Android 图形系统之gralloc

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Gralloc1::Loader 与 gralloc 模块加载

Gralloc1::Loader 用于加载 HAL gralloc 模块。其类定义（位于 frameworks/native/include/ui/Gralloc1.h）如下：

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class Loader
{
public:
    Loader();
    ~Loader();
    std::unique_ptr<Device> getDevice();
private:
    static std::unique_ptr<Gralloc1On0Adapter> mAdapter;
    std::unique_ptr<Device> mDevice;
};

这个类只包含构造函数，析够函数和一个 Get 函数用于获得 Gralloc1::Device。该类全部方法实现（位于frameworks/native/libs/ui/Gralloc1.cpp）如下：

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Loader::Loader()
  : mDevice(nullptr)
{
    hw_module_t const* module;
    int err = hw_get_module(GRALLOC_HARDWARE_MODULE_ID, &module);
    uint8_t majorVersion = (module->module_api_version >> 8) & 0xFF;
    uint8_t minorVersion = module->module_api_version & 0xFF;
    gralloc1_device_t* device = nullptr;
    if (majorVersion == 1) {
        gralloc1_open(module, &device);
    } else {
        if (!mAdapter) {
            mAdapter = std::make_unique<Gralloc1On0Adapter>(module);
        }
        device = mAdapter->getDevice();
    }
    mDevice = std::make_unique<Gralloc1::Device>(device);
}
Loader::~Loader() {}
std::unique_ptr<Device> Loader::getDevice()
{
    return std::move(mDevice);
}

Gralloc1::Loader 在其构造函数中通过 hw_get_module() 函数完成 HAL gralloc 模块的加载，随后创建一个 Gralloc1::Device 。当 gralloc API 版本为 1 时，通过 frameworks/native/include/ui/Gralloc1.h 中定义的 gralloc1_open() 函数创建：

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static inline int gralloc1_open(const struct hw_module_t* module,
        gralloc1_device_t** device) {
    return module->methods->open(module, GRALLOC_HARDWARE_MODULE_ID,
            (struct hw_device_t**) device);
}

当 gralloc API 版本不为 1 时，则创建 Gralloc1On0Adapter 作为 Gralloc1::Device，相关函数（位于frameworks/native/include/ui/Gralloc1On0Adapter.h）如下：

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class Gralloc1On0Adapter : public gralloc1_device_t
{
public:
    Gralloc1On0Adapter(const hw_module_t* module);
    ~Gralloc1On0Adapter();
    gralloc1_device_t* getDevice() {
        return static_cast<gralloc1_device_t*>(this);
    }

Gralloc1::Loader 的构造函数为 hw_get_module() 传入模块 ID 和 hw_module_t 指针 module 加载 gralloc 模块，其中 module 用于接收加载的结果。gralloc 的模块 ID 定义（位于 hardware/libhardware/include/hardware/gralloc1.h）如下：

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#define GRALLOC_HARDWARE_MODULE_ID "gralloc"

hw_get_module() 函数用于加载 HAL gralloc 模块，其定义（位于 hardware/libhardware/hardware.c）如下：

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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]));
. . . . . . 
int hw_get_module_by_class(const char *class_id, const char *inst,
                           const struct hw_module_t **module)
{
    int i = 0;
    char prop[PATH_MAX] = {0};
    char path[PATH_MAX] = {0};
    char name[PATH_MAX] = {0};
    char prop_name[PATH_MAX] = {0};
    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.
     */
    /* First try a property specific to the class and possibly instance */
    snprintf(prop_name, sizeof(prop_name), "ro.hardware.%s", name);
    if (property_get(prop_name, prop, NULL) > 0) {
        if (hw_module_exists(path, sizeof(path), name, prop) == 0) {
            goto found;
        }
    }
    /* Loop through the configuration variants looking for a module */
    for (i=0 ; i<HAL_VARIANT_KEYS_COUNT; i++) {
        if (property_get(variant_keys[i], prop, NULL) == 0) {
            continue;
        }
        if (hw_module_exists(path, sizeof(path), name, prop) == 0) {
            goto found;
        }
    }
    /* Nothing found, try the default */
    if (hw_module_exists(path, sizeof(path), name, "default") == 0) {
        goto found;
    }
    return -ENOENT;
found:
    /* load the module, if this fails, we're doomed, and we should not try
     * to load a different variant. */
    return load(class_id, path, module);
}
int hw_get_module(const char *id, const struct hw_module_t **module)
{
    return hw_get_module_by_class(id, NULL, module);
}

hw_get_module() 调用 hw_get_module_by_class() 实现其功能。hw_get_module_by_class() 通过两个步骤完成 gralloc 模块的加载：

  1. 找到 gralloc 模块文件的路径。
  2. 加载 gralloc 模块文件。

hw_get_module_by_class() 首先通过模块 ID 和模块实例名称得到模块名称，对于 gralloc 而言，模块 ID为 gralloc，模块实例名称为空，然后获得模块子名称。模块子名称来自于系统属性。hw_get_module_by_class() 依次尝试从如下几个系统属性中获得模块子名称：

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ro.hardware.gralloc
ro.hardware
ro.product.board
ro.board.platform
ro.arch

对于 Pixel 设备而言，系统属性 ro.hardware.gralloc 和 ro.arch 未定义，其它几个系统属性值如下：

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[ro.hardware]: [sailfish]
[ro.product.board]: [sailfish]
[ro.board.platform]: [msm8996]

无法由从系统属性获得的模块子名称找到所对应的模块文件时，则以 default 作为模块子名称继续查找。

有了模块名称和模块子名称，hw_get_module_by_class() 则依次在 /odm -> /vendor -> /system 等目录下查找相应的模块文件：

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#if defined(__LP64__)
#define HAL_LIBRARY_PATH1 "/system/lib64/hw"
#define HAL_LIBRARY_PATH2 "/vendor/lib64/hw"
#define HAL_LIBRARY_PATH3 "/odm/lib64/hw"
#else
#define HAL_LIBRARY_PATH1 "/system/lib/hw"
#define HAL_LIBRARY_PATH2 "/vendor/lib/hw"
#define HAL_LIBRARY_PATH3 "/odm/lib/hw"
#endif
. . . . . .
static int hw_module_exists(char *path, size_t path_len, const char *name,
                            const char *subname)
{
    snprintf(path, path_len, "%s/%s.%s.so",
             HAL_LIBRARY_PATH3, name, subname);
    if (access(path, R_OK) == 0)
        return 0;
    snprintf(path, path_len, "%s/%s.%s.so",
             HAL_LIBRARY_PATH2, name, subname);
    if (access(path, R_OK) == 0)
        return 0;
    snprintf(path, path_len, "%s/%s.%s.so",
             HAL_LIBRARY_PATH1, name, subname);
    if (access(path, R_OK) == 0)
        return 0;
    return -ENOENT;
}

