Android Camera預覽過程資料流淺析

標籤：android   camera   

硬體平台：Atmel SAMA5D3 SoC + OV2640 Camera SensorAndroid版本：4.2.2
mediaserver進程是Camera Service的容器進程，它會動態載入Camera HAL和Gralloc HAL。視頻資料幀首先必須從Camera驅動程式到達Camera硬體抽象層。在Camera硬體抽象層，視頻資料幀被從video capture buffer拷貝到gralloc buffer。
surfaceflinger進程作為顯示伺服器會動態載入HWComposer HAL和Gralloc HAL。在HWComposer硬體抽象層，會把資料幀從gralloc buffer拷貝到video output buffer。
經過上述過程，Camera Sensor採集的映像最終通過LCDC HEO顯示在顯示屏上。

圖中紅色實線為視頻資料幀流向，不帶箭頭的紅線串連的兩端為同一塊記憶體。
涉及三塊記憶體，分別如下：video capture buffer /dev/video1gralloc buffer       匿名共用記憶體 mediaserver進程和surfacelinger進程都可以訪問這塊記憶體 video output buffer /dev/video0
進行了兩次資料拷貝操作，如下：media server進程Camera HAL    video capture buffer -> gralloc buffer
surfaceflinger進程HWCompser HAL   gralloc buffer -> video output buffer

video capture buffer的分配與記憶體映射https://github.com/Android4SAM/platform_hardware_atmel/blob/android4sam_v4.0/camera/CameraHardwareSam.cpphttps://github.com/Android4SAM/platform_hardware_atmel/blob/android4sam_v4.0/camera/V4L2Camera.cpp
申請video capture buffer
CameraHardwareSam ::startPreviewInternal -> V4L2Camera ::startPreview -> isi_v4l2_reqbufs

    ret = ioctl(fp, VIDIOC_REQBUFS, &req);

記憶體映射CameraHardwareSam ::startPreviewInternal

    mPreviewHeap = mGetMemoryCb((int)mV4L2Camera->getCameraFd(),                                aligned_buffer_size,                                kBufferCount,                                0); 

獲得gralloc buffer以及視頻資料幀從video capture buffer到gralloc buffer的拷貝https://github.com/Android4SAM/platform_hardware_atmel/blob/android4sam_v4.0/camera/CameraHardwareSam.cpp

CameraHardwareSam ::previewThread

    if (mPreviewWindow && mGrallocHal) {        buffer_handle_t *buf_handle;        int stride;        if (0 != mPreviewWindow->dequeue_buffer(mPreviewWindow, &buf_handle, &stride)) {            ALOGE("Could not dequeue gralloc buffer!\n");            goto callbacks;        }        void *vaddr;        if (!mGrallocHal->lock(mGrallocHal,                               *buf_handle,                               GRALLOC_USAGE_SW_WRITE_OFTEN,                               0, 0, width, height, &vaddr)) {            char *frame = ((char *)mPreviewHeap->data) + offset;            // the code below assumes YUV, not RGB            {                int h;                char *src = frame;                char *ptr = (char *)vaddr;                memcpy(ptr, src, frame_size);                //YUY2toYV12(frame, vaddr, width, height);            }            mGrallocHal->unlock(mGrallocHal, *buf_handle);        }        else            ALOGE("%s: could not obtain gralloc buffer", __func__);        if (0 != mPreviewWindow->enqueue_buffer(mPreviewWindow, buf_handle)) {            ALOGE("Could not enqueue gralloc buffer!\n");            goto callbacks;        }    }

video output buffer的分配與記憶體映射https://github.com/Android4SAM/platform_hardware_atmel/blob/android4sam_v4.0/hwcomposer/hwcomposer.cpphttps://github.com/Android4SAM/platform_hardware_atmel/blob/android4sam_v4.0/hwcomposer/v4l2_utils.cpp

申請video ouput buffer hwc_prepare -> assign_heo_overlay_window -> v4l2_overlay_req_buf

    ret = ioctl(win->fd, VIDIOC_REQBUFS, &reqbuf);

記憶體映射 hwc_prepare -> assign_heo_overlay_window ->  v4l2_overlay_map_buf

    *len = buf.length;    *start = mmap(NULL, buf.length, PROT_READ | PROT_WRITE, MAP_SHARED,            fd, buf.m.offset);

視頻資料幀從gralloc buffer到video output buffer的拷貝
hwc_set ->  copy_heo_src_content

    for (unsigned int i = 0; i < cur_layer->visibleRegionScreen.numRects; i++) {        uint8_t *cur_dst_addr = dst_addr;        uint8_t *cur_src_addr = src_addr;        for (int j = 0; j < h ; j++) {            memcpy(cur_dst_addr, cur_src_addr, cpy_size);            cur_dst_addr = &cur_dst_addr[cpy_size];            cur_src_addr = &cur_src_addr[(cur_layer->displayFrame.right - cur_layer->displayFrame.left) * (prev_handle->uiBpp / 8)];        }        cur_rect++;    }