前面分析了 UVC 摄像头的硬件模型和描述符，对于一个 usb 摄像头来说，内部大概分为一个 VC 接口和一个 VS 接口，VC 接口内部有许多 unit 和 terminal 用来“控制”摄像头，比如我们可以通过 Process unit 设置白平衡、曝光等等。对于 VS 接口来说，标准 VS 接口往往含有许多个设置，每一个设置都包含一个实时传输端点，虽然它们的端点地址可能相同，但是它们的最大传输包大小不同，在 Class specific VS 接口中，包含多个 Format ，每一个 Format 包含多个 Frame ,Format 指的 YUYV MJPG 等等，Frame 就是各种分辨率 480*320 640 * 480 等等。以上这些信息，都是通过分析描述符来获得。
VideoStreaming Requests
参考 UVC 1.5 Class specification 4.3 节
我们需要使用控制传输来和VS通信，Probe and commit 设置，请求格式参考上图。
bmRequestType 请求类型，参考标准USB协议
bRequest 子类，定义在 Table A-8
CS ，Control Selector ，定义在 Table A-16 ，例如是probe 还是 commit
wIndex 高字节为0，低字节为接口号
wLength 和 Data 和标准USB协议一样，为数据长度和数据
参数设置的过程需要主机和USB设备进行协商， 协商的过程大致如下图所示：
Host 先将期望的设置发送给USB设备(PROBE)
设备将Host期望设置在自身能力范围之内进行修改，返回给Host(PROBE)
Host 认为设置可行的话，Commit 提交(COMMIT)
设置接口的当前设置为某一个设置
那么协商哪些数据？这些数据在哪里定义？参考Table 4-75 ,里面包含了使用哪一个Frame 哪一个 Frame 帧频率，一次传输包大小等等的信息。
参考代码：
static int myuvc_try_streaming_params(struct myuvc_streaming_control *ctrl)
{
__u8 *data;
__u16 size;
int ret;
__u8 type = USB_TYPE_CLASS | USB_RECIP_INTERFACE;
unsigned int pipe;
memset(ctrl, 0, sizeof *ctrl);
ctrl->bmHint = 1; /* dwFrameInterval */
ctrl->bFormatIndex = 1;
ctrl->bFrameIndex = 1;
ctrl->dwFrameInterval = 333333;
ctrl->dwClockFrequency = 48000000;
ctrl->wCompQuality = 61;
size = 34;
data = kzalloc(size, GFP_KERNEL);
if (data == NULL)
return -ENOMEM;
*(__le16 *)&data[0] = cpu_to_le16(ctrl->bmHint);
data[2] = ctrl->bFormatIndex;
data[3] = ctrl->bFrameIndex;
*(__le32 *)&data[4] = cpu_to_le32(ctrl->dwFrameInterval);
*(__le16 *)&data[8] = cpu_to_le16(ctrl->wKeyFrameRate);
*(__le16 *)&data[10] = cpu_to_le16(ctrl->wPFrameRate);
*(__le16 *)&data[12] = cpu_to_le16(ctrl->wCompQuality);
*(__le16 *)&data[14] = cpu_to_le16(ctrl->wCompWindowSize);
*(__le16 *)&data[16] = cpu_to_le16(ctrl->wDelay);
put_unaligned_le32(ctrl->dwMaxVideoFrameSize, &data[18]);
put_unaligned_le32(ctrl->dwMaxPayloadTransferSize, &data[22]);
if (size == 34) {
put_unaligned_le32(ctrl->dwClockFrequency, &data[26]);
data[30] = ctrl->bmFramingInfo;
data[31] = ctrl->bPreferedVersion;
data[32] = ctrl->bMinVersion;
data[33] = ctrl->bMaxVersion;
}
pipe = usb_sndctrlpipe(myuvc_udev, 0);
type |= USB_DIR_OUT;
ret = usb_control_msg(myuvc_udev, pipe, 0x01, type, 0x01 << 8,
0 << 8 | myuvc_streaming_intf, data, size, 5000);
kfree(data);
return (ret < 0) ? ret : 0;
}
static int myuvc_get_streaming_params(struct myuvc_streaming_control *ctrl)
{
__u8 *data;
__u16 size;
int ret;
__u8 type = USB_TYPE_CLASS | USB_RECIP_INTERFACE;
unsigned int pipe;
size = 34;
data = kmalloc(size, GFP_KERNEL);
if (data == NULL)
return -ENOMEM;
pipe = usb_rcvctrlpipe(myuvc_udev, 0);
type |= USB_DIR_IN;
ret = usb_control_msg(myuvc_udev, pipe, 0x81, type, 0x01 << 8,
0 << 8 | myuvc_streaming_intf, data, size, 5000);
if (ret < 0)
goto done;
ctrl->bmHint = le16_to_cpup((__le16 *)&data[0]);
ctrl->bFormatIndex = data[2];
ctrl->bFrameIndex = data[3];
ctrl->dwFrameInterval = le32_to_cpup((__le32 *)&data[4]);