对于 Pixel 设备，可以在 /system/lib64/hw/ 目录下找到如下的 gralloc 模块文件：

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1|sailfish:/ # ls /vendor/lib64/hw/ | grep gralloc                           
gralloc.default.so
gralloc.msm8996.so

最终将采用 /vendor/lib64/hw/gralloc.msm8996.so 作为 gralloc 模块文件。关于这一点，可以从 surfacefinger 进程的内存映射镜像中看到：

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7105377000-710537d000 r-xp 00000000 fd:00 1575                           /system/lib64/hw/gralloc.msm8996.so
710537d000-710537e000 r--p 00005000 fd:00 1575                           /system/lib64/hw/gralloc.msm8996.so
710537e000-710537f000 rw-p 00006000 fd:00 1575                           /system/lib64/hw/gralloc.msm8996.so

hw_get_module_by_class() 通过 load() 函数来加载模块文件：

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static int load(const char *id,
        const char *path,
        const struct hw_module_t **pHmi)
{
    int status = -EINVAL;
    void *handle = NULL;
    struct hw_module_t *hmi = NULL;
    /*
     * 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
     */
    handle = dlopen(path, RTLD_NOW);
    if (handle == NULL) {
        char const *err_str = dlerror();
        ALOGE("load: module=%s\n%s", path, err_str?err_str:"unknown");
        status = -EINVAL;
        goto done;
    }
    /* 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;
    }
    /* 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;
    }
    hmi->dso = handle;
    /* success */
    status = 0;
    done:
    if (status != 0) {
        hmi = NULL;
        if (handle != NULL) {
            dlclose(handle);
            handle = NULL;
        }
    } else {
        ALOGV("loaded HAL id=%s path=%s hmi=%p handle=%p",
                id, path, *pHmi, handle);
    }
    *pHmi = hmi;
    return status;
}

加载过程主要是加载动态链接库文件，找到其中名为 HAL_MODULE_INFO_SYM_AS_STR，类型为 struct hw_module_t 的变量，并返回给调用者。

HAL 层要求 HAL 模块中都需要定义一个名为 HAL_MODULE_INFO_SYM_AS_STR 的 struct hw_module_t 变量。HAL_MODULE_INFO_SYM_AS_STR 定义（位于 hardware/libhardware/include/hardware/hardware.h）如下：

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/**
 * Name of the hal_module_info
 */
#define HAL_MODULE_INFO_SYM         HMI
/**
 * Name of the hal_module_info as a string
 */
#define HAL_MODULE_INFO_SYM_AS_STR  "HMI"

前面看到的 gralloc 实现 gralloc.msm8996.so 位于（ gralloc 接口实现） hardware/qcom/display/msm8996/libgralloc，或位于 hardware/qcom/display/msm8996/libgralloc1（ gralloc1 接口实现）。

gralloc.default.so 实现位于 hardware/libhardware/modules/gralloc，其 struct hw_module_t 定义（位于 hardware/libhardware/modules/gralloc/gralloc.cpp）如下：

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struct private_module_t HAL_MODULE_INFO_SYM = {
    .base = {
        .common = {
            .tag = HARDWARE_MODULE_TAG,
            .version_major = 1,
            .version_minor = 0,
            .id = GRALLOC_HARDWARE_MODULE_ID,
            .name = "Graphics Memory Allocator Module",
            .author = "The Android Open Source Project",
            .methods = &gralloc_module_methods
        },
        .registerBuffer = gralloc_register_buffer,
        .unregisterBuffer = gralloc_unregister_buffer,
        .lock = gralloc_lock,
        .unlock = gralloc_unlock,
    },
    .framebuffer = 0,
    .flags = 0,
    .numBuffers = 0,
    .bufferMask = 0,
    .lock = PTHREAD_MUTEX_INITIALIZER,
    .currentBuffer = 0,
};

不同类型的 HAL 模块，不同类型 HAL 模块的具体实现通常会定义自己的 struct hw_module_t 结构，如 gralloc.default.so 的 struct private_module_t，其定义（位于 hardware/libhardware/modules/gralloc/gralloc_priv.h）如下：

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struct private_module_t {
    gralloc_module_t base;
    private_handle_t* framebuffer;
    uint32_t flags;
    uint32_t numBuffers;
    uint32_t bufferMask;
    pthread_mutex_t lock;
    buffer_handle_t currentBuffer;
    int pmem_master;
    void* pmem_master_base;
    struct fb_var_screeninfo info;
    struct fb_fix_screeninfo finfo;
    float xdpi;
    float ydpi;
    float fps;
};

struct private_module_t 的成员 base 类型为 gralloc_module_t，该类型定义如下：

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typedef struct gralloc_module_t {
    struct hw_module_t common;
    
    int (*registerBuffer)(struct gralloc_module_t const* module,
            buffer_handle_t handle);
    int (*unregisterBuffer)(struct gralloc_module_t const* module,
            buffer_handle_t handle);
    
    int (*lock)(struct gralloc_module_t const* module,
            buffer_handle_t handle, int usage,
            int l, int t, int w, int h,
            void** vaddr);
    int (*unlock)(struct gralloc_module_t const* module,
            buffer_handle_t handle);
    int (*perform)(struct gralloc_module_t const* module,
            int operation, ... );
    int (*lock_ycbcr)(struct gralloc_module_t const* module,
            buffer_handle_t handle, int usage,
            int l, int t, int w, int h,
            struct android_ycbcr *ycbcr);
    int (*lockAsync)(struct gralloc_module_t const* module,
            buffer_handle_t handle, int usage,
            int l, int t, int w, int h,
            void** vaddr, int fenceFd);
    int (*unlockAsync)(struct gralloc_module_t const* module,
            buffer_handle_t handle, int* fenceFd);
    int (*lockAsync_ycbcr)(struct gralloc_module_t const* module,
            buffer_handle_t handle, int usage,
            int l, int t, int w, int h,
            struct android_ycbcr *ycbcr, int fenceFd);
    /* reserved for future use */
    void* reserved_proc[3];
} gralloc_module_t;

gralloc_module_t 类型的第一个成员 common 类型为 struct hw_module_t。这实际是 C 语言对于继承的实现 —— 父结构体总是作为子结构体的第一个成员存在。对于 HAL gralloc 模块，其 struct hw_module_t 将总是 struct gralloc_module_t 结构的子结构，各个 gralloc 模块实现，通常又都会定义自己的私有 struct gralloc_module_t 结构。

