[media] v4l: vivi: port to videobuf2

Make vivi use videobuf2 in place of videobuf.

Signed-off-by: Pawel Osciak <p.osciak@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
CC: Pawel Osciak <pawel@osciak.com>
Reviewed-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

1 files changed, 199 insertions, 158 deletions

diff --git a/drivers/media/video/vivi.c b/drivers/media/video/vivi.c
index c49c39386bd..9ed3831b34b 100644
--- a/drivers/media/video/vivi.c
+++ b/drivers/media/video/vivi.c
@@ -7,6 +7,9 @@
  *      John Sokol <sokol--a.t--videotechnology.com>
  *      http://v4l.videotechnology.com/
  *
+ *      Conversion to videobuf2 by Pawel Osciak & Marek Szyprowski
+ *      Copyright (c) 2010 Samsung Electronics
+ *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the BSD Licence, GNU General Public License
  * as published by the Free Software Foundation; either version 2 of the
@@ -23,10 +26,8 @@
 #include <linux/mutex.h>
 #include <linux/videodev2.h>
 #include <linux/kthread.h>
-#if LINUX_VERSION_CODE >= KERNEL_VERSION(2, 6, 20)
 #include <linux/freezer.h>
-#endif
-#include <media/videobuf-vmalloc.h>
+#include <media/videobuf2-vmalloc.h>
 #include <media/v4l2-device.h>
 #include <media/v4l2-ioctl.h>
 #include <media/v4l2-common.h>
@@ -42,7 +43,7 @@
 #define MAX_HEIGHT 1200
 
 #define VIVI_MAJOR_VERSION 0
-#define VIVI_MINOR_VERSION 7
+#define VIVI_MINOR_VERSION 8
 #define VIVI_RELEASE 0
 #define VIVI_VERSION \
         KERNEL_VERSION(VIVI_MAJOR_VERSION, VIVI_MINOR_VERSION, VIVI_RELEASE)
@@ -133,16 +134,11 @@ static struct vivi_fmt *get_format(struct v4l2_format *f)
         return &formats[k];
 }
 
-struct sg_to_addr {
-        int pos;
-        struct scatterlist *sg;
-};
-
 /* buffer for one video frame */
 struct vivi_buffer {
         /* common v4l buffer stuff -- must be first */
-        struct videobuf_buffer vb;
-
+        struct vb2_buffer       vb;
+        struct list_head        list;
         struct vivi_fmt        *fmt;
 };
 
@@ -190,9 +186,11 @@ struct vivi_dev {
         /* video capture */
         struct vivi_fmt            *fmt;
         unsigned int               width, height;
-        struct videobuf_queue      vb_vidq;
+        struct vb2_queue           vb_vidq;
+        enum v4l2_field            field;
+        unsigned int               field_count;
 
-        unsigned long              generating;
+        unsigned int               open_count;
         u8                         bars[9][3];
         u8                         line[MAX_WIDTH * 4];
 };
@@ -443,10 +441,10 @@ static void gen_text(struct vivi_dev *dev, char *basep,
 
 static void vivi_fillbuff(struct vivi_dev *dev, struct vivi_buffer *buf)
 {
-        int hmax = buf->vb.height;
-        int wmax = buf->vb.width;
+        int wmax = dev->width;
+        int hmax = dev->height;
         struct timeval ts;
-        void *vbuf = videobuf_to_vmalloc(&buf->vb);
+        void *vbuf = vb2_plane_vaddr(&buf->vb, 0);
         unsigned ms;
         char str[100];
         int h, line = 1;
@@ -483,11 +481,11 @@ static void vivi_fillbuff(struct vivi_dev *dev, struct vivi_buffer *buf)
 
         dev->mv_count += 2;
 
-        /* Advice that buffer was filled */
-        buf->vb.field_count++;
+        buf->vb.v4l2_buf.field = dev->field;
+        dev->field_count++;
+        buf->vb.v4l2_buf.sequence = dev->field_count >> 1;
         do_gettimeofday(&ts);
-        buf->vb.ts = ts;
-        buf->vb.state = VIDEOBUF_DONE;
+        buf->vb.v4l2_buf.timestamp = ts;
 }
 
 static void vivi_thread_tick(struct vivi_dev *dev)
@@ -504,23 +502,17 @@ static void vivi_thread_tick(struct vivi_dev *dev)
                 goto unlock;
         }
 
