V4L2 How to capture fresh frame? (i.MX mxc capture , select() with timeout = 0)

[ilyakosharov]

Hi!

I'm working with board based on Nitrogen6x using v4l2 for capturing mt9v131 Aptina sensor through mxc capture drivers.

Sensor is connected through csi and ipu and works with mxc_v4l2_capture.c subsystem.

As it has FRAME_NUM = 10 buffer frames i have a video stream with delay of 10 frames. One of solutions i have found to have fresh frame is to dequeue and queue buffers as long as it possible with timeout = 0 while select returns something with Read descriptor. When select returns an error you have a latest frame or, as it could be, have to wait for timeout != 0 (corresponding with your FPS).

So, the problem is that select with timeout = 0

res = select(handlePrivate->devHandle + 1, &readFDSet, NULL, NULL, 0);

everytime returns 1, means that something is to be readed. It happens even in endless loop with select request only.

The questions are:

- Is the camera filedescriptor has something to read? What could it be? Not only frames?

- Maybe there is a more elegant way to get latest frame with v4l2?

Thanx for suggestions!

P.S. I've tried this solution with USB webcam and it worked fine.

[ilyakosharov · Accepted solution]

I've found that poll() and select() works correct with timeout == 0 when device is opened with O_NONBLOCK flag.

That's all.

[EricNelson]

Hello Ilya,

If you're using mfw_v4l2_capture.c, this is the right approach (poll() with zero timeout and ioctl(DQBUF)).

I wouldn't use select(), since it's pretty clunky. poll() is much easier to use.

[ilyakosharov]

EricNelsonThank you for your answer! poll() is really easier to use.

Using this I've found mxc_poll(...) function in mxc_v4l2_capture.c that performs poll():

static unsigned int mxc_poll(struct file *file, struct poll_table_struct *wait)

{

struct video_device *dev = video_devdata(file);

cam_data *cam = video_get_drvdata(dev);

wait_queue_head_t *queue = NULL;

int res = POLLIN | POLLRDNORM;

pr_debug("In MVC:mxc_poll\n");

if (down_interruptible(&cam->busy_lock))

return -EINTR;

queue = &cam->enc_queue;

poll_wait(file, queue, wait);

up(&cam->busy_lock);

return res;

}

As I understand, poll_wait(...) waits for something in queue. So it just waits for new frame and returns POLLIN | POLLRDNORM everytime.

So, we are not able to reach the condition then all buffers are red out and we wait for fresh one, as mxc_poll returns only POLLIN | POLLRDNORM.

So, how to skip all buffers but last? =)