Prio handling and v4l2_device release callback

index b5eb1f3..c3ccd47 100644
@@ -81,69 +81,6 @@ MODULE_LICENSE("GPL");
  *  Video Standard Operations (contributed by Michael Schimek)
  */
 
-
-/* ----------------------------------------------------------------- */
-/* priority handling                                                 */
-
-#define V4L2_PRIO_VALID(val) (val == V4L2_PRIORITY_BACKGROUND   || \
-			      val == V4L2_PRIORITY_INTERACTIVE  || \
-			      val == V4L2_PRIORITY_RECORD)
-
-void v4l2_prio_init(struct v4l2_prio_state *global)
-{
-	memset(global, 0, sizeof(*global));
-}
-EXPORT_SYMBOL(v4l2_prio_init);
-
-int v4l2_prio_change(struct v4l2_prio_state *global, enum v4l2_priority *local,
-		     enum v4l2_priority new)
-{
-	if (!V4L2_PRIO_VALID(new))
-		return -EINVAL;
-	if (*local == new)
-		return 0;
-
-	atomic_inc(&global->prios[new]);
-	if (V4L2_PRIO_VALID(*local))
-		atomic_dec(&global->prios[*local]);
-	*local = new;
-	return 0;
-}
-EXPORT_SYMBOL(v4l2_prio_change);
-
-void v4l2_prio_open(struct v4l2_prio_state *global, enum v4l2_priority *local)
-{
-	v4l2_prio_change(global, local, V4L2_PRIORITY_DEFAULT);
-}
-EXPORT_SYMBOL(v4l2_prio_open);
-
-void v4l2_prio_close(struct v4l2_prio_state *global, enum v4l2_priority local)
-{
-	if (V4L2_PRIO_VALID(local))
-		atomic_dec(&global->prios[local]);
-}
-EXPORT_SYMBOL(v4l2_prio_close);
-
-enum v4l2_priority v4l2_prio_max(struct v4l2_prio_state *global)
-{
-	if (atomic_read(&global->prios[V4L2_PRIORITY_RECORD]) > 0)
-		return V4L2_PRIORITY_RECORD;
-	if (atomic_read(&global->prios[V4L2_PRIORITY_INTERACTIVE]) > 0)
-		return V4L2_PRIORITY_INTERACTIVE;
-	if (atomic_read(&global->prios[V4L2_PRIORITY_BACKGROUND]) > 0)
-		return V4L2_PRIORITY_BACKGROUND;
-	return V4L2_PRIORITY_UNSET;
-}
-EXPORT_SYMBOL(v4l2_prio_max);
-
-int v4l2_prio_check(struct v4l2_prio_state *global, enum v4l2_priority local)
-{
-	return (local < v4l2_prio_max(global)) ? -EBUSY : 0;
-}
-EXPORT_SYMBOL(v4l2_prio_check);
-
-/* ----------------------------------------------------------------- */
-
 /* Helper functions for control handling			     */
 
 /* Check for correctness of the ctrl's value based on the data from

70 lines not on this page · raw message

index 239125a..4c70d15 100644
@@ -80,21 +80,6 @@
 
 /* ------------------------------------------------------------------------- */
 
-/* Priority helper functions */
-
-struct v4l2_prio_state {
-	atomic_t prios[4];
-};
-void v4l2_prio_init(struct v4l2_prio_state *global);
-int v4l2_prio_change(struct v4l2_prio_state *global, enum v4l2_priority *local,
-		     enum v4l2_priority new);
-void v4l2_prio_open(struct v4l2_prio_state *global, enum v4l2_priority *local);
-void v4l2_prio_close(struct v4l2_prio_state *global, enum v4l2_priority local);
-enum v4l2_priority v4l2_prio_max(struct v4l2_prio_state *global);
-int v4l2_prio_check(struct v4l2_prio_state *global, enum v4l2_priority local);
-
-/* ------------------------------------------------------------------------- */
-
 /* Control helper functions */
 
 int v4l2_ctrl_check(struct v4l2_ext_control *ctrl, struct v4l2_queryctrl *qctrl,

index 15802a0..861f323 100644
@@ -34,6 +34,21 @@ struct v4l2_ctrl_handler;
 #define V4L2_FL_REGISTERED	(0)
 #define V4L2_FL_USES_V4L2_FH	(1)
 