按照接口约定，实现了 gralloc1 接口的 HAL gralloc 模块，其 module->methods->open() 函数应该通过参数 device 返回一个类型为 gralloc1_device_t 的结构，一个 struct hw_device_t 结构的子结构；而实现了 gralloc 接口的 HAL gralloc 模块，则应该通过相同的方法，返回 struct hw_device_t 结构的子结构 struct alloc_device_t。

gralloc1_device_t 定义（位于 hardware/libhardware/include/hardware/gralloc1.h）如下：

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typedef struct gralloc1_device {
    /* Must be the first member of this struct, since a pointer to this struct
     * will be generated by casting from a hw_device_t* */
    struct hw_device_t common;
    void (*getCapabilities)(struct gralloc1_device* device, uint32_t* outCount,
            int32_t* /*gralloc1_capability_t*/ outCapabilities);
    gralloc1_function_pointer_t (*getFunction)(struct gralloc1_device* device,
            int32_t /*gralloc1_function_descriptor_t*/ descriptor);
} gralloc1_device_t;

gralloc1_device_t 新定义了两个函数指针成员，它们也是 gralloc1 的实现模块必须要提供的函数。其中 getCapabilities 用于获得设备支持的 capabilities 的列表，getFunction 则用于获得实现不同功能的函数的指针。

struct alloc_device_t 接口定义如下：

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typedef struct alloc_device_t {
    struct hw_device_t common;
    int (*alloc)(struct alloc_device_t* dev,
            int w, int h, int format, int usage,
            buffer_handle_t* handle, int* stride);
    int (*free)(struct alloc_device_t* dev,
            buffer_handle_t handle);
    void (*dump)(struct alloc_device_t *dev, char *buff, int buff_len);
    void* reserved_proc[7];
} alloc_device_t;

struct alloc_device_t 新定义了用于分配和释放图形缓冲区的接口。

Gralloc1::Loader 的构造函数中，当 majorVersion 不为 1 时，将创建 Gralloc1On0Adapter 把 gralloc 接口下的 struct alloc_device_t 转为 gralloc1 接口下的 gralloc1_device_t。

前面看到的 gralloc.default.so，它实现了 gralloc 接口，而不是 gralloc1。

Gralloc1::Device

Gralloc1::Device 是对 gralloc1_device_t 的一个包装。Gralloc1::Device 定义（位于 frameworks/native/include/ui/Gralloc1.h）如下：

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class Device {
    friend class Gralloc1::Descriptor;
public:
    Device(gralloc1_device_t* device);
    bool hasCapability(gralloc1_capability_t capability) const;
    std::string dump();
    std::shared_ptr<Descriptor> createDescriptor();
    gralloc1_error_t getStride(buffer_handle_t buffer, uint32_t* outStride);
    gralloc1_error_t allocate(
            const std::vector<std::shared_ptr<const Descriptor>>& descriptors,
            std::vector<buffer_handle_t>* outBuffers);
    gralloc1_error_t allocate(
            const std::shared_ptr<const Descriptor>& descriptor,
            gralloc1_backing_store_t id, buffer_handle_t* outBuffer);
    gralloc1_error_t retain(buffer_handle_t buffer);
    gralloc1_error_t retain(const GraphicBuffer* buffer);
    gralloc1_error_t release(buffer_handle_t buffer);
    gralloc1_error_t getNumFlexPlanes(buffer_handle_t buffer,
            uint32_t* outNumPlanes);
    gralloc1_error_t lock(buffer_handle_t buffer,
            gralloc1_producer_usage_t producerUsage,
            gralloc1_consumer_usage_t consumerUsage,
            const gralloc1_rect_t* accessRegion, void** outData,
            const sp<Fence>& acquireFence);
    gralloc1_error_t lockFlex(buffer_handle_t buffer,
            gralloc1_producer_usage_t producerUsage,
            gralloc1_consumer_usage_t consumerUsage,
            const gralloc1_rect_t* accessRegion,
            struct android_flex_layout* outData, const sp<Fence>& acquireFence);
    gralloc1_error_t lockYCbCr(buffer_handle_t buffer,
            gralloc1_producer_usage_t producerUsage,
            gralloc1_consumer_usage_t consumerUsage,
            const gralloc1_rect_t* accessRegion, struct android_ycbcr* outData,
            const sp<Fence>& acquireFence);
    gralloc1_error_t unlock(buffer_handle_t buffer, sp<Fence>* outFence);
private:
    std::unordered_set<gralloc1_capability_t> loadCapabilities();
    bool loadFunctions();
    template <typename LockType, typename OutType>
    gralloc1_error_t lockHelper(LockType pfn, buffer_handle_t buffer,
            gralloc1_producer_usage_t producerUsage,
            gralloc1_consumer_usage_t consumerUsage,
            const gralloc1_rect_t* accessRegion, OutType* outData,
            const sp<Fence>& acquireFence) {
        int32_t intError = pfn(mDevice, buffer,
                static_cast<uint64_t>(producerUsage),
                static_cast<uint64_t>(consumerUsage), accessRegion, outData,
                acquireFence->dup());
        return static_cast<gralloc1_error_t>(intError);
    }
    gralloc1_device_t* const mDevice;
    const std::unordered_set<gralloc1_capability_t> mCapabilities;
    template <typename PFN, gralloc1_function_descriptor_t descriptor>
    struct FunctionLoader {
        FunctionLoader() : pfn(nullptr) {}
        bool load(gralloc1_device_t* device, bool errorIfNull) {
            gralloc1_function_pointer_t rawPointer =
                    device->getFunction(device, descriptor);
            pfn = reinterpret_cast<PFN>(rawPointer);
            if (errorIfNull && !rawPointer) {
                ALOG(LOG_ERROR, GRALLOC1_LOG_TAG,
                        "Failed to load function pointer %d", descriptor);
            }
            return rawPointer != nullptr;
        }
        template <typename ...Args>
        typename std::result_of<PFN(Args...)>::type operator()(Args... args) {
            return pfn(args...);
        }
        PFN pfn;
    };
    // Function pointers
    struct Functions {
        FunctionLoader<GRALLOC1_PFN_DUMP, GRALLOC1_FUNCTION_DUMP> dump;
        FunctionLoader<GRALLOC1_PFN_CREATE_DESCRIPTOR,
                GRALLOC1_FUNCTION_CREATE_DESCRIPTOR> createDescriptor;
        FunctionLoader<GRALLOC1_PFN_DESTROY_DESCRIPTOR,
                GRALLOC1_FUNCTION_DESTROY_DESCRIPTOR> destroyDescriptor;
        FunctionLoader<GRALLOC1_PFN_SET_CONSUMER_USAGE,
                GRALLOC1_FUNCTION_SET_CONSUMER_USAGE> setConsumerUsage;
        FunctionLoader<GRALLOC1_PFN_SET_DIMENSIONS,
                GRALLOC1_FUNCTION_SET_DIMENSIONS> setDimensions;
        FunctionLoader<GRALLOC1_PFN_SET_FORMAT,
                GRALLOC1_FUNCTION_SET_FORMAT> setFormat;
        FunctionLoader<GRALLOC1_PFN_SET_PRODUCER_USAGE,
                GRALLOC1_FUNCTION_SET_PRODUCER_USAGE> setProducerUsage;
        FunctionLoader<GRALLOC1_PFN_GET_BACKING_STORE,
                GRALLOC1_FUNCTION_GET_BACKING_STORE> getBackingStore;
        FunctionLoader<GRALLOC1_PFN_GET_CONSUMER_USAGE,
                GRALLOC1_FUNCTION_GET_CONSUMER_USAGE> getConsumerUsage;
        FunctionLoader<GRALLOC1_PFN_GET_DIMENSIONS,
                GRALLOC1_FUNCTION_GET_DIMENSIONS> getDimensions;
        FunctionLoader<GRALLOC1_PFN_GET_FORMAT,
                GRALLOC1_FUNCTION_GET_FORMAT> getFormat;
        FunctionLoader<GRALLOC1_PFN_GET_PRODUCER_USAGE,
                GRALLOC1_FUNCTION_GET_PRODUCER_USAGE> getProducerUsage;
        FunctionLoader<GRALLOC1_PFN_GET_STRIDE,
                GRALLOC1_FUNCTION_GET_STRIDE> getStride;
        FunctionLoader<GRALLOC1_PFN_ALLOCATE,
                GRALLOC1_FUNCTION_ALLOCATE> allocate;
        FunctionLoader<GRALLOC1_PFN_RETAIN,
                GRALLOC1_FUNCTION_RETAIN> retain;
        FunctionLoader<GRALLOC1_PFN_RELEASE,
                GRALLOC1_FUNCTION_RELEASE> release;
        FunctionLoader<GRALLOC1_PFN_GET_NUM_FLEX_PLANES,
                GRALLOC1_FUNCTION_GET_NUM_FLEX_PLANES> getNumFlexPlanes;
        FunctionLoader<GRALLOC1_PFN_LOCK,
                GRALLOC1_FUNCTION_LOCK> lock;
        FunctionLoader<GRALLOC1_PFN_LOCK_FLEX,
                GRALLOC1_FUNCTION_LOCK_FLEX> lockFlex;
        FunctionLoader<GRALLOC1_PFN_LOCK_YCBCR,
                GRALLOC1_FUNCTION_LOCK_YCBCR> lockYCbCr;
        FunctionLoader<GRALLOC1_PFN_UNLOCK,
                GRALLOC1_FUNCTION_UNLOCK> unlock;
        // Adapter-only functions
        FunctionLoader<GRALLOC1_PFN_RETAIN_GRAPHIC_BUFFER,
                GRALLOC1_FUNCTION_RETAIN_GRAPHIC_BUFFER> retainGraphicBuffer;
        FunctionLoader<GRALLOC1_PFN_ALLOCATE_WITH_ID,
                GRALLOC1_FUNCTION_ALLOCATE_WITH_ID> allocateWithId;
    } mFunctions;
}; // class android::Gralloc1::Device

