From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from perceval.ideasonboard.com (perceval.ideasonboard.com [213.167.242.64]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CE9B019CCF7; Thu, 1 Oct 2026 12:55:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=213.167.242.64 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790859332; cv=none; b=ahF+/Vedv+U8OYT7Kholw7wGt0u9DeUoXATscOkj/dqb0DBTHcicVz/5C3WSAWsUHT8A/ykY2WUmH5kV5DO9ryezGDgpifHHao7oozuf1qupKhvWv9KeeIvOgLWTFFq9n43rd8hWrly/9NewSNCigxGiFpQHKn/SDtm92b8c/HQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790859332; c=relaxed/simple; bh=vlHdojXD6/U0CaTQzSxImp/LqfQZ7lB0i8akVbu0Lso=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=QfvnorsB7ZkD9KCNFyi8qAXjAQTOTxF0Q51PHmC+yPQRNs+aLgCbTwZg9Vw17H/cseE0lQZIBrqJWgghFHHJDgD9x+jkaYKo5Ok/q1JP+NbXPo8Ii71EvVVSP77OvoC2blmB+Of0zHxzU5O6KSLeFIyhmMOpNjKzbtJc0sVLSVQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=ideasonboard.com; spf=pass smtp.mailfrom=ideasonboard.com; dkim=pass (1024-bit key) header.d=ideasonboard.com header.i=@ideasonboard.com header.b=bZZPm9le; arc=none smtp.client-ip=213.167.242.64 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=ideasonboard.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=ideasonboard.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=ideasonboard.com header.i=@ideasonboard.com header.b="bZZPm9le" Received: from killaraus.ideasonboard.com (2001-14ba-70f3-e800--a06.rev.dnainternet.fi [IPv6:2001:14ba:70f3:e800::a06]) by perceval.ideasonboard.com (Postfix) with ESMTPSA id AD79F593; Thu, 1 Oct 2026 14:53:29 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com; s=mail; t=1790859209; bh=vlHdojXD6/U0CaTQzSxImp/LqfQZ7lB0i8akVbu0Lso=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=bZZPm9leAUT2pZzt8vJavHoPH0A8jWggi4NJPBvfHRL5ebnRSDvSm7/T0cgkEhOZy 1R2Ez8ECr0nRXnVlmDsbFnTA3x+QWn9Hr9w5Uh3qe8T9mxv/ucs9H0uyrrgepqYlYg rCMgcEOdKf30+GfNMt6EuPkRfuv3NN5irIVnBbpo= Date: Thu, 1 Oct 2026 15:55:20 +0300 From: Laurent Pinchart To: Paul Elder Cc: Ali Nasrollahi , linux-media@vger.kernel.org, linux-kernel@vger.kernel.org, mchehab@kernel.org, Frank.Li@nxp.com, hverkuil+cisco@kernel.org, guoniu.zhou@oss.nxp.com Subject: Re: [PATCH] media: v4l2-core: make number of video devices configurable Message-ID: <20261001125520.GL944070@killaraus.ideasonboard.com> References: <20260810162406.118981-2-A.Nasrolahi01@gmail.com> <20260810162858.GA3012183@killaraus.ideasonboard.com> <20260811040459.GE3046075@killaraus.ideasonboard.com> <179081652958.2882041.3075142352161432960@neptunite.rasen.tech> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <179081652958.2882041.3075142352161432960@neptunite.rasen.tech> On Thu, Oct 01, 2026 at 10:02:09AM +0900, Paul Elder wrote: > Quoting Laurent Pinchart (2026-08-11 13:04:59) > > 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 ? > > I do :) > > On the Renesas R-Car X5H, if I enable all the VSP-I and VSP-B and VSP-D (a) > (with uapi mode enabled) and VSP-D (b) (with uapi mode enabled) then I get 101 > video devices and 155 subdevs and thus a few of the VSP instances fail to > probe with -ENFILE. I've always said that I like to be proven wrong. Here we go :-) Technically speaking, in production, the 10 VSPD instances won't be exposed to userspace so you won't hit the limit, but for development it's useful to get them to register a full media graph. > > > > > 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 > > > > > --- > > > > > 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 > > > > > #include > > > > > > > > > > -#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