;
#define version_major module_api_version
/**
* version_major/version_minor defines are supplied here for temporary
* source code compatibility. They will be removed in the next version.
* ALL clients must convert to the new version format.
*/
/**
* The API version of the HAL module interface. This is meant to
* version the hw_module_t, hw_module_methods_t, and hw_device_t
* structures and definitions.
*
* The HAL interface owns this field. Module users/implementations
* must NOT rely on this value for version information.
*
* Presently, 0 is the only valid value.
*/
uint16_t hal_api_version;
#define version_minor hal_api_version
/** Identifier of module */
const char *id;
/** Name of this module */
const char *name;
/** Author/owner/implementor of the module */
const char *author;
/** Modules methods */
struct hw_module_methods_t* methods;
/** module"s dso */
void* dso;
#ifdef __LP64__
uint64_t reserved[32-7];
#else
/** padding to 128 bytes, reserved for future use */
uint32_t reserved[32-7];
#endif
} hw_module_t;
/**
* Name of the hal_module_info
*/
#define HAL_MODULE_INFO_SYM HMI
每个module必须提供一个名字为HMI的结构体HMI结构体的第一个字段必须是hw_module_t
#define MAKE_TAG_CONSTANT(A,B,C,D) (((A) << 24) | ((B) << 16) | ((C) << 8) | (D))
#define HARDWARE_MODULE_TAG MAKE_TAG_CONSTANT("H", "W", "M", "T")
#define HARDWARE_DEVICE_TAG MAKE_TAG_CONSTANT("H", "W", "D", "T")
APIversion.当moduelinterface改变后moduleowner需要更新APIversion.moduleuser必须解释APIversion以决定是否与该module实现进行互操作。例如:SurfaceFlinger必须负责确保它知道如何管理不同版本的gralloc-moduleAPI.APIversion包含一个major和一个minorcomponent.例如:version0可以由0x0100表示.这种格式表示的范围:0x0100-0x01ff未来,libhardware将会实现一个hw_get_module_version函数,该函数接受minmux/maximumsupportedversions作为参数,并且可以拒绝超出范围的version.
TheAPIversionoftheHALmoduleinterface.指示hw_module_t,hw_module_methods_t,hw_device_t.HALinterface拥有次字段.Moduleusers/implementations必须不依赖于它.目前,0是非法的.
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;
hw_module_methods_t指定了module的入口函数.
/**
* Every device data structure must begin with hw_device_t
* followed by module specific public methods and attributes.
*/
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 */
#ifdef __LP64__
uint64_t reserved[12];
#else
uint32_t reserved[12];
#endif
/** Close this device */
int (*close)(struct hw_device_t* device);
} hw_device_t;
Versionofthemodule-specificdeviceAPI.derived-moduleuser使用该字段管理不同的deviceimplementatsion.不同版本的module可以支持多个设备.在设备接口以不兼容的方式改变时,version字段就会非常有用,但是同时支持较老的实现是必要的.例如Camera0API.version字段由moduleuser解释,并且被HALinterface忽略.
typedef struct camera_device {
/**
* camera_device.common.version must be in the range
* HARDWARE_DEVICE_API_VERSION(0,0)-(1,FF). CAMERA_DEVICE_API_VERSION_1_0 is
* recommended.
*/
hw_device_t common;
camera_device_ops_t *ops;
void *priv;
} camera_device_t;
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