Android Camera预览过程数据流浅析

硬件平台：Atmel SAMA5D3 SoC + OV2640 Camera Sensor  Android 版本：4.2.2 
mediaserver进程是Camera Service的容器进程，它会动态加载Camera HAL和Gralloc HAL。 视频数据帧首先必须从Camera驱动程序到达Camera硬件抽象层。 在Camera硬件抽象层，视频数据帧被从video capture buffer拷贝到gralloc buffer。 
surfaceflinger进程作为显示服务器会动态加载HWComposer HAL和Gralloc HAL。 在HWComposer硬件抽象层，会把数据帧从gralloc buffer拷贝到video output buffer。 
经过上述过程，Camera Sensor采集的图像最终通过LCDC HEO显示在显示屏上。

图中红色实线为视频数据帧流向，不带箭头的红线连接的两端为同一块内存。 
涉及三块内存，分别如下： video capture buffer /dev/video1 gralloc buffer 匿名共享内存 mediaserver进程和surfacelinger进程都可以访问这块内存 video output buffer /dev/video0 
进行了两次数据拷贝操作，如下： media server进程 Camera HAL video capture buffer -> gralloc buffer 
surfaceflinger进程 HWCompser HAL gralloc buffer -> video output buffer 

video capture buffer的分配与内存映射 https://github.com/Android4SAM/platform_hardware_atmel/blob/android4sam_v4.0/camera/CameraHardwareSam.cpp https://github.com/Android4SAM喎�"/kf/ware/vc/" target="_blank" class="keylink">vcGxhdGZvcm1faGFyZHdhcmVfYXRtZWwvYmxvYi9hbmRyb2lkNHNhbV92NC4wL2NhbWVyYS9WNEwyQ2FtZXJhLmNwcAo8YnI+CgoKyerH63ZpZGVvIGNhcHR1cmUgYnVmZmVyCjxicj4KCkNhbWVyYUhhcmR3YXJlU2FtPHN0cm9uZz46Ojwvc3Ryb25nPnN0YXJ0UHJldmlld0ludGVybmFsCiAtPiBWNEwyQ2FtZXJhPHN0cm9uZz46Ojwvc3Ryb25nPnN0YXJ0UHJldmlldwogLT4gaXNpX3Y0bDJfcmVxYnVmcwo8YnI+Cgo8cHJlIGNsYXNzPQ=="brush:java;"> ret = ioctl(fp, VIDIOC_REQBUFS, &req); 
内存映射 CameraHardwareSam :: startPreviewInternal 

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

获得gralloc buffer以及视频数据帧从video capture buffer到gralloc buffer的拷贝 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的分配与内存映射 https://github.com/Android4SAM/platform_hardware_atmel/blob/android4sam_v4.0/hwcomposer/hwcomposer.cpp https://github.com/Android4SAM/platform_hardware_atmel/blob/android4sam_v4.0/hwcomposer/v4l2_utils.cpp

申请video ouput buffer  hwc_prepare ->  assign_heo_overlay_window -> v4l2_overlay_req_buf

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

内存映射  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);

视频数据帧从gralloc buffer到video output buffer的拷贝  
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++;
}