Re: [PATCH] media: v4l2-core: make number of video devices configurable

[Ali Nasrollahi <a.nasrolahi01@gmail.com>]

On 26/08/11 07:04AM, Laurent Pinchart wrote:
> On Tue, Aug 11, 2026 at 07:29:36AM +0330, Ali Nasrollahi wrote:
> > On 26/08/10 07:28PM, Laurent Pinchart wrote:
> > > On Mon, Aug 10, 2026 at 07:54:07PM +0330, Ali Nasrollahi wrote:
> > > > The V4L2 core currently limits the number of registered video devices to
> > > > 256. This can become a restriction for workloads that create a large
> > > > number of virtual video devices, such as v4l2loopback instances.
> > > 
> > > We don't support v4l2loopback upstream though.
> > 
> > Yes, you are correct.
> > 
> > But perhaps my description was a bit misleading. The main idea of this
> > patch is not to support out-of-tree modules by any means, but to allow a
> > configurable number of V4L2 devices to be created and used, whether
> > through loopback devices or other means.
> 
> Do you have a use case where you're reaching the limit without
> out-of-tree drivers ?
> 

My workloads depend on loopback devices which is how I've reached the
limit, however I haven't had a usecase with a such number of deivces
through physical hardware.

> > > > Make the maximum number of video devices configurable through a module
> > > > parameter. The default and minimum remain at the existing limit of 256,
> > > > while the maximum is capped at 4096.
> > > > 
> > > > The parameter is only available when CONFIG_VIDEO_FIXED_MINOR_RANGES is
> > > > disabled, as the fixed minor allocation scheme relies on the existing
> > > > minor ranges.
> > > > 
> > > > The device array is allocated according to the configured limit, while
> > > > the allocation bitmaps remain statically sized to the maximum supported
> > > > limit of 4096.
> > > > 
> > > > Signed-off-by: Ali Nasrollahi <A.Nasrolahi01@gmail.com>
> > > > ---
> > > >  drivers/media/v4l2-core/v4l2-dev.c | 70 +++++++++++++++++++++++-------
> > > >  1 file changed, 55 insertions(+), 15 deletions(-)
> > > > 
> > > > diff --git a/drivers/media/v4l2-core/v4l2-dev.c b/drivers/media/v4l2-core/v4l2-dev.c
> > > > index fd267fb74905..5a526b577970 100644
> > > > --- a/drivers/media/v4l2-core/v4l2-dev.c
> > > > +++ b/drivers/media/v4l2-core/v4l2-dev.c
> > > > @@ -31,7 +31,8 @@
> > > >  #include <media/v4l2-ioctl.h>
> > > >  #include <media/v4l2-event.h>
> > > >  
> > > > -#define VIDEO_NUM_DEVICES	256
> > > > +#define VIDEO_MIN_NUM_DEVICES	256
> > > > +#define VIDEO_MAX_NUM_DEVICES	4096
> > > >  #define VIDEO_NAME              "video4linux"
> > > >  
> > > >  #define dprintk(fmt, arg...) do {					\
> > > > @@ -39,6 +40,27 @@
> > > >  		       __func__, ##arg);				\
> > > >  } while (0)
> > > >  
> > > > +static unsigned int video_nr_devices = VIDEO_MIN_NUM_DEVICES;
> > > > +
> > > > +#ifndef CONFIG_VIDEO_FIXED_MINOR_RANGES
> > > > +static int param_set_video_nr_devices(const char *val,
> > > > +				      const struct kernel_param *kp)
> > > > +{
> > > > +	return param_set_uint_minmax(val, kp, VIDEO_MIN_NUM_DEVICES,
> > > > +				     VIDEO_MAX_NUM_DEVICES);
> > > > +}
> > > > +
> > > > +static const struct kernel_param_ops param_ops_video_nr_devices = {
> > > > +	.set = param_set_video_nr_devices,
> > > > +	.get = param_get_uint,
> > > > +};
> > > > +
> > > > +module_param_cb(video_nr_devices, &param_ops_video_nr_devices,
> > > > +		&video_nr_devices, 0444);
> > > > +
> > > > +MODULE_PARM_DESC(video_nr_devices, "Maximum number of V4L2 devices (256-4096)");
> > > > +#endif
> > > > +
> > > >  /*
> > > >   *	sysfs stuff
> > > >   */
> > > > @@ -98,9 +120,14 @@ static struct dentry *v4l2_debugfs_root_dir;
> > > >  /*
> > > >   *	Active devices
> > > >   */
> > > > -static struct video_device *video_devices[VIDEO_NUM_DEVICES];
> > > > +#ifdef CONFIG_VIDEO_FIXED_MINOR_RANGES
> > > > +static struct video_device *video_devices[VIDEO_MIN_NUM_DEVICES];
> > > > +static DECLARE_BITMAP(devnode_nums[VFL_TYPE_MAX], VIDEO_MIN_NUM_DEVICES);
> > > > +#else
> > > > +static struct video_device **video_devices;
> > > > +static DECLARE_BITMAP(devnode_nums[VFL_TYPE_MAX], VIDEO_MAX_NUM_DEVICES);
> > > > +#endif
> > > >  static DEFINE_MUTEX(videodev_lock);
> > > > -static DECLARE_BITMAP(devnode_nums[VFL_TYPE_MAX], VIDEO_NUM_DEVICES);
> > > >  
> > > >  /* Device node utility functions */
> > > >  
> > > > @@ -504,7 +531,7 @@ static const struct file_operations v4l2_fops = {