Gralloc1::Device 定义了接口以提供图形内存的分配/释放、lock/unlock 操作，所有的这些操作都依赖于从 gralloc1_device_t 获得的函数指针实现。在
Gralloc1::Device 构造期间，会加载相关的函数指针，以及设备的 capabilities：

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Device::Device(gralloc1_device_t* device)
  : mDevice(device),
    mCapabilities(loadCapabilities()),
    mFunctions()
{
    if (!loadFunctions()) {
        ALOGE("Failed to load a required function, aborting");
        abort();
    }
}
. . . . . . 
std::unordered_set<gralloc1_capability_t> Device::loadCapabilities()
{
    std::vector<int32_t> intCapabilities;
    uint32_t numCapabilities = 0;
    mDevice->getCapabilities(mDevice, &numCapabilities, nullptr);
    intCapabilities.resize(numCapabilities);
    mDevice->getCapabilities(mDevice, &numCapabilities, intCapabilities.data());
    std::unordered_set<gralloc1_capability_t> capabilities;
    for (const auto intCapability : intCapabilities) {
        capabilities.emplace(static_cast<gralloc1_capability_t>(intCapability));
    }
    return capabilities;
}
bool Device::loadFunctions()
{
    // Functions which must always be present
    if (!mFunctions.dump.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.createDescriptor.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.destroyDescriptor.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.setConsumerUsage.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.setDimensions.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.setFormat.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.setProducerUsage.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.getBackingStore.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.getConsumerUsage.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.getDimensions.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.getFormat.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.getProducerUsage.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.getStride.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.retain.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.release.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.getNumFlexPlanes.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.lock.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.lockFlex.load(mDevice, true)) {
        return false;
    }
    if (!mFunctions.unlock.load(mDevice, true)) {
        return false;
    }
    if (hasCapability(GRALLOC1_CAPABILITY_ON_ADAPTER)) {
        // These should always be present on the adapter
        if (!mFunctions.retainGraphicBuffer.load(mDevice, true)) {
            return false;
        }
        if (!mFunctions.lockYCbCr.load(mDevice, true)) {
            return false;
        }
        // allocateWithId may not be present if we're only able to map in this
        // process
        mFunctions.allocateWithId.load(mDevice, false);
    } else {
        // allocate may not be present if we're only able to map in this process
        mFunctions.allocate.load(mDevice, false);
    }
    return true;
}

