Android HAL

1 HAL產生的原因

Android HAL(Hardware Abstraction Libraries)是處於user space的最下層，是Android定義的要求Linux內核空間來具體實現的驅動接口。根據Google的說法，使用user-space HAL的原因爲：

1 Not all components have standardizedkernel driver interface

2 Kernel driver are GPL which exposes anyproprietary IP

3 Android has specific requirements forhardware drivers

但這其中最重要的原因爲第二個，把控制硬件的動作都放到了user space中，在kernel driver裏面只有最簡單的讀寫寄存器的操作。Android user space的代碼就可以不必受到GPL的束縛而封裝成庫，避免了暴露硬件廠商使用的硬件型號。

2 HAL的重要性

HAL的重要性主要表現在兩個方面：

1 從學習的角度看，HAL通常是Linux內核開發者想學習整個Android系統架構時選擇的突破口，這是由於HAL所處在整個系統架構的位置決定的；同樣Android user-space的開發者，想要從APP和中間件進一步深入貫通到內核，HAL也是必經之路。如果把Kernel比作關內，Android user-space比作關外，HAL就是進出關的唯一通道，兵家必爭之地。

2 從使用的頻度來看，隨着對SOC性能要求的進一步提高，越來越多的模塊將使用硬件加速，甚至以前使用DSP來加速的一些模塊，由於性能和功耗的要求，都改用硬件實現，特別是多媒體相關的領域，GPU也幾乎成了最近2年出來的SOC的標配，Android也集成了方便使用硬件加速的標準OpenMAX, OpenGL等。

3 HAL的2種模式

3.1 直接調用(舊的模式)

1 源碼路徑：\hardware\libhardware_legacy

2 模式簡介：libhardware_legacy是將 *.so 文件當作shared library來使用，JNI以 direct function call 使用 HAL module。通過直接函數調用的方式，來操作驅動程序。

3 實例：PowerManager

JNI層：android_os_Power.cpp(frameworks\base\core\jni)

static void
acquireWakeLock(JNIEnv *env, jobject clazz, jint lock, jstring idObj)
{
    if (idObj == NULL) {
        throw_NullPointerException(env, "id is null");
        return ;
    }

    const char *id = env->GetStringUTFChars(idObj, NULL);

    acquire_wake_lock(lock, id);

    env->ReleaseStringUTFChars(idObj, id);
}

static void acquireWakeLock(JNIEnv *env, jobject clazz, jint lock, jstring idObj) { if (idObj == NULL) { throw_NullPointerException(env, "id is null"); return ; } const char *id = env->GetStringUTFChars(idObj, NULL); acquire_wake_lock(lock, id); env->ReleaseStringUTFChars(idObj, id); }
HAL層：Power.c(hardware\libhardware_legacy\power)

int
acquire_wake_lock(int lock, const char* id)
{
    initialize_fds();

//    LOGI("acquire_wake_lock lock=%d id='%s'\n", lock, id);

    if (g_error) return g_error;

    int fd;

    if (lock == PARTIAL_WAKE_LOCK) {
        fd = g_fds[ACQUIRE_PARTIAL_WAKE_LOCK];
    }
    else {
        return EINVAL;
    }

    return write(fd, id, strlen(id));
}

int acquire_wake_lock(int lock, const char* id) { initialize_fds(); // LOGI("acquire_wake_lock lock=%d id='%s'\n", lock, id); if (g_error) return g_error; int fd; if (lock == PARTIAL_WAKE_LOCK) { fd = g_fds[ACQUIRE_PARTIAL_WAKE_LOCK]; } else { return EINVAL; } return write(fd, id, strlen(id)); }

3.2 通過回調函數調用並統一接口(新的模式)

1 源碼路徑：\hardware\libhardware

2 模式簡介：hardware.c和hardware.h(hardware\libhardware\include\hardware)提供了使用這種模式的HAL的方式，並規定統一接口。上層通過hardware.c中的函數hw_get_module獲取相應module的stub，然後通過stub來訪問HAL。

3 接口源碼hardware.c和hardware.h說明

(1)Hardware.h用2個結構體規定了統一接口

/**
 * 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;

    /** major version number for the module */
    uint16_t version_major;

    /** minor version number of the module */
    uint16_t version_minor;

    /** 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;

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

} hw_module_t;

/** * 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; /** major version number for the module */ uint16_t version_major; /** minor version number of the module */ uint16_t version_minor; /** 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; /** padding to 128 bytes, reserved for future use */ uint32_t reserved[32-7]; } hw_module_t;

由註釋可知，每個HAL模塊都必須繼承這個結構體，繼承方式爲，定義一個結構體，其第一個元素爲hw_module_t，之後是表徵自己模塊獨有的信息的變量。

/**
 * 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 number for hw_device_t */
    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;

/** * 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 number for hw_device_t */ 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;

由註釋可知，每個HAL模塊必須繼承這個結構體，繼承方式類似地，定義一個結構體，其第一個元素爲hw_device_t，之後是自己模塊獨有的方法和屬性。

(2)hardware.c使用hw_get_module函數，通過id查找並加載相應HAL模塊，並獲得相應模塊的Stub。相關的解釋見這篇文章http://my.unix-center.net/~Simon_fu/?p=630。

