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/V4L2Camera.cpp
申请video capture buffer
CameraHardwareSam::startPreviewInternal
-> V4L2Camera::startPreview
-> isi_v4l2_reqbufs
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
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
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++;
}
Android Camera预览过程数据流浅析