+/* Priority helper functions */
+
+struct v4l2_prio_state {
+	atomic_t prios[4];
+};
+
+void v4l2_prio_init(struct v4l2_prio_state *global);
+int v4l2_prio_change(struct v4l2_prio_state *global, enum v4l2_priority *local,
+		     enum v4l2_priority new);
+void v4l2_prio_open(struct v4l2_prio_state *global, enum v4l2_priority *local);
+void v4l2_prio_close(struct v4l2_prio_state *global, enum v4l2_priority local);
+enum v4l2_priority v4l2_prio_max(struct v4l2_prio_state *global);
+int v4l2_prio_check(struct v4l2_prio_state *global, enum v4l2_priority local);
+
+
 struct v4l2_file_operations {
 	struct module *owner;
 	ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);

index 8698fe4..c8f6ae1 100644
@@ -543,6 +543,12 @@ static int __video_register_device(struct video_device *vdev, int type, int nr,
 			vdev->parent = vdev->v4l2_dev->dev;
 		if (vdev->ctrl_handler == NULL)
 			vdev->ctrl_handler = vdev->v4l2_dev->ctrl_handler;
+		/* If the prio state pointer is NULL, and if the driver doesn't
+		   handle priorities itself, then use the v4l2_device prio
+		   state. */
+		if (vdev->prio == NULL && vdev->ioctl_ops &&
+				vdev->ioctl_ops->vidioc_s_priority == NULL)
+			vdev->prio = &vdev->v4l2_dev->prio;
 	}
 
 	/* Part 2: find a free minor, device node number and device index. */

index 7fe6f92..e12844c 100644
@@ -36,6 +36,7 @@ int v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev)
 	INIT_LIST_HEAD(&v4l2_dev->subdevs);
 	spin_lock_init(&v4l2_dev->lock);
 	mutex_init(&v4l2_dev->ioctl_lock);
+	v4l2_prio_init(&v4l2_dev->prio);
 	v4l2_dev->dev = dev;
 	if (dev == NULL) {
 		/* If dev == NULL, then name must be filled in by the caller */

index 861f323..15dd756 100644
@@ -83,6 +83,9 @@ struct video_device
 	/* Control handler associated with this device node. May be NULL. */
 	struct v4l2_ctrl_handler *ctrl_handler;
 
+	/* Priority state. If NULL, then v4l2_dev->prio will be used. */
+	struct v4l2_prio_state *prio;
+
 	/* device info */
 	char name[32];
 	int vfl_type;

index b16f307..fd5d450 100644
@@ -22,6 +22,7 @@
 #define _V4L2_DEVICE_H
 
 #include <media/v4l2-subdev.h>
+#include <media/v4l2-dev.h>
 
 /* Each instance of a V4L2 device should create the v4l2_device struct,
    either stand-alone or embedded in a larger struct.

@@ -51,6 +52,8 @@ struct v4l2_device {
 			unsigned int notification, void *arg);
 	/* The control handler. May be NULL. */
 	struct v4l2_ctrl_handler *ctrl_handler;
+	/* Device's priority state */
+	struct v4l2_prio_state prio;
 	/* BKL replacement mutex. Temporary solution only. */
 	struct mutex ioctl_lock;
 };

index d78f184..78a1608 100644
@@ -33,6 +33,8 @@ int v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev)
 	fh->vdev = vdev;
 	INIT_LIST_HEAD(&fh->list);
 	set_bit(V4L2_FL_USES_V4L2_FH, &fh->vdev->flags);
+	fh->prio = V4L2_PRIORITY_UNSET;
+	BUG_ON(vdev->prio == NULL);
 
 	/*
 	 * fh->events only needs to be initialized if the driver

@@ -51,6 +53,7 @@ void v4l2_fh_add(struct v4l2_fh *fh)
 {
 	unsigned long flags;
 
+	v4l2_prio_open(fh->vdev->prio, &fh->prio);
 	spin_lock_irqsave(&fh->vdev->fh_lock, flags);
 	list_add(&fh->list, &fh->vdev->fh_list);
 	spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);