4 實例：overlay

Service層：DisplayHardware.cpp(frameworks\base\services\surfaceflinger\displayhardware)

void DisplayHardware::init(uint32_t dpy)
{
    mNativeWindow = new FramebufferNativeWindow();
    framebuffer_device_t const * fbDev = mNativeWindow->getDevice();
    mDpiX = mNativeWindow->xdpi;
    mDpiY = mNativeWindow->ydpi;
    mRefreshRate = fbDev->fps;

    mOverlayEngine = NULL;
    hw_module_t const* module;
    if (hw_get_module(OVERLAY_HARDWARE_MODULE_ID, &module) == 0) {
        overlay_control_open(module, &mOverlayEngine);
    }
    ....
}

void DisplayHardware::init(uint32_t dpy) { mNativeWindow = new FramebufferNativeWindow(); framebuffer_device_t const * fbDev = mNativeWindow->getDevice(); mDpiX = mNativeWindow->xdpi; mDpiY = mNativeWindow->ydpi; mRefreshRate = fbDev->fps; mOverlayEngine = NULL; hw_module_t const* module; if (hw_get_module(OVERLAY_HARDWARE_MODULE_ID, &module) == 0) { overlay_control_open(module, &mOverlayEngine); } .... }

HAL層：Overlay.h(hardware\libhardware\include\hardware)

/**
 * 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.
 */
struct overlay_module_t {
    struct hw_module_t common;
};

/** * 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. */ struct overlay_module_t { struct hw_module_t common; };

/**
 * Every device data structure must begin with hw_device_t
 * followed by module specific public methods and attributes.
 */

struct overlay_control_device_t {
    struct hw_device_t common;

    /* get static informations about the capabilities of the overlay engine */
    int (*get)(struct overlay_control_device_t *dev, int name);

    /* creates an overlay matching the given parameters as closely as possible.
     * returns an error if no more overlays are available. The actual
     * size and format is returned in overlay_t. */
    overlay_t* (*createOverlay)(struct overlay_control_device_t *dev,
            uint32_t w, uint32_t h, int32_t format);

    /* destroys an overlay. This call releases all
     * resources associated with overlay_t and make it invalid */
    void (*destroyOverlay)(struct overlay_control_device_t *dev,
            overlay_t* overlay);

    /* set position and scaling of the given overlay as closely as possible.
     * if scaling cannot be performed, overlay must be centered. */
    int (*setPosition)(struct overlay_control_device_t *dev,
            overlay_t* overlay,
            int x, int y, uint32_t w, uint32_t h);

    /* returns the actual position and size of the overlay */
    int (*getPosition)(struct overlay_control_device_t *dev,
            overlay_t* overlay,
            int* x, int* y, uint32_t* w, uint32_t* h);

    /* sets configurable parameters for this overlay. returns an error if not
     * supported. */
    int (*setParameter)(struct overlay_control_device_t *dev,
            overlay_t* overlay, int param, int value);

    int (*stage)(struct overlay_control_device_t *dev, overlay_t* overlay);
    int (*commit)(struct overlay_control_device_t *dev, overlay_t* overlay);
};

/** * Every device data structure must begin with hw_device_t * followed by module specific public methods and attributes. */ struct overlay_control_device_t { struct hw_device_t common; /* get static informations about the capabilities of the overlay engine */ int (*get)(struct overlay_control_device_t *dev, int name); /* creates an overlay matching the given parameters as closely as possible. * returns an error if no more overlays are available. The actual * size and format is returned in overlay_t. */ overlay_t* (*createOverlay)(struct overlay_control_device_t *dev, uint32_t w, uint32_t h, int32_t format); /* destroys an overlay. This call releases all * resources associated with overlay_t and make it invalid */ void (*destroyOverlay)(struct overlay_control_device_t *dev, overlay_t* overlay); /* set position and scaling of the given overlay as closely as possible. * if scaling cannot be performed, overlay must be centered. */ int (*setPosition)(struct overlay_control_device_t *dev, overlay_t* overlay, int x, int y, uint32_t w, uint32_t h); /* returns the actual position and size of the overlay */ int (*getPosition)(struct overlay_control_device_t *dev, overlay_t* overlay, int* x, int* y, uint32_t* w, uint32_t* h); /* sets configurable parameters for this overlay. returns an error if not * supported. */ int (*setParameter)(struct overlay_control_device_t *dev, overlay_t* overlay, int param, int value); int (*stage)(struct overlay_control_device_t *dev, overlay_t* overlay); int (*commit)(struct overlay_control_device_t *dev, overlay_t* overlay); };

Overlay.cpp (hardware\libhardware\modules\overlay)

static int overlay_device_open(const struct hw_module_t* module, const char* name,
        struct hw_device_t** device);

static struct hw_module_methods_t overlay_module_methods = {
    open: overlay_device_open
};

struct overlay_module_t HAL_MODULE_INFO_SYM = {
    common: {
        tag: HARDWARE_MODULE_TAG,
        version_major: 1,
        version_minor: 0,
        id: OVERLAY_HARDWARE_MODULE_ID,
        name: "Sample Overlay module",
        author: "The Android Open Source Project",
        methods: &overlay_module_methods,
    }
};

static int overlay_device_open(const struct hw_module_t* module, const char* name, struct hw_device_t** device); static struct hw_module_methods_t overlay_module_methods = { open: overlay_device_open }; struct overlay_module_t HAL_MODULE_INFO_SYM = { common: { tag: HARDWARE_MODULE_TAG, version_major: 1, version_minor: 0, id: OVERLAY_HARDWARE_MODULE_ID, name: "Sample Overlay module", author: "The Android Open Source Project", methods: &overlay_module_methods, } };

4 後記

在理解HAL的時候，2種模式的區別萬不可糾結於上層是用JNI調用，manager調用，還是用service調用，mokoid在這方面也許會造成一些誤導。其關鍵爲：舊的模式爲直接調用，新的模式爲通過hw_get_module獲取到相應HAL模塊的stub，以回調的方式進行調用。

5 Reference

http://android.tgbus.com/Android/tutorial/201104/350065.shtml

http://blog.csdn.net/k229650014/article/details/5801397

http://my.unix-center.net/~Simon_fu/?p=630