-        buf = list_entry(dma_q->active.next,
-                         struct vivi_buffer, vb.queue);
-
-        /* Nobody is waiting on this buffer, return */
-        if (!waitqueue_active(&buf->vb.done))
-                goto unlock;
+        buf = list_entry(dma_q->active.next, struct vivi_buffer, list);
+        list_del(&buf->list);
 
-        list_del(&buf->vb.queue);
-
-        do_gettimeofday(&buf->vb.ts);
+        do_gettimeofday(&buf->vb.v4l2_buf.timestamp);
 
         /* Fill buffer */
         vivi_fillbuff(dev, buf);
         dprintk(dev, 1, "filled buffer %p\n", buf);
 
-        wake_up(&buf->vb.done);
-        dprintk(dev, 2, "[%p/%d] wakeup\n", buf, buf->vb. i);
+        vb2_buffer_done(&buf->vb, VB2_BUF_STATE_DONE);
+        dprintk(dev, 2, "[%p/%d] done\n", buf, buf->vb.v4l2_buf.index);
 unlock:
         spin_unlock_irqrestore(&dev->slock, flags);
 }
@@ -571,17 +563,12 @@ static int vivi_thread(void *data)
         return 0;
 }
 
-static void vivi_start_generating(struct file *file)
+static int vivi_start_generating(struct vivi_dev *dev)
 {
-        struct vivi_dev *dev = video_drvdata(file);
         struct vivi_dmaqueue *dma_q = &dev->vidq;
 
         dprintk(dev, 1, "%s\n", __func__);
 
-        if (test_and_set_bit(0, &dev->generating))
-                return;
-        file->private_data = dev;
-
         /* Resets frame counters */
         dev->ms = 0;
         dev->mv_count = 0;
@@ -593,146 +580,200 @@ static void vivi_start_generating(struct file *file)
 
         if (IS_ERR(dma_q->kthread)) {
                 v4l2_err(&dev->v4l2_dev, "kernel_thread() failed\n");
-                clear_bit(0, &dev->generating);
-                return;
+                return PTR_ERR(dma_q->kthread);
         }
         /* Wakes thread */
         wake_up_interruptible(&dma_q->wq);
 
         dprintk(dev, 1, "returning from %s\n", __func__);
+        return 0;
 }
 
-static void vivi_stop_generating(struct file *file)
+static void vivi_stop_generating(struct vivi_dev *dev)
 {
-        struct vivi_dev *dev = video_drvdata(file);
         struct vivi_dmaqueue *dma_q = &dev->vidq;
 
         dprintk(dev, 1, "%s\n", __func__);
 
-        if (!file->private_data)
-                return;
-        if (!test_and_clear_bit(0, &dev->generating))
-                return;
-
         /* shutdown control thread */
         if (dma_q->kthread) {
                 kthread_stop(dma_q->kthread);
                 dma_q->kthread = NULL;
         }
-        videobuf_stop(&dev->vb_vidq);
-        videobuf_mmap_free(&dev->vb_vidq);
-}
 
-static int vivi_is_generating(struct vivi_dev *dev)
-{
-        return test_bit(0, &dev->generating);
+        /*
+         * Typical driver might need to wait here until dma engine stops.
+         * In this case we can abort imiedetly, so it's just a noop.
+         */
+
+        /* Release all active buffers */
+        while (!list_empty(&dma_q->active)) {
+                struct vivi_buffer *buf;
+                buf = list_entry(dma_q->active.next, struct vivi_buffer, list);
+                list_del(&buf->list);
+                vb2_buffer_done(&buf->vb, VB2_BUF_STATE_ERROR);
+                dprintk(dev, 2, "[%p/%d] done\n", buf, buf->vb.v4l2_buf.index);
+        }
 }
-
 /* ------------------------------------------------------------------
         Videobuf operations
    ------------------------------------------------------------------*/
-static int
-buffer_setup(struct videobuf_queue *vq, unsigned int *count, unsigned int *size)
+static int queue_setup(struct vb2_queue *vq, unsigned int *nbuffers,
+                                unsigned int *nplanes, unsigned long sizes[],
+                                void *alloc_ctxs[])
 {
-        struct vivi_dev *dev = vq->priv_data;
+        struct vivi_dev *dev = vb2_get_drv_priv(vq);
+        unsigned long size;
+
+        size = dev->width * dev->height * 2;
 