ctrl->wKeyFrameRate = le16_to_cpup((__le16 *)&data[8]);
ctrl->wPFrameRate = le16_to_cpup((__le16 *)&data[10]);
ctrl->wCompQuality = le16_to_cpup((__le16 *)&data[12]);
ctrl->wCompWindowSize = le16_to_cpup((__le16 *)&data[14]);
ctrl->wDelay = le16_to_cpup((__le16 *)&data[16]);
ctrl->dwMaxVideoFrameSize = get_unaligned_le32(&data[18]);
ctrl->dwMaxPayloadTransferSize = get_unaligned_le32(&data[22]);
if (size == 34) {
ctrl->dwClockFrequency = get_unaligned_le32(&data[26]);
ctrl->bmFramingInfo = data[30];
ctrl->bPreferedVersion = data[31];
ctrl->bMinVersion = data[32];
ctrl->bMaxVersion = data[33];
} else {
//ctrl->dwClockFrequency = video->dev->clock_frequency;
ctrl->bmFramingInfo = 0;
ctrl->bPreferedVersion = 0;
ctrl->bMinVersion = 0;
ctrl->bMaxVersion = 0;
}
done:
kfree(data);
return (ret < 0) ? ret : 0;
}
static int myuvc_set_streaming_params(struct myuvc_streaming_control *ctrl)
{
__u8 *data;
__u16 size;
int ret;
__u8 type = USB_TYPE_CLASS | USB_RECIP_INTERFACE;
unsigned int pipe;
size = 34;
data = kzalloc(size, GFP_KERNEL);
if (data == NULL)
return -ENOMEM;
*(__le16 *)&data[0] = cpu_to_le16(ctrl->bmHint);
data[2] = ctrl->bFormatIndex;
data[3] = ctrl->bFrameIndex;
*(__le32 *)&data[4] = cpu_to_le32(ctrl->dwFrameInterval);
*(__le16 *)&data[8] = cpu_to_le16(ctrl->wKeyFrameRate);
*(__le16 *)&data[10] = cpu_to_le16(ctrl->wPFrameRate);
*(__le16 *)&data[12] = cpu_to_le16(ctrl->wCompQuality);
*(__le16 *)&data[14] = cpu_to_le16(ctrl->wCompWindowSize);
*(__le16 *)&data[16] = cpu_to_le16(ctrl->wDelay);
put_unaligned_le32(ctrl->dwMaxVideoFrameSize, &data[18]);
put_unaligned_le32(ctrl->dwMaxPayloadTransferSize, &data[22]);
if (size == 34) {
put_unaligned_le32(ctrl->dwClockFrequency, &data[26]);
data[30] = ctrl->bmFramingInfo;
data[31] = ctrl->bPreferedVersion;
data[32] = ctrl->bMinVersion;
data[33] = ctrl->bMaxVersion;
}
pipe = usb_sndctrlpipe(myuvc_udev, 0);
type |= USB_DIR_OUT;
ret = usb_control_msg(myuvc_udev, pipe, 0x01, type, 0x02 << 8,
0 << 8 | myuvc_streaming_intf, data, size, 5000);
kfree(data);
return (ret < 0) ? ret : 0;
}
VideoControl Requests
这里我们主要分析 ＶＣ 接口里的 Processing Unit Control Requests 以亮度为例：
static void myuvc_set_le_value(__s32 value, __u8 *data)
{
int bits = 16;
int offset = 0;
__u8 mask;
data += offset / 8;
offset &= 7;
for (; bits > 0; data++) {
mask = ((1LL << bits) - 1) << offset;
*data = (*data & ~mask) | ((value << offset) & mask);
value >>= offset ? offset : 8;
bits -= 8 - offset;
offset = 0;
}
}
static __s32 myuvc_get_le_value(const __u8 *data)
{
int bits = 16;
int offset = 0;
__s32 value = 0;
__u8 mask;
data += offset / 8;
offset &= 7;
mask = ((1LL << bits) - 1) << offset;
for (; bits > 0; data++) {
__u8 byte = *data & mask;
value |= offset > 0 ? (byte >> offset) : (byte << (-offset));
bits -= 8 - (offset > 0 ? offset : 0);
offset -= 8;
mask = (1 << bits) - 1;
}
/* Sign-extend the value if needed. */