@@ -64,6 +67,7 @@ void v4l2_fh_del(struct v4l2_fh *fh)
 	spin_lock_irqsave(&fh->vdev->fh_lock, flags);
 	list_del_init(&fh->list);
 	spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);
+	v4l2_prio_close(fh->vdev->prio, fh->prio);
 }
 EXPORT_SYMBOL_GPL(v4l2_fh_del);
 

index 1d72dde..5fc5ba9 100644
@@ -35,6 +35,7 @@ struct v4l2_fh {
 	struct list_head	list;
 	struct video_device	*vdev;
 	struct v4l2_events      *events; /* events, pending and subscribed */
+	enum v4l2_priority	prio;
 };
 
 /*

index c8f6ae1..96ec954 100644
@@ -380,6 +380,15 @@ static int v4l2_open(struct inode *inode, struct file *filp)
 	/* and increase the device refcount */
 	video_get(vdev);
 	mutex_unlock(&videodev_lock);
+
+	if (test_bit(V4L2_FL_USES_V4L2_PRIO, &vdev->flags)) {
+		enum v4l2_priority prio =
+			(enum v4l2_priority)filp->private_data;
+
+		v4l2_prio_open(vdev->prio, &prio);
+		filp->private_data = (void *)prio;
+	}
+
 	if (vdev->fops->open) {
 		if (vdev->lock && mutex_lock_interruptible(vdev->lock)) {
 			ret = -ERESTARTSYS;

@@ -404,8 +413,13 @@ err:
 static int v4l2_release(struct inode *inode, struct file *filp)
 {
 	struct video_device *vdev = video_devdata(filp);
-	int ret = 0;
 
+	if (test_bit(V4L2_FL_USES_V4L2_PRIO, &vdev->flags)) {
+		enum v4l2_priority prio =
+			(enum v4l2_priority)filp->private_data;
+
+		v4l2_prio_close(vdev->prio, prio);
+	}
 	if (vdev->fops->release) {
 		if (vdev->lock)
 			mutex_lock(vdev->lock);

@@ -417,7 +431,7 @@ static int v4l2_release(struct inode *inode, struct file *filp)
 	/* decrease the refcount unconditionally since the release()
 	   return value is ignored. */
 	video_put(vdev);
-	return ret;
+	return 0;
 }
 
 static const struct file_operations v4l2_fops = {

@@ -612,6 +626,11 @@ static int __video_register_device(struct video_device *vdev, int type, int nr,
 	vdev->index = get_index(vdev);
 	mutex_unlock(&videodev_lock);
 
+	/* Determine if we can store v4l2_priority in file->private_data */
+	if (vdev->fops->open == NULL && vdev->fops->release == NULL &&
+	    vdev->prio != NULL)
+		set_bit(V4L2_FL_USES_V4L2_PRIO, &vdev->flags);
+
 	/* Part 3: Initialize the character device */
 	vdev->cdev = cdev_alloc();
 	if (vdev->cdev == NULL) {

index 15dd756..045dffa 100644
@@ -32,7 +32,13 @@ struct v4l2_ctrl_handler;
    Drivers can clear this flag if they want to block all future
    device access. It is cleared by video_unregister_device. */
 #define V4L2_FL_REGISTERED	(0)
+/* Flag to mark that the driver stores a pointer to struct v4l2_fh in
+   file->private_data. Is automatically detected. */
 #define V4L2_FL_USES_V4L2_FH	(1)
+/* Flag to mark that the driver stores the local v4l2_priority in
+   file->private_data. Will be set if open and release are both NULL.
+   Drivers can also set it manually. */
+#define V4L2_FL_USES_V4L2_PRIO	(2)
 
 /* Priority helper functions */
 

index 726d367..444b4cf 100644
@@ -224,7 +224,8 @@ static int radio_si4713_s_frequency(struct file *file, void *p,
 							s_frequency, vf);
 }
 
-static long radio_si4713_default(struct file *file, void *p, int cmd, void *arg)
+static long radio_si4713_default(struct file *file, void *p,
+				bool valid_prio, int cmd, void *arg)
 {
 	return v4l2_device_call_until_err(get_v4l2_dev(file), 0, core,
 							ioctl, cmd, arg);

index 7150195..6b5bb3e 100644
@@ -1056,7 +1056,8 @@ static int cx18_log_status(struct file *file, void *fh)
 	return 0;
 }
 