-        *size = dev->width * dev->height * 2;
+        if (0 == *nbuffers)
+                *nbuffers = 32;
 
-        if (0 == *count)
-                *count = 32;
+        while (size * *nbuffers > vid_limit * 1024 * 1024)
+                (*nbuffers)--;
 
-        while (*size * *count > vid_limit * 1024 * 1024)
-                (*count)--;
+        *nplanes = 1;
 
-        dprintk(dev, 1, "%s, count=%d, size=%d\n", __func__,
-                *count, *size);
+        sizes[0] = size;
+
+        /*
+         * videobuf2-vmalloc allocator is context-less so no need to set
+         * alloc_ctxs array.
+         */
+
+        dprintk(dev, 1, "%s, count=%d, size=%ld\n", __func__,
+                *nbuffers, size);
 
         return 0;
 }
 
-static void free_buffer(struct videobuf_queue *vq, struct vivi_buffer *buf)
+static int buffer_init(struct vb2_buffer *vb)
 {
-        struct vivi_dev *dev = vq->priv_data;
+        struct vivi_dev *dev = vb2_get_drv_priv(vb->vb2_queue);
 
-        dprintk(dev, 1, "%s, state: %i\n", __func__, buf->vb.state);
+        BUG_ON(NULL == dev->fmt);
 
-        videobuf_vmalloc_free(&buf->vb);
-        dprintk(dev, 1, "free_buffer: freed\n");
-        buf->vb.state = VIDEOBUF_NEEDS_INIT;
+        /*
+         * This callback is called once per buffer, after its allocation.
+         *
+         * Vivi does not allow changing format during streaming, but it is
+         * possible to do so when streaming is paused (i.e. in streamoff state).
+         * Buffers however are not freed when going into streamoff and so
+         * buffer size verification has to be done in buffer_prepare, on each
+         * qbuf.
+         * It would be best to move verification code here to buf_init and
+         * s_fmt though.
+         */
+
+        return 0;
 }
 
-static int
-buffer_prepare(struct videobuf_queue *vq, struct videobuf_buffer *vb,
-                                                enum v4l2_field field)
+static int buffer_prepare(struct vb2_buffer *vb)
 {
-        struct vivi_dev *dev = vq->priv_data;
+        struct vivi_dev *dev = vb2_get_drv_priv(vb->vb2_queue);
         struct vivi_buffer *buf = container_of(vb, struct vivi_buffer, vb);
-        int rc;
+        unsigned long size;
 
-        dprintk(dev, 1, "%s, field=%d\n", __func__, field);
+        dprintk(dev, 1, "%s, field=%d\n", __func__, vb->v4l2_buf.field);
 
         BUG_ON(NULL == dev->fmt);
 
+        /*
+         * Theses properties only change when queue is idle, see s_fmt.
+         * The below checks should not be performed here, on each
+         * buffer_prepare (i.e. on each qbuf). Most of the code in this function
+         * should thus be moved to buffer_init and s_fmt.
+         */
         if (dev->width  < 48 || dev->width  > MAX_WIDTH ||
             dev->height < 32 || dev->height > MAX_HEIGHT)
                 return -EINVAL;
 
-        buf->vb.size = dev->width * dev->height * 2;
-        if (0 != buf->vb.baddr && buf->vb.bsize < buf->vb.size)
+        size = dev->width * dev->height * 2;
+        if (vb2_plane_size(vb, 0) < size) {
+                dprintk(dev, 1, "%s data will not fit into plane (%lu < %lu)\n",
+                                __func__, vb2_plane_size(vb, 0), size);
                 return -EINVAL;
+        }
+
+        vb2_set_plane_payload(&buf->vb, 0, size);
 
-        /* These properties only change when queue is idle, see s_fmt */
-        buf->fmt       = dev->fmt;
-        buf->vb.width  = dev->width;
-        buf->vb.height = dev->height;
-        buf->vb.field  = field;
+        buf->fmt = dev->fmt;
 
         precalculate_bars(dev);
         precalculate_line(dev);
 
