Re: [PATCH] media: v4l2-core: make number of video devices configurable
From: Laurent Pinchart
Date: Mon Aug 10 2026 - 12:35:48 EST
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.
> 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@xxxxxxxxx>
> ---
> 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, ¶m_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