-static long cx18_default(struct file *file, void *fh, int cmd, void *arg)
+static long cx18_default(struct file *file, void *fh, bool valid_prio,
+							int cmd, void *arg)
 {
 	struct cx18 *cx = ((struct cx18_open_id *)fh)->cx;
 

index 353eada..71e961e 100644
@@ -1719,7 +1719,7 @@ unlock_out:
 
 
 static long vpfe_param_handler(struct file *file, void *priv,
-		int cmd, void *param)
+		bool valid_prio, int cmd, void *param)
 {
 	struct vpfe_device *vpfe_dev = video_drvdata(file);
 	int ret = 0;

index b686da5..d9386a7 100644
@@ -1795,7 +1795,8 @@ static int ivtv_decoder_ioctls(struct file *filp, unsigned int cmd, void *arg)
 	return 0;
 }
 
-static long ivtv_default(struct file *file, void *fh, int cmd, void *arg)
+static long ivtv_default(struct file *file, void *fh, bool valid_prio,
+						int cmd, void *arg)
 {
 	struct ivtv *itv = ((struct ivtv_open_id *)fh)->itv;
 

index 48d2c24..b09a3c8 100644
@@ -1547,7 +1547,8 @@ static int vidioc_streamoff(struct file *file, void *fh, enum v4l2_buf_type i)
 	return 0;
 }
 
-static long vidioc_default(struct file *file, void *fh, int cmd, void *arg)
+static long vidioc_default(struct file *file, void *fh, bool valid_prio,
+						int cmd, void *arg)
 {
 	switch (cmd) {
 	case MEYEIOC_G_PARAMS:

index 4e8fd96..507bed2 100644
@@ -643,7 +643,8 @@ static int vidioc_s_register(struct file *file, void *fh, struct v4l2_dbg_regist
 }
 #endif
 
-static long vidioc_default(struct file *file, void *fh, int cmd, void *arg)
+static long vidioc_default(struct file *file, void *fh, bool valid_prio,
+							int cmd, void *arg)
 {
 	struct saa7146_dev *dev = ((struct saa7146_fh *)fh)->dev;
 	struct mxb *mxb = (struct mxb *)dev->ext_priv;

116 lines not on this page · raw message

index 67df375..4d6303b 100644
@@ -254,7 +254,7 @@ struct v4l2_ioctl_ops {
 
 	/* For other private ioctls */
 	long (*vidioc_default)	       (struct file *file, void *fh,
-					int cmd, void *arg);
+					bool valid_prio, int cmd, void *arg);
 };
 
 

index 04bacdb..84bdf0f 100644
@@ -383,7 +383,6 @@ struct ivtv_open_id {
 	u32 open_id;                    /* unique ID for this file descriptor */
 	int type;                       /* stream type */
 	int yuv_frames;                 /* 1: started OUT_UDMA_YUV output mode */
-	enum v4l2_priority prio;        /* priority */
 	struct ivtv *itv;
 };
 

@@ -710,7 +709,6 @@ struct ivtv {
 
 	/* Miscellaneous */
 	u32 open_id;			/* incremented each time an open occurs, is >= 1 */
-	struct v4l2_prio_state prio;    /* priority state */
 	int search_pack_header;         /* 1 if ivtv_copy_buf_to_user() is scanning for a pack header (0xba) */
 	int speed;                      /* current playback speed setting */
 	u8 speed_mute_audio;            /* 1 if audio should be muted when fast forward */

index c57a585..4463bf4 100644
@@ -856,7 +856,6 @@ int ivtv_v4l2_close(struct file *filp)
 
 	IVTV_DEBUG_FILE("close %s\n", s->name);
 
-	v4l2_prio_close(&itv->prio, id->prio);
 	v4l2_fh_del(fh);
 	v4l2_fh_exit(fh);
 

@@ -973,7 +972,6 @@ static int ivtv_serialized_open(struct ivtv_stream *s, struct file *filp)
 	}
 	item->itv = itv;
 	item->type = s->type;
-	v4l2_prio_open(&itv->prio, &item->prio);
 
 	item->open_id = itv->open_id++;
 	filp->private_data = &item->fh;