-        if (VIDEOBUF_NEEDS_INIT == buf->vb.state) {
-                rc = videobuf_iolock(vq, &buf->vb, NULL);
-                if (rc < 0)
-                        goto fail;
-        }
+        return 0;
+}
 
-        buf->vb.state = VIDEOBUF_PREPARED;
+static int buffer_finish(struct vb2_buffer *vb)
+{
+        struct vivi_dev *dev = vb2_get_drv_priv(vb->vb2_queue);
+        dprintk(dev, 1, "%s\n", __func__);
         return 0;
+}
+
+static void buffer_cleanup(struct vb2_buffer *vb)
+{
+        struct vivi_dev *dev = vb2_get_drv_priv(vb->vb2_queue);
+        dprintk(dev, 1, "%s\n", __func__);
 
-fail:
-        free_buffer(vq, buf);
-        return rc;
 }
 
-static void
-buffer_queue(struct videobuf_queue *vq, struct videobuf_buffer *vb)
+static void buffer_queue(struct vb2_buffer *vb)
 {
-        struct vivi_dev *dev = vq->priv_data;
+        struct vivi_dev *dev = vb2_get_drv_priv(vb->vb2_queue);
         struct vivi_buffer *buf = container_of(vb, struct vivi_buffer, vb);
         struct vivi_dmaqueue *vidq = &dev->vidq;
+        unsigned long flags = 0;
 
         dprintk(dev, 1, "%s\n", __func__);
 
-        buf->vb.state = VIDEOBUF_QUEUED;
-        list_add_tail(&buf->vb.queue, &vidq->active);
+        spin_lock_irqsave(&dev->slock, flags);
+        list_add_tail(&buf->list, &vidq->active);
+        spin_unlock_irqrestore(&dev->slock, flags);
 }
 
-static void buffer_release(struct videobuf_queue *vq,
-                           struct videobuf_buffer *vb)
+static int start_streaming(struct vb2_queue *vq)
 {
-        struct vivi_dev *dev = vq->priv_data;
-        struct vivi_buffer *buf  = container_of(vb, struct vivi_buffer, vb);
+        struct vivi_dev *dev = vb2_get_drv_priv(vq);
+        dprintk(dev, 1, "%s\n", __func__);
+        return vivi_start_generating(dev);
+}
 
+/* abort streaming and wait for last buffer */
+static int stop_streaming(struct vb2_queue *vq)
+{
+        struct vivi_dev *dev = vb2_get_drv_priv(vq);
         dprintk(dev, 1, "%s\n", __func__);
+        vivi_stop_generating(dev);
+        return 0;
+}
+
+static void vivi_lock(struct vb2_queue *vq)
+{
+        struct vivi_dev *dev = vb2_get_drv_priv(vq);
+        mutex_lock(&dev->mutex);
+}
 
-        free_buffer(vq, buf);
+static void vivi_unlock(struct vb2_queue *vq)
+{
+        struct vivi_dev *dev = vb2_get_drv_priv(vq);
+        mutex_unlock(&dev->mutex);
 }
 
-static struct videobuf_queue_ops vivi_video_qops = {
-        .buf_setup      = buffer_setup,
-        .buf_prepare    = buffer_prepare,
-        .buf_queue      = buffer_queue,
-        .buf_release    = buffer_release,
+
+static struct vb2_ops vivi_video_qops = {
+        .queue_setup            = queue_setup,
+        .buf_init               = buffer_init,
+        .buf_prepare            = buffer_prepare,
+        .buf_finish             = buffer_finish,
+        .buf_cleanup            = buffer_cleanup,
+        .buf_queue              = buffer_queue,
+        .start_streaming        = start_streaming,
+        .stop_streaming         = stop_streaming,
+        .wait_prepare           = vivi_unlock,
+        .wait_finish            = vivi_lock,
 };
 
 /* ------------------------------------------------------------------
@@ -774,7 +815,7 @@ static int vidioc_g_fmt_vid_cap(struct file *file, void *priv,
 
         f->fmt.pix.width        = dev->width;
         f->fmt.pix.height       = dev->height;
-        f->fmt.pix.field        = dev->vb_vidq.field;
+        f->fmt.pix.field        = dev->field;
         f->fmt.pix.pixelformat  = dev->fmt->fourcc;
         f->fmt.pix.bytesperline =
                 (f->fmt.pix.width * dev->fmt->depth) >> 3;
@@ -820,82 +861,60 @@ static int vidioc_s_fmt_vid_cap(struct file *file, void *priv,
                                         struct v4l2_format *f)
 {
         struct vivi_dev *dev = video_drvdata(file);
+        struct vb2_queue *q = &dev->vb_vidq;
 
         int ret = vidioc_try_fmt_vid_cap(file, priv, f);
         if (ret < 0)
                 return ret;
 