Gralloc1::Device 中定义的 FunctionLoader 模板，即可以加载函数指针，同时它实现了 operator()，也是一个函数对象：

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template <typename PFN, gralloc1_function_descriptor_t descriptor>
struct FunctionLoader {
    FunctionLoader() : pfn(nullptr) {}
    bool load(gralloc1_device_t* device, bool errorIfNull) {
        gralloc1_function_pointer_t rawPointer =
                device->getFunction(device, descriptor);
        pfn = reinterpret_cast<PFN>(rawPointer);
        if (errorIfNull && !rawPointer) {
            ALOG(LOG_ERROR, GRALLOC1_LOG_TAG,
                    "Failed to load function pointer %d", descriptor);
        }
        return rawPointer != nullptr;
    }
    template <typename ...Args>
    typename std::result_of<PFN(Args...)>::type operator()(Args... args) {
        return pfn(args...);
    }
    PFN pfn;
};

FunctionLoader 模板藉由 gralloc1_device_t 的 getFunction() 接口获得函数指针。Gralloc1::Device 的函数函数类型定义像下面这样：

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typedef void (*GRALLOC1_PFN_DUMP)(gralloc1_device_t* device, uint32_t* outSize,
        char* outBuffer);
typedef int32_t /*gralloc1_error_t*/ (*GRALLOC1_PFN_CREATE_DESCRIPTOR)(
        gralloc1_device_t* device, gralloc1_buffer_descriptor_t* outDescriptor);
typedef int32_t /*gralloc1_error_t*/ (*GRALLOC1_PFN_DESTROY_DESCRIPTOR)(
        gralloc1_device_t* device, gralloc1_buffer_descriptor_t descriptor);
typedef int32_t /*gralloc1_error_t*/ (*GRALLOC1_PFN_SET_CONSUMER_USAGE)(
        gralloc1_device_t* device, gralloc1_buffer_descriptor_t descriptor,
        uint64_t /*gralloc1_consumer_usage_t*/ usage);
typedef int32_t /*gralloc1_error_t*/ (*GRALLOC1_PFN_SET_DIMENSIONS)(
        gralloc1_device_t* device, gralloc1_buffer_descriptor_t descriptor,
        uint32_t width, uint32_t height);
typedef int32_t /*gralloc1_error_t*/ (*GRALLOC1_PFN_SET_FORMAT)(
        gralloc1_device_t* device, gralloc1_buffer_descriptor_t descriptor,
        int32_t /*android_pixel_format_t*/ format);
typedef int32_t /*gralloc1_error_t*/ (*GRALLOC1_PFN_SET_PRODUCER_USAGE)(
        gralloc1_device_t* device, gralloc1_buffer_descriptor_t descriptor,
        uint64_t /*gralloc1_producer_usage_t*/ usage);
typedef int32_t /*gralloc1_error_t*/ (*GRALLOC1_PFN_GET_BACKING_STORE)(
        gralloc1_device_t* device, buffer_handle_t buffer,
        gralloc1_backing_store_t* outStore);
typedef int32_t /*gralloc1_error_t*/ (*GRALLOC1_PFN_GET_CONSUMER_USAGE)(
        gralloc1_device_t* device, buffer_handle_t buffer,
        uint64_t* /*gralloc1_consumer_usage_t*/ outUsage);
typedef int32_t /*gralloc1_error_t*/ (*GRALLOC1_PFN_GET_DIMENSIONS)(
        gralloc1_device_t* device, buffer_handle_t buffer, uint32_t* outWidth,
        uint32_t* outHeight);
typedef int32_t /*gralloc1_error_t*/ (*GRALLOC1_PFN_GET_FORMAT)(
        gralloc1_device_t* device, buffer_handle_t descriptor,
        int32_t* outFormat);
typedef int32_t /*gralloc1_error_t*/ (*GRALLOC1_PFN_GET_PRODUCER_USAGE)(
        gralloc1_device_t* device, buffer_handle_t buffer,
        uint64_t* /*gralloc1_producer_usage_t*/ outUsage);
typedef int32_t /*gralloc1_error_t*/ (*GRALLOC1_PFN_GET_STRIDE)(
        gralloc1_device_t* device, buffer_handle_t buffer, uint32_t* outStride);
typedef int32_t /*gralloc1_error_t*/ (*GRALLOC1_PFN_ALLOCATE)(
        gralloc1_device_t* device, uint32_t numDescriptors,
        const gralloc1_buffer_descriptor_t* descriptors,
        buffer_handle_t* outBuffers);
typedef int32_t /*gralloc1_error_t*/ (*GRALLOC1_PFN_RETAIN)(
        gralloc1_device_t* device, buffer_handle_t buffer);
typedef int32_t /*gralloc1_error_t*/ (*GRALLOC1_PFN_RELEASE)(
        gralloc1_device_t* device, buffer_handle_t buffer);

而所谓的函数 descriptor 则是用于标识函数的整数：

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typedef enum {
    GRALLOC1_FUNCTION_INVALID = 0,
    GRALLOC1_FUNCTION_DUMP = 1,
    GRALLOC1_FUNCTION_CREATE_DESCRIPTOR = 2,
    GRALLOC1_FUNCTION_DESTROY_DESCRIPTOR = 3,
    GRALLOC1_FUNCTION_SET_CONSUMER_USAGE = 4,
    GRALLOC1_FUNCTION_SET_DIMENSIONS = 5,
    GRALLOC1_FUNCTION_SET_FORMAT = 6,
    GRALLOC1_FUNCTION_SET_PRODUCER_USAGE = 7,
    GRALLOC1_FUNCTION_GET_BACKING_STORE = 8,
    GRALLOC1_FUNCTION_GET_CONSUMER_USAGE = 9,
    GRALLOC1_FUNCTION_GET_DIMENSIONS = 10,
    GRALLOC1_FUNCTION_GET_FORMAT = 11,
    GRALLOC1_FUNCTION_GET_PRODUCER_USAGE = 12,
    GRALLOC1_FUNCTION_GET_STRIDE = 13,
    GRALLOC1_FUNCTION_ALLOCATE = 14,
    GRALLOC1_FUNCTION_RETAIN = 15,
    GRALLOC1_FUNCTION_RELEASE = 16,
    GRALLOC1_FUNCTION_GET_NUM_FLEX_PLANES = 17,
    GRALLOC1_FUNCTION_LOCK = 18,
    GRALLOC1_FUNCTION_LOCK_FLEX = 19,
    GRALLOC1_FUNCTION_UNLOCK = 20,
    GRALLOC1_LAST_FUNCTION = 20,
} gralloc1_function_descriptor_t;