82 lines not on this page · raw message

index 4463bf4..ae6e266 100644
@@ -507,9 +507,7 @@ ssize_t ivtv_v4l2_read(struct file * filp, char __user *buf, size_t count, loff_
 
 	IVTV_DEBUG_HI_FILE("read %zd bytes from %s\n", count, s->name);
 
-	mutex_lock(&itv->serialize_lock);
 	rc = ivtv_start_capture(id);
-	mutex_unlock(&itv->serialize_lock);
 	if (rc)
 		return rc;
 	return ivtv_read_pos(s, buf, count, pos, filp->f_flags & O_NONBLOCK);

@@ -584,9 +582,7 @@ ssize_t ivtv_v4l2_write(struct file *filp, const char __user *user_buf, size_t c
 	set_bit(IVTV_F_S_APPL_IO, &s->s_flags);
 
 	/* Start decoder (returns 0 if already started) */
-	mutex_lock(&itv->serialize_lock);
 	rc = ivtv_start_decoding(id, itv->speed);
-	mutex_unlock(&itv->serialize_lock);
 	if (rc) {
 		IVTV_DEBUG_WARN("Failed start decode stream %s\n", s->name);
 

@@ -755,9 +751,7 @@ unsigned int ivtv_v4l2_enc_poll(struct file *filp, poll_table * wait)
 	if (!eof && !test_bit(IVTV_F_S_STREAMING, &s->s_flags)) {
 		int rc;
 
-		mutex_lock(&itv->serialize_lock);
 		rc = ivtv_start_capture(id);
-		mutex_unlock(&itv->serialize_lock);
 		if (rc) {
 			IVTV_DEBUG_INFO("Could not start capture for %s (%d)\n",
 					s->name, rc);

@@ -868,7 +862,6 @@ int ivtv_v4l2_close(struct file *filp)
 	/* 'Unclaim' this stream */
 
 	/* Stop radio */
-	mutex_lock(&itv->serialize_lock);
 	if (id->type == IVTV_ENC_STREAM_TYPE_RAD) {
 		/* Closing radio device, return to TV mode */
 		ivtv_mute(itv);

@@ -906,7 +899,6 @@ int ivtv_v4l2_close(struct file *filp)
 		ivtv_stop_capture(id, 0);
 	}
 	kfree(id);
-	mutex_unlock(&itv->serialize_lock);
 	return 0;
 }
 

@@ -1026,7 +1018,6 @@ static int ivtv_serialized_open(struct ivtv_stream *s, struct file *filp)
 
 int ivtv_v4l2_open(struct file *filp)
 {
-	int res;
 	struct ivtv *itv = NULL;
 	struct ivtv_stream *s = NULL;
 	struct video_device *vdev = video_devdata(filp);

@@ -1034,16 +1025,12 @@ int ivtv_v4l2_open(struct file *filp)
 	s = video_get_drvdata(vdev);
 	itv = s->itv;
 
-	mutex_lock(&itv->serialize_lock);
 	if (ivtv_init_on_first_open(itv)) {
 		IVTV_ERR("Failed to initialize on device %s\n",
 			 video_device_node_name(vdev));
-		mutex_unlock(&itv->serialize_lock);
 		return -ENXIO;
 	}
-	res = ivtv_serialized_open(s, filp);
-	mutex_unlock(&itv->serialize_lock);
-	return res;
+	return ivtv_serialized_open(s, filp);
 }
 
 void ivtv_mute(struct ivtv *itv)

index 6fb1837..d65c4ed 100644
@@ -1831,8 +1831,7 @@ static long ivtv_default(struct file *file, void *fh, bool valid_prio,
 	return 0;
 }
 
-static long ivtv_serialized_ioctl(struct ivtv *itv, struct file *filp,
-		unsigned int cmd, unsigned long arg)
+long ivtv_v4l2_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 {
 	struct video_device *vfd = video_devdata(filp);
 	long ret;

@@ -1844,21 +1843,6 @@ static long ivtv_serialized_ioctl(struct ivtv *itv, struct file *filp,
 	return ret;
 }
 