-        if (vivi_is_generating(dev)) {
+        if (vb2_is_streaming(q)) {
                 dprintk(dev, 1, "%s device busy\n", __func__);
-                ret = -EBUSY;
-                goto out;
+                return -EBUSY;
         }
 
         dev->fmt = get_format(f);
         dev->width = f->fmt.pix.width;
         dev->height = f->fmt.pix.height;
-        dev->vb_vidq.field = f->fmt.pix.field;
-        ret = 0;
-out:
-        return ret;
+        dev->field = f->fmt.pix.field;
+
+        return 0;
 }
 
 static int vidioc_reqbufs(struct file *file, void *priv,
                           struct v4l2_requestbuffers *p)
 {
         struct vivi_dev *dev = video_drvdata(file);
-
-        return videobuf_reqbufs(&dev->vb_vidq, p);
+        return vb2_reqbufs(&dev->vb_vidq, p);
 }
 
 static int vidioc_querybuf(struct file *file, void *priv, struct v4l2_buffer *p)
 {
         struct vivi_dev *dev = video_drvdata(file);
-
-        return videobuf_querybuf(&dev->vb_vidq, p);
+        return vb2_querybuf(&dev->vb_vidq, p);
 }
 
 static int vidioc_qbuf(struct file *file, void *priv, struct v4l2_buffer *p)
 {
         struct vivi_dev *dev = video_drvdata(file);
-
-        return videobuf_qbuf(&dev->vb_vidq, p);
+        return vb2_qbuf(&dev->vb_vidq, p);
 }
 
 static int vidioc_dqbuf(struct file *file, void *priv, struct v4l2_buffer *p)
 {
         struct vivi_dev *dev = video_drvdata(file);
-
-        return videobuf_dqbuf(&dev->vb_vidq, p,
-                                file->f_flags & O_NONBLOCK);
+        return vb2_dqbuf(&dev->vb_vidq, p, file->f_flags & O_NONBLOCK);
 }
 
 static int vidioc_streamon(struct file *file, void *priv, enum v4l2_buf_type i)
 {
         struct vivi_dev *dev = video_drvdata(file);
-        int ret;
-
-        if (i != V4L2_BUF_TYPE_VIDEO_CAPTURE)
-                return -EINVAL;
-        ret = videobuf_streamon(&dev->vb_vidq);
-        if (ret)
-                return ret;
-
-        vivi_start_generating(file);
-        return 0;
+        return vb2_streamon(&dev->vb_vidq, i);
 }
 
 static int vidioc_streamoff(struct file *file, void *priv, enum v4l2_buf_type i)
 {
         struct vivi_dev *dev = video_drvdata(file);
-        int ret;
-
-        if (i != V4L2_BUF_TYPE_VIDEO_CAPTURE)
-                return -EINVAL;
-        ret = videobuf_streamoff(&dev->vb_vidq);
-        if (!ret)
-                vivi_stop_generating(file);
-        return ret;
+        return vb2_streamoff(&dev->vb_vidq, i);
 }
 
 static int vidioc_s_std(struct file *file, void *priv, v4l2_std_id *i)
@@ -1018,26 +1037,33 @@ static int vidioc_s_ctrl(struct file *file, void *priv,
         File operations for the device
    ------------------------------------------------------------------*/
 
+static int vivi_open(struct file *file)
+{
+        struct vivi_dev *dev = video_drvdata(file);
+
+        dprintk(dev, 1, "%s, %p\n", __func__, file);
+        dev->open_count++;
+        return 0;
+}
+
 static ssize_t
 vivi_read(struct file *file, char __user *data, size_t count, loff_t *ppos)
 {
         struct vivi_dev *dev = video_drvdata(file);
 