> > > >   * in the video_device array, but it was able to obtain a minor number.
> > > >   *
> > > >   * This means that we can always obtain a free stream index number since
> > > > - * the worst case scenario is that there are VIDEO_NUM_DEVICES - 1 slots in
> > > > + * the worst case scenario is that there are video_nr_devices - 1 slots in
> > > >   * use of the video_device array.
> > > >   *
> > > >   * Returns a free index number.
> > > > @@ -513,19 +540,19 @@ static int get_index(struct video_device *vdev)
> > > >  {
> > > >  	/* This can be static since this function is called with the global
> > > >  	   videodev_lock held. */
> > > > -	static DECLARE_BITMAP(used, VIDEO_NUM_DEVICES);
> > > > +	static DECLARE_BITMAP(used, VIDEO_MAX_NUM_DEVICES);
> > > >  	int i;
> > > >  
> > > > -	bitmap_zero(used, VIDEO_NUM_DEVICES);
> > > > +	bitmap_zero(used, VIDEO_MAX_NUM_DEVICES);
> > > >  
> > > > -	for (i = 0; i < VIDEO_NUM_DEVICES; i++) {
> > > > +	for (i = 0; i < video_nr_devices; i++) {
> > > >  		if (video_devices[i] != NULL &&
> > > >  		    video_devices[i]->v4l2_dev == vdev->v4l2_dev) {
> > > >  			__set_bit(video_devices[i]->index, used);
> > > >  		}
> > > >  	}
> > > >  
> > > > -	return find_first_zero_bit(used, VIDEO_NUM_DEVICES);
> > > > +	return find_first_zero_bit(used, video_nr_devices);
> > > >  }
> > > >  
> > > >  #define SET_VALID_IOCTL(ops, cmd, op) \
> > > > @@ -915,7 +942,7 @@ int __video_register_device(struct video_device *vdev,
> > > >  	int i = 0;
> > > >  	int ret;
> > > >  	int minor_offset = 0;
> > > > -	int minor_cnt = VIDEO_NUM_DEVICES;
> > > > +	int minor_cnt = video_nr_devices;
> > > >  	const char *name_base;
> > > >  
> > > >  	/* A minor value of -1 marks this video device as never
> > > > @@ -1020,10 +1047,10 @@ int __video_register_device(struct video_device *vdev,
> > > >  #else
> > > >  	/* The device node number and minor numbers are independent, so
> > > >  	   we just find the first free minor number. */
> > > > -	for (i = 0; i < VIDEO_NUM_DEVICES; i++)
> > > > +	for (i = 0; i < video_nr_devices; i++)
> > > >  		if (video_devices[i] == NULL)
> > > >  			break;
> > > > -	if (i == VIDEO_NUM_DEVICES) {
> > > > +	if (i == video_nr_devices) {
> > > >  		mutex_unlock(&videodev_lock);
> > > >  		pr_err("could not get a free minor\n");
> > > >  		return -ENFILE;
> > > > @@ -1032,7 +1059,7 @@ int __video_register_device(struct video_device *vdev,
> > > >  	vdev->minor = i + minor_offset;
> > > >  	vdev->num = nr;
> > > >  
> > > > -	if (WARN_ON(vdev->minor >= VIDEO_NUM_DEVICES)) {
> > > > +	if (WARN_ON(vdev->minor >= video_nr_devices)) {
> > > >  		mutex_unlock(&videodev_lock);
> > > >  		return -EINVAL;
> > > >  	}
> > > > @@ -1241,7 +1268,7 @@ static int __init videodev_init(void)
> > > >  	int ret;
> > > >  
> > > >  	pr_info("Linux video capture interface: v2.00\n");
> > > > -	ret = register_chrdev_region(dev, VIDEO_NUM_DEVICES, VIDEO_NAME);
> > > > +	ret = register_chrdev_region(dev, video_nr_devices, VIDEO_NAME);
> > > >  	if (ret < 0) {
> > > >  		pr_warn("videodev: unable to get major %d\n",
> > > >  				VIDEO_MAJOR);
> > > > @@ -1250,11 +1277,21 @@ static int __init videodev_init(void)
> > > >  
> > > >  	ret = class_register(&video_class);
> > > >  	if (ret < 0) {
> > > > -		unregister_chrdev_region(dev, VIDEO_NUM_DEVICES);
> > > > +		unregister_chrdev_region(dev, video_nr_devices);
> > > >  		pr_warn("video_dev: class_register failed\n");
> > > >  		return -EIO;
> > > >  	}
> > > >  
> > > > +#ifndef CONFIG_VIDEO_FIXED_MINOR_RANGES
> > > > +	video_devices = kzalloc_objs(*video_devices, video_nr_devices);
> > > > +	if (!video_devices) {
> > > > +		class_unregister(&video_class);
> > > > +		unregister_chrdev_region(dev, video_nr_devices);
> > > > +		pr_warn("video_dev: failed to allocate video_devices\n");
> > > > +		return -ENOMEM;
> > > > +	}
> > > > +#endif
> > > > +
> > > >  	return 0;
> > > >  }
> > > >  
> > > > @@ -1262,8 +1299,11 @@ static void __exit videodev_exit(void)
> > > >  {
> > > >  	dev_t dev = MKDEV(VIDEO_MAJOR, 0);
> > > >  
> > > > +#ifndef CONFIG_VIDEO_FIXED_MINOR_RANGES
> > > > +	kvfree(video_devices);
> > > > +#endif
> > > >  	class_unregister(&video_class);
> > > > -	unregister_chrdev_region(dev, VIDEO_NUM_DEVICES);
> > > > +	unregister_chrdev_region(dev, video_nr_devices);
> > > >  	debugfs_remove_recursive(v4l2_debugfs_root_dir);
> > > >  	v4l2_debugfs_root_dir = NULL;
> > > >  }
> 
> -- 
> Regards,
> 
> Laurent Pinchart