value |= -(value & (1 << (16 - 1)));
return value;
}
/* 参考:uvc_query_v4l2_ctrl */
int myuvc_vidioc_queryctrl (struct file *file, void *fh,
struct v4l2_queryctrl *ctrl)
{
__u8 type = USB_TYPE_CLASS | USB_RECIP_INTERFACE;
unsigned int pipe;
int ret;
u8 data[2];
if (ctrl->id != V4L2_CID_BRIGHTNESS)
return -EINVAL;
memset(ctrl, 0, sizeof *ctrl);
ctrl->id = V4L2_CID_BRIGHTNESS;
ctrl->type = V4L2_CTRL_TYPE_INTEGER;
strcpy(ctrl->name, "MyUVC_BRIGHTNESS");
ctrl->flags = 0;
pipe = usb_rcvctrlpipe(udev, 0);
type |= USB_DIR_IN;
/* 发起USB传输确定这些值 */
ret = usb_control_msg(udev, pipe, GET_MIN, type, PU_BRIGHTNESS_CONTROL << 8,
ProcessingUnitID << 8 | myuvc_control_intf, data, 2, 5000);
if (ret != 2)
return -EIO;
ctrl->minimum = myuvc_get_le_value(data); /* Note signedness */
ret = usb_control_msg(udev, pipe, GET_MAX, type, PU_BRIGHTNESS_CONTROL << 8,
ProcessingUnitID << 8 | myuvc_control_intf, data, 2, 5000);
if (ret != 2)
return -EIO;
ctrl->maximum = myuvc_get_le_value(data); /* Note signedness */
ret = usb_control_msg(udev, pipe, GET_RES, type, PU_BRIGHTNESS_CONTROL << 8,
ProcessingUnitID << 8 | myuvc_control_intf, data, 2, 5000);
if (ret != 2)
return -EIO;
ctrl->step = myuvc_get_le_value(data); /* Note signedness */
ret = usb_control_msg(udev, pipe, GET_DEF, type, PU_BRIGHTNESS_CONTROL << 8,
ProcessingUnitID << 8 | myuvc_control_intf, data, 2, 5000);
if (ret != 2)
return -EIO;
ctrl->default_value = myuvc_get_le_value(data); /* Note signedness */
printk("Brightness: min =%d, max = %d, step = %d, default = %d\n", ctrl->minimum, ctrl->maximum, ctrl->step, ctrl->default_value);
return 0;
}
/* 参考 : uvc_ctrl_get */
int myuvc_vidioc_g_ctrl (struct file *file, void *fh,
struct v4l2_control *ctrl)
{
__u8 type = USB_TYPE_CLASS | USB_RECIP_INTERFACE;
unsigned int pipe;
int ret;
u8 data[2];
if (ctrl->id != V4L2_CID_BRIGHTNESS)
return -EINVAL;
pipe = usb_rcvctrlpipe(udev, 0);
type |= USB_DIR_IN;
ret = usb_control_msg(udev, pipe, GET_CUR, type, PU_BRIGHTNESS_CONTROL << 8,
ProcessingUnitID << 8 | myuvc_control_intf, data, 2, 5000);
if (ret != 2)
return -EIO;
ctrl->value = myuvc_get_le_value(data); /* Note signedness */
return 0;
}
/* 参考: uvc_ctrl_set/uvc_ctrl_commit */
int myuvc_vidioc_s_ctrl (struct file *file, void *fh,
struct v4l2_control *ctrl)
{
__u8 type = USB_TYPE_CLASS | USB_RECIP_INTERFACE;
unsigned int pipe;
int ret;
u8 data[2];
if (ctrl->id != V4L2_CID_BRIGHTNESS)
return -EINVAL;
myuvc_set_le_value(ctrl->value, data);
pipe = usb_sndctrlpipe(udev, 0);
type |= USB_DIR_OUT;
ret = usb_control_msg(udev, pipe, SET_CUR, type, PU_BRIGHTNESS_CONTROL << 8,
ProcessingUnitID << 8 | myuvc_control_intf, data, 2, 5000);
if (ret != 2)
return -EIO;
return 0;
}
数据采集
数据采集时，我们需要和标准VS接口设置里的实时端点通信获取数据，分配、设置、提交URB。
以 640*320 分辨率的图像为例，每一个像素16bit，因此一帧图像占用空间 640*320*2字节 ，对于我的摄像头一次传输，3060字节。需要注意的是，标准UVC摄像头驱动中限制了一个URB的Buffer数量最大为32个，因此一个URB能承载的数据为 3060 * 32 ,经过计算，一帧图像数据需要多个 URB 来传输。因此在将图像数据拷贝到用户空间Buffer时候，可能在某一个URB包含两帧图像的数据，需要不要处理完前一帧就把第二帧图像数据丢弃了，那样会造成图像数据丢失。