-        vivi_start_generating(file);
-        return videobuf_read_stream(&dev->vb_vidq, data, count, ppos, 0,
-                                        file->f_flags & O_NONBLOCK);
+        dprintk(dev, 1, "read called\n");
+        return vb2_read(&dev->vb_vidq, data, count, ppos,
+                       file->f_flags & O_NONBLOCK);
 }
 
 static unsigned int
 vivi_poll(struct file *file, struct poll_table_struct *wait)
 {
         struct vivi_dev *dev = video_drvdata(file);
-        struct videobuf_queue *q = &dev->vb_vidq;
+        struct vb2_queue *q = &dev->vb_vidq;
 
         dprintk(dev, 1, "%s\n", __func__);
-
-        vivi_start_generating(file);
-        return videobuf_poll_stream(file, q, wait);
+        return vb2_poll(q, file, wait);
 }
 
 static int vivi_close(struct file *file)
@@ -1045,10 +1071,11 @@ static int vivi_close(struct file *file)
         struct video_device  *vdev = video_devdata(file);
         struct vivi_dev *dev = video_drvdata(file);
 
-        vivi_stop_generating(file);
+        dprintk(dev, 1, "close called (dev=%s), file %p\n",
+                video_device_node_name(vdev), file);
 
-        dprintk(dev, 1, "close called (dev=%s)\n",
-                video_device_node_name(vdev));
+        if (--dev->open_count == 0)
+                vb2_queue_release(&dev->vb_vidq);
         return 0;
 }
 
@@ -1059,8 +1086,7 @@ static int vivi_mmap(struct file *file, struct vm_area_struct *vma)
 
         dprintk(dev, 1, "mmap called, vma=0x%08lx\n", (unsigned long)vma);
 
-        ret = videobuf_mmap_mapper(&dev->vb_vidq, vma);
-
+        ret = vb2_mmap(&dev->vb_vidq, vma);
         dprintk(dev, 1, "vma start=0x%08lx, size=%ld, ret=%d\n",
                 (unsigned long)vma->vm_start,
                 (unsigned long)vma->vm_end - (unsigned long)vma->vm_start,
@@ -1070,6 +1096,7 @@ static int vivi_mmap(struct file *file, struct vm_area_struct *vma)
 
 static const struct v4l2_file_operations vivi_fops = {
         .owner          = THIS_MODULE,
+        .open           = vivi_open,
         .release        = vivi_close,
         .read           = vivi_read,
         .poll           = vivi_poll,
@@ -1136,6 +1163,7 @@ static int __init vivi_create_instance(int inst)
 {
         struct vivi_dev *dev;
         struct video_device *vfd;
+        struct vb2_queue *q;
         int ret;
 
         dev = kzalloc(sizeof(*dev), GFP_KERNEL);
@@ -1159,12 +1187,20 @@ static int __init vivi_create_instance(int inst)
 
         /* initialize locks */
         spin_lock_init(&dev->slock);
-        mutex_init(&dev->mutex);
 
-        videobuf_queue_vmalloc_init(&dev->vb_vidq, &vivi_video_qops,
-                        NULL, &dev->slock, V4L2_BUF_TYPE_VIDEO_CAPTURE,
-                        V4L2_FIELD_INTERLACED,
-                        sizeof(struct vivi_buffer), dev, &dev->mutex);
+        /* initialize queue */
+        q = &dev->vb_vidq;
+        memset(q, 0, sizeof(dev->vb_vidq));
+        q->type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+        q->io_modes = VB2_MMAP | VB2_USERPTR | VB2_READ;
+        q->drv_priv = dev;
+        q->buf_struct_size = sizeof(struct vivi_buffer);
+        q->ops = &vivi_video_qops;
+        q->mem_ops = &vb2_vmalloc_memops;
+
+        vb2_queue_init(q);
+
+        mutex_init(&dev->mutex);
 
         /* init video dma queues */
         INIT_LIST_HEAD(&dev->vidq.active);
@@ -1178,6 +1214,11 @@ static int __init vivi_create_instance(int inst)
         *vfd = vivi_template;
         vfd->debug = debug;
         vfd->v4l2_dev = &dev->v4l2_dev;
+
+        /*
+         * Provide a mutex to v4l2 core. It will be used to protect
+         * all fops and v4l2 ioctls.
+         */
         vfd->lock = &dev->mutex;
 
         ret = video_register_device(vfd, VFL_TYPE_GRABBER, video_nr);