Gralloc1::Device 的所有功能实现，都依赖于从 gralloc1_device_t 的函数指针：

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std::string Device::dump()
{
    uint32_t length = 0;
    mFunctions.dump(mDevice, &length, nullptr);
    std::vector<char> output;
    output.resize(length);
    mFunctions.dump(mDevice, &length, output.data());
    return std::string(output.cbegin(), output.cend());
}
std::shared_ptr<Descriptor> Device::createDescriptor()
{
    gralloc1_buffer_descriptor_t descriptorId;
    int32_t intError = mFunctions.createDescriptor(mDevice, &descriptorId);
    auto error = static_cast<gralloc1_error_t>(intError);
    if (error != GRALLOC1_ERROR_NONE) {
        return nullptr;
    }
    auto descriptor = std::make_shared<Descriptor>(*this, descriptorId);
    return descriptor;
}
gralloc1_error_t Device::getStride(buffer_handle_t buffer, uint32_t* outStride)
{
    int32_t intError = mFunctions.getStride(mDevice, buffer, outStride);
    return static_cast<gralloc1_error_t>(intError);
}
static inline bool allocationSucceded(gralloc1_error_t error)
{
    return error == GRALLOC1_ERROR_NONE || error == GRALLOC1_ERROR_NOT_SHARED;
}
gralloc1_error_t Device::allocate(
        const std::vector<std::shared_ptr<const Descriptor>>& descriptors,
        std::vector<buffer_handle_t>* outBuffers)
{
    if (mFunctions.allocate.pfn == nullptr) {
        // Allocation is not supported on this device
        return GRALLOC1_ERROR_UNSUPPORTED;
    }
    std::vector<gralloc1_buffer_descriptor_t> deviceIds;
    for (const auto& descriptor : descriptors) {
        deviceIds.emplace_back(descriptor->getDeviceId());
    }
    std::vector<buffer_handle_t> buffers(descriptors.size());
    int32_t intError = mFunctions.allocate(mDevice,
            static_cast<uint32_t>(descriptors.size()), deviceIds.data(),
            buffers.data());
    auto error = static_cast<gralloc1_error_t>(intError);
    if (allocationSucceded(error)) {
        *outBuffers = std::move(buffers);
    }
    return error;
}
gralloc1_error_t Device::allocate(
        const std::shared_ptr<const Descriptor>& descriptor,
        gralloc1_backing_store_t id, buffer_handle_t* outBuffer)
{
    gralloc1_error_t error = GRALLOC1_ERROR_NONE;
    if (hasCapability(GRALLOC1_CAPABILITY_ON_ADAPTER)) {
        buffer_handle_t buffer = nullptr;
        int32_t intError = mFunctions.allocateWithId(mDevice,
                descriptor->getDeviceId(), id, &buffer);
        error = static_cast<gralloc1_error_t>(intError);
        if (allocationSucceded(error)) {
            *outBuffer = buffer;
        }
    } else {
        std::vector<std::shared_ptr<const Descriptor>> descriptors;
        descriptors.emplace_back(descriptor);
        std::vector<buffer_handle_t> buffers;
        error = allocate(descriptors, &buffers);
        if (allocationSucceded(error)) {
            *outBuffer = buffers[0];
        }
    }
    return error;
}
gralloc1_error_t Device::retain(buffer_handle_t buffer)
{
    int32_t intError = mFunctions.retain(mDevice, buffer);
    return static_cast<gralloc1_error_t>(intError);
}
gralloc1_error_t Device::retain(const GraphicBuffer* buffer)
{
    if (hasCapability(GRALLOC1_CAPABILITY_ON_ADAPTER)) {
        return mFunctions.retainGraphicBuffer(mDevice, buffer);
    } else {
        return retain(buffer->getNativeBuffer()->handle);
    }
}
gralloc1_error_t Device::release(buffer_handle_t buffer)
{
    int32_t intError = mFunctions.release(mDevice, buffer);
    return static_cast<gralloc1_error_t>(intError);
}
gralloc1_error_t Device::getNumFlexPlanes(buffer_handle_t buffer,
        uint32_t* outNumPlanes)
{
    uint32_t numPlanes = 0;
    int32_t intError = mFunctions.getNumFlexPlanes(mDevice, buffer, &numPlanes);
    auto error = static_cast<gralloc1_error_t>(intError);
    if (error == GRALLOC1_ERROR_NONE) {
        *outNumPlanes = numPlanes;
    }
    return error;
}
gralloc1_error_t Device::lock(buffer_handle_t buffer,
        gralloc1_producer_usage_t producerUsage,
        gralloc1_consumer_usage_t consumerUsage,
        const gralloc1_rect_t* accessRegion, void** outData,
        const sp<Fence>& acquireFence)
{
    ALOGV("Calling lock(%p)", buffer);
    return lockHelper(mFunctions.lock.pfn, buffer, producerUsage,
            consumerUsage, accessRegion, outData, acquireFence);
}
gralloc1_error_t Device::lockFlex(buffer_handle_t buffer,
        gralloc1_producer_usage_t producerUsage,
        gralloc1_consumer_usage_t consumerUsage,
        const gralloc1_rect_t* accessRegion,
        struct android_flex_layout* outData,
        const sp<Fence>& acquireFence)
{
    ALOGV("Calling lockFlex(%p)", buffer);
    return lockHelper(mFunctions.lockFlex.pfn, buffer, producerUsage,
            consumerUsage, accessRegion, outData, acquireFence);
}
gralloc1_error_t Device::lockYCbCr(buffer_handle_t buffer,
        gralloc1_producer_usage_t producerUsage,
        gralloc1_consumer_usage_t consumerUsage,
        const gralloc1_rect_t* accessRegion,
        struct android_ycbcr* outData,
        const sp<Fence>& acquireFence)
{
    ALOGV("Calling lockYCbCr(%p)", buffer);
    return lockHelper(mFunctions.lockYCbCr.pfn, buffer, producerUsage,
            consumerUsage, accessRegion, outData, acquireFence);
}
gralloc1_error_t Device::unlock(buffer_handle_t buffer, sp<Fence>* outFence)
{
    int32_t fenceFd = -1;
    int32_t intError = mFunctions.unlock(mDevice, buffer, &fenceFd);
    auto error = static_cast<gralloc1_error_t>(intError);
    if (error == GRALLOC1_ERROR_NONE) {
        *outFence = new Fence(fenceFd);
    }
    return error;
}

我们可以看一下 Gralloc1On0Adapter，来简单了解 HAL gralloc 模块，gralloc1_device_t 可能的实现和功能。Gralloc1On0Adapter 继承自 gralloc1_device_t：

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class Gralloc1On0Adapter : public gralloc1_device_t
{
public:
    Gralloc1On0Adapter(const hw_module_t* module);
    ~Gralloc1On0Adapter();
    gralloc1_device_t* getDevice() {
        return static_cast<gralloc1_device_t*>(this);
    }