static int myuvc_alloc_init_urbs(void)
{
u16 psize;
u32 size;
int npackets;
int i,j;
struct urb *urb;
//struct urb *urb;
psize = 3060; /* 实时传输端点一次能传输的最大字节数 */
size = 614400; /* 一帧数据的最大长度 */
npackets = DIV_ROUND_UP(size, psize);
if (npackets > 32)
npackets = 32;
myprintk("psize %d npackets %d\n",psize,npackets);
size = psize * npackets;
for (i = 0; i < 5; ++i) {
/* 1. 分配usb_buffers */
urb_buffer[i] = usb_alloc_coherent(
myuvc_udev, size,
GFP_KERNEL | __GFP_NOWARN, &urb_dma[i]);
/* 2. 分配urb */
myurb[i] = usb_alloc_urb(npackets, GFP_KERNEL);
if (!urb_buffer[i] || !myurb[i])
{
//myuvc_uninit_urbs();
myprintk("alloc buffer or urb failed\n");
return -ENOMEM;
}
}
/* 3. 设置urb */
for (i = 0; i < 5; ++i) {
urb = myurb[i];
urb->dev = myuvc_udev;
urb->context = NULL;
urb->pipe = usb_rcvisocpipe(myuvc_udev, 0x81);
urb->transfer_flags = URB_ISO_ASAP | URB_NO_TRANSFER_DMA_MAP;
urb->interval = 1;
urb->transfer_buffer = urb_buffer[i];
urb->transfer_dma = urb_dma[i];
urb->complete = myuvc_video_complete;
urb->number_of_packets = npackets;
urb->transfer_buffer_length = size;
for (j = 0; j < npackets; ++j) {
urb->iso_frame_desc[j].offset = j * psize;
urb->iso_frame_desc[j].length = psize;
}
}
return 0;
}
static void myuvc_video_complete(struct urb *urb)
{
u8 *src;
//u8 *dest;
int ret, i;
int len;
int maxlen;
// int nbytes;
// struct myuvc_buffer *buf;
myprintk("video complete\n");
switch (urb->status) {
case 0:
break;
default:
myprintk("Non-zero status (%d) in video "
"completion handler.\n", urb->status);
return;
}
for (i = 0; i < urb->number_of_packets; ++i) {
if (urb->iso_frame_desc[i].status < 0) {
myprintk("USB isochronous frame "
"lost (%d).\n", urb->iso_frame_desc[i].status);
continue;
}
src = urb->transfer_buffer + urb->iso_frame_desc[i].offset;
//dest = myuvc_queue.mem + buf->buf.m.offset + buf->buf.bytesused;
len = urb->iso_frame_desc[i].actual_length;
myprintk("len %d\n",urb->iso_frame_desc[i].actual_length);
/* 判断数据是否有效 */
/* URB数据含义:
* data[0] : 头部长度
* data[1] : 错误状态
*/
if (len < 2 || src[0] < 2 || src[0] > len)
continue;
/* Skip payloads marked with the error bit ("error frames"). */
if (src[1] & UVC_STREAM_ERR) {
myprintk("Dropping payload (error bit set).\n");
continue;
}
/* 除去头部后的数据长度 */
len -= src[0];
/* 缓冲区最多还能存多少数据 */
//maxlen = buf->buf.length - buf->buf.bytesused;
//nbytes = min(len, maxlen);
/* 复制数据 */
//memcpy(dest, src + src[0], nbytes);
//buf->buf.bytesused += nbytes;
/* 判断一帧数据是否已经全部接收到 */
if (len > maxlen) {
//buf->state = VIDEOBUF_DONE;
}
/* Mark the buffer as done if the EOF marker is set. */
if (src[1] & UVC_STREAM_EOF) {
myprintk("Frame complete (EOF found).\n");
if (len == 0)
myprintk("EOF in empty payload.\n");
//buf->state = VIDEOBUF_DONE;
}
}
/* 再次提交URB */
if ((ret = usb_submit_urb(urb, GFP_ATOMIC)) < 0) {
myprintk("Failed to resubmit video URB (%d).\n", ret);
}
}
---------------------