-long ivtv_v4l2_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
-{
-	struct ivtv_open_id *id = fh2id(filp->private_data);
-	struct ivtv *itv = id->itv;
-	long res;
-
-	/* DQEVENT can block, so this should not run with the serialize lock */
-	if (cmd == VIDIOC_DQEVENT)
-		return ivtv_serialized_ioctl(itv, filp, cmd, arg);
-	mutex_lock(&itv->serialize_lock);
-	res = ivtv_serialized_ioctl(itv, filp, cmd, arg);
-	mutex_unlock(&itv->serialize_lock);
-	return res;
-}
-
 static const struct v4l2_ioctl_ops ivtv_ioctl_ops = {
 	.vidioc_querycap    		    = ivtv_querycap,
 	.vidioc_s_audio     		    = ivtv_s_audio,

index 512607e..b588ffc 100644
@@ -214,6 +214,7 @@ static int ivtv_prep_dev(struct ivtv *itv, int type)
 	s->vdev->fops = ivtv_stream_info[type].fops;
 	s->vdev->release = video_device_release;
 	s->vdev->tvnorms = V4L2_STD_ALL;
+	s->vdev->lock = &itv->serialize_lock;
 	ivtv_set_funcs(s->vdev);
 	return 0;
 }

index 96ec954..c83d8a9 100644
@@ -143,6 +143,7 @@ static inline void video_put(struct video_device *vdev)
 static void v4l2_device_release(struct device *cd)
 {
 	struct video_device *vdev = to_video_device(cd);
+	struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
 
 	mutex_lock(&videodev_lock);
 	if (video_device[vdev->minor] != vdev) {

@@ -169,6 +170,10 @@ static void v4l2_device_release(struct device *cd)
 	/* Release video_device and perform other
 	   cleanups as needed. */
 	vdev->release(vdev);
+
+	/* Decrease v4l2_device refcount */
+	if (v4l2_dev)
+		v4l2_device_put(v4l2_dev);
 }
 
 static struct class video_class = {

@@ -670,6 +675,9 @@ static int __video_register_device(struct video_device *vdev, int type, int nr,
 		printk(KERN_WARNING "%s: requested %s%d, got %s\n", __func__,
 			name_base, nr, video_device_node_name(vdev));
 
+	/* Increase v4l2_device refcount */
+	if (vdev->v4l2_dev)
+		v4l2_device_get(vdev->v4l2_dev);
 	/* Part 5: Activate this minor. The char device can now be used. */
 	set_bit(V4L2_FL_REGISTERED, &vdev->flags);
 	mutex_lock(&videodev_lock);

index e12844c..4124016 100644
@@ -55,6 +55,21 @@ int v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev)
 }
 EXPORT_SYMBOL_GPL(v4l2_device_register);
 
+static void v4l2_device_release(struct kref *ref)
+{
+	struct v4l2_device *v4l2_dev =
+		container_of(ref, struct v4l2_device, ref);
+
+	if (v4l2_dev->release)
+		v4l2_dev->release(v4l2_dev);
+}
+
+int v4l2_device_put(struct v4l2_device *v4l2_dev)
+{
+	return kref_put(&v4l2_dev->ref, v4l2_device_release);
+}
+EXPORT_SYMBOL_GPL(v4l2_device_put);
+
 int v4l2_device_set_name(struct v4l2_device *v4l2_dev, const char *basename,
 						atomic_t *instance)
 {

index fd5d450..44763bc 100644
@@ -56,8 +56,19 @@ struct v4l2_device {
 	struct v4l2_prio_state prio;
 	/* BKL replacement mutex. Temporary solution only. */
 	struct mutex ioctl_lock;
+	/* Keep track of the references to this struct. */
+	struct kref ref;
+	/* Release function that is called when the ref count goes to 0. */
+	void (*release)(struct v4l2_device *v4l2_dev);
 };
 
+static inline void v4l2_device_get(struct v4l2_device *v4l2_dev)
+{
+	kref_get(&v4l2_dev->ref);
+}
+
+int v4l2_device_put(struct v4l2_device *v4l2_dev);
+
 /* Initialize v4l2_dev and make dev->driver_data point to v4l2_dev.
    dev may be NULL in rare cases (ISA devices). In that case you
    must fill in the v4l2_dev->name field before calling this function. */