Gralloc1On0Adapter 需要提供 gralloc1_device_t 所必需的 getCapabilities 和 getFunction 这两个回调：

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namespace android {
Gralloc1On0Adapter::Gralloc1On0Adapter(const hw_module_t* module)
  : mModule(reinterpret_cast<const gralloc_module_t*>(module)),
    mMinorVersion(mModule->common.module_api_version & 0xFF),
    mDevice(nullptr)
{
    ALOGV("Constructing");
    getCapabilities = getCapabilitiesHook;
    getFunction = getFunctionHook;
    int error = ::gralloc_open(&(mModule->common), &mDevice);
    if (error) {
        ALOGE("Failed to open gralloc0 module: %d", error);
    }
    ALOGV("Opened gralloc0 device %p", mDevice);
}

这两个回调的实现在定义类的头文件里：

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private:
    static inline Gralloc1On0Adapter* getAdapter(gralloc1_device_t* device) {
        return static_cast<Gralloc1On0Adapter*>(device);
    }
    // getCapabilities
    void doGetCapabilities(uint32_t* outCount,
            int32_t* /*gralloc1_capability_t*/ outCapabilities);
    static void getCapabilitiesHook(gralloc1_device_t* device,
            uint32_t* outCount,
            int32_t* /*gralloc1_capability_t*/ outCapabilities) {
        getAdapter(device)->doGetCapabilities(outCount, outCapabilities);
    };
    // getFunction
    gralloc1_function_pointer_t doGetFunction(
            int32_t /*gralloc1_function_descriptor_t*/ descriptor);
    static gralloc1_function_pointer_t getFunctionHook(
            gralloc1_device_t* device,
            int32_t /*gralloc1_function_descriptor_t*/ descriptor) {
        return getAdapter(device)->doGetFunction(descriptor);
    }

两个回调最终通过 doGetCapabilities() 和 doGetFunction() 两个成员函数完成其功能，这两个函数实现如下：

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void Gralloc1On0Adapter::doGetCapabilities(uint32_t* outCount,
        int32_t* outCapabilities)
{
    if (outCapabilities == nullptr) {
        *outCount = 1;
        return;
    }
    if (*outCount >= 1) {
        *outCapabilities = GRALLOC1_CAPABILITY_ON_ADAPTER;
        *outCount = 1;
    }
}
gralloc1_function_pointer_t Gralloc1On0Adapter::doGetFunction(
        int32_t intDescriptor)
{
    constexpr auto lastDescriptor =
            static_cast<int32_t>(GRALLOC1_LAST_ADAPTER_FUNCTION);
    if (intDescriptor < 0 || intDescriptor > lastDescriptor) {
        ALOGE("Invalid function descriptor");
        return nullptr;
    }
    auto descriptor =
            static_cast<gralloc1_function_descriptor_t>(intDescriptor);
    switch (descriptor) {
        case GRALLOC1_FUNCTION_DUMP:
            return asFP<GRALLOC1_PFN_DUMP>(dumpHook);
        case GRALLOC1_FUNCTION_CREATE_DESCRIPTOR:
            return asFP<GRALLOC1_PFN_CREATE_DESCRIPTOR>(createDescriptorHook);
        case GRALLOC1_FUNCTION_DESTROY_DESCRIPTOR:
            return asFP<GRALLOC1_PFN_DESTROY_DESCRIPTOR>(destroyDescriptorHook);
        case GRALLOC1_FUNCTION_SET_CONSUMER_USAGE:
            return asFP<GRALLOC1_PFN_SET_CONSUMER_USAGE>(setConsumerUsageHook);
        case GRALLOC1_FUNCTION_SET_DIMENSIONS:
            return asFP<GRALLOC1_PFN_SET_DIMENSIONS>(setDimensionsHook);
        case GRALLOC1_FUNCTION_SET_FORMAT:
            return asFP<GRALLOC1_PFN_SET_FORMAT>(setFormatHook);
        case GRALLOC1_FUNCTION_SET_PRODUCER_USAGE:
            return asFP<GRALLOC1_PFN_SET_PRODUCER_USAGE>(setProducerUsageHook);
        case GRALLOC1_FUNCTION_GET_BACKING_STORE:
            return asFP<GRALLOC1_PFN_GET_BACKING_STORE>(
                    bufferHook<decltype(&Buffer::getBackingStore),
                    &Buffer::getBackingStore, gralloc1_backing_store_t*>);
        case GRALLOC1_FUNCTION_GET_CONSUMER_USAGE:
            return asFP<GRALLOC1_PFN_GET_CONSUMER_USAGE>(getConsumerUsageHook);
        case GRALLOC1_FUNCTION_GET_DIMENSIONS:
            return asFP<GRALLOC1_PFN_GET_DIMENSIONS>(
                    bufferHook<decltype(&Buffer::getDimensions),
                    &Buffer::getDimensions, uint32_t*, uint32_t*>);
        case GRALLOC1_FUNCTION_GET_FORMAT:
            return asFP<GRALLOC1_PFN_GET_FORMAT>(
                    bufferHook<decltype(&Buffer::getFormat),
                    &Buffer::getFormat, int32_t*>);
        case GRALLOC1_FUNCTION_GET_PRODUCER_USAGE:
            return asFP<GRALLOC1_PFN_GET_PRODUCER_USAGE>(getProducerUsageHook);
        case GRALLOC1_FUNCTION_GET_STRIDE:
            return asFP<GRALLOC1_PFN_GET_STRIDE>(
                    bufferHook<decltype(&Buffer::getStride),
                    &Buffer::getStride, uint32_t*>);
        case GRALLOC1_FUNCTION_ALLOCATE:
            // Not provided, since we'll use ALLOCATE_WITH_ID
            return nullptr;
        case GRALLOC1_FUNCTION_ALLOCATE_WITH_ID:
            if (mDevice != nullptr) {
                return asFP<GRALLOC1_PFN_ALLOCATE_WITH_ID>(allocateWithIdHook);
            } else {
                return nullptr;
            }
        case GRALLOC1_FUNCTION_RETAIN:
            return asFP<GRALLOC1_PFN_RETAIN>(
                    managementHook<&Gralloc1On0Adapter::retain>);
        case GRALLOC1_FUNCTION_RELEASE:
            return asFP<GRALLOC1_PFN_RELEASE>(
                    managementHook<&Gralloc1On0Adapter::release>);
        case GRALLOC1_FUNCTION_RETAIN_GRAPHIC_BUFFER:
            return asFP<GRALLOC1_PFN_RETAIN_GRAPHIC_BUFFER>(
                    retainGraphicBufferHook);
        case GRALLOC1_FUNCTION_GET_NUM_FLEX_PLANES:
            return asFP<GRALLOC1_PFN_GET_NUM_FLEX_PLANES>(
                    bufferHook<decltype(&Buffer::getNumFlexPlanes),
                    &Buffer::getNumFlexPlanes, uint32_t*>);
        case GRALLOC1_FUNCTION_LOCK:
            return asFP<GRALLOC1_PFN_LOCK>(
                    lockHook<void*, &Gralloc1On0Adapter::lock>);
        case GRALLOC1_FUNCTION_LOCK_FLEX:
            return asFP<GRALLOC1_PFN_LOCK_FLEX>(
                    lockHook<struct android_flex_layout,
                    &Gralloc1On0Adapter::lockFlex>);
        case GRALLOC1_FUNCTION_LOCK_YCBCR:
            return asFP<GRALLOC1_PFN_LOCK_YCBCR>(
                    lockHook<struct android_ycbcr,
                    &Gralloc1On0Adapter::lockYCbCr>);
        case GRALLOC1_FUNCTION_UNLOCK:
            return asFP<GRALLOC1_PFN_UNLOCK>(unlockHook);
        case GRALLOC1_FUNCTION_INVALID:
            ALOGE("Invalid function descriptor");
            return nullptr;
    }
    ALOGE("Unknown function descriptor: %d", intDescriptor);
    return nullptr;
}

最终 Gralloc1On0Adapter 的这些功能函数有将依赖于 alloc_device_t 和 gralloc_module_t 提供的那些回调来完成其功能，如 alloc() 函数依赖于 alloc_device_t：

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gralloc1_error_t Gralloc1On0Adapter::allocate(
        const std::shared_ptr<Descriptor>& descriptor,
        gralloc1_backing_store_t store,
        buffer_handle_t* outBufferHandle)
{
    ALOGV("allocate(%" PRIu64 ", %#" PRIx64 ")", descriptor->id, store);
    // If this function is being called, it's because we handed out its function
    // pointer, which only occurs when mDevice has been loaded successfully and
    // we are permitted to allocate
    int usage = static_cast<int>(descriptor->producerUsage) |
            static_cast<int>(descriptor->consumerUsage);
    buffer_handle_t handle = nullptr;
    int stride = 0;
    ALOGV("Calling alloc(%p, %u, %u, %i, %u)", mDevice, descriptor->width,
            descriptor->height, descriptor->format, usage);
    auto error = mDevice->alloc(mDevice,
            static_cast<int>(descriptor->width),
            static_cast<int>(descriptor->height), descriptor->format,
            usage, &handle, &stride);
    if (error != 0) {
        ALOGE("gralloc0 allocation failed: %d (%s)", error,
                strerror(-error));
        return GRALLOC1_ERROR_NO_RESOURCES;
    }
    *outBufferHandle = handle;
    auto buffer = std::make_shared<Buffer>(handle, store, *descriptor, stride,
            true);
    std::lock_guard<std::mutex> lock(mBufferMutex);
    mBuffers.emplace(handle, std::move(buffer));
    return GRALLOC1_ERROR_NONE;
}

lock/unlock 操作则都依赖于 gralloc_module_t，以 lock 为例：

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gralloc1_error_t Gralloc1On0Adapter::lock(
        const std::shared_ptr<Buffer>& buffer,
        gralloc1_producer_usage_t producerUsage,
        gralloc1_consumer_usage_t consumerUsage,
        const gralloc1_rect_t& accessRegion, void** outData,
        const sp<Fence>& acquireFence)
{
    if (mMinorVersion >= 3) {
        int result = mModule->lockAsync(mModule, buffer->getHandle(),
                static_cast<int32_t>(producerUsage | consumerUsage),
                accessRegion.left, accessRegion.top, accessRegion.width,
                accessRegion.height, outData, acquireFence->dup());
        if (result != 0) {
            return GRALLOC1_ERROR_UNSUPPORTED;
        }
    } else {
        acquireFence->waitForever("Gralloc1On0Adapter::lock");
        int result = mModule->lock(mModule, buffer->getHandle(),
                static_cast<int32_t>(producerUsage | consumerUsage),
                accessRegion.left, accessRegion.top, accessRegion.width,
                accessRegion.height, outData);
        ALOGV("gralloc0 lock returned %d", result);
        if (result != 0) {
            return GRALLOC1_ERROR_UNSUPPORTED;
        }
    }
    return GRALLOC1_ERROR_NONE;
}

关于 HAL gralloc 模块更具体的实现，这里不再深追。

总结一下 Android 图形系统中，对于 gralloc 接口的情况，与图形缓冲区分配、映射有关的组件之间的结构，如下图所示：

如果是 gralloc1 接口的情况，则稍微有一点不同：

此外，在前面 BufferQueueCore 中创建 IGraphicBufferAlloc 时，我们看到这个对象总是会通过 surfacefinger 创建，然后通过 Binder IPC 传递给创建 BufferQueue 的进程一个句柄。后续在创建 BufferQueue 的进程中的生产者要分配 GraphicBuffer，则这一分配过程实际也将发生在 surfaceflinger 进程中，创建的 GraphicBuffer经过序列化之后，传回给创建 BufferQueue 的进程。在 GraphicBuffer 对象真正创建的时候，也会直接为其分配图形缓冲区。也就是说，在 Android 系统中，真正会分配图形缓冲区的进程只有 surfaceflinger，尽管可能会创建 BufferQueue 的进程有多个。

GraphicBufferAllocator 和 GraphicBufferMapper 在对象创建的时候，都会通过 Gralloc1::Loader 加载 HAL gralloc。只有需要分配图形缓冲区的进程才需要创建 GraphicBufferAllocator，只有需要访问图形缓冲区的进程，才需要创建 GraphicBufferMapper 对象。从 Android 的日志分析，可以看到，只有 surfaceflinger 进程中，同时发生了为创建 GraphicBufferAllocator 和 GraphicBufferMapper 而加载 HAL gralloc。而为创建 GraphicBufferMapper 而加载 HAL gralloc 则发生在 zygote、bootanimation 等多个进程。可见能够访问图形缓冲区的进程包括 Android Java 应用等多个进程。

打赏

Done.

Android OpenGL 图形系统分析系列文章

在 Android 中使用 OpenGL
Android 图形驱动初始化
EGL Context 创建
Android 图形系统之图形缓冲区分配
Android 图形系统之gralloc

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