Re: [RFC] Discussion: VIDEO_NUM_DEVICES limit in v4l2-dev.c

[Hans Verkuil <hverkuil+cisco@kernel.org>]

On 02/08/2026 20:25, Ali Nasrolahi wrote:
> On 26/08/02 12:57PM, Mauro Carvalho Chehab wrote:
>> On Sun, 2 Aug 2026 08:24:29 +0330
>> Ali Nasrolahi <a.nasrolahi01@gmail.com> wrote:
>>> I'm working on a project involving high density video processing on a
>>> single server using a large number of v4l2 loopback devices to route
>>> virtual video streams to tenants.
>>>
>>> While doing so, I ran into the hard limit imposed by `VIDEO_NUM_DEVICES`
>>> (currently 256) in `drivers/media/v4l2-core/v4l2-dev.c`. Looking through
>>> the code and git history, this limit has been there for quite a long time.
>>> However, after searching the mailing list archives, I couldn't find any
>>> previous discussion regarding this limitation.
>>>
>>> May I ask:
>>>
>>> 1. Is there any internal design constraint that requires limiting the
>>>    number of V4L2 devices to 256 on modern kernels?
>>
>> There was a constraint at the very beginning due to the way minors
>> were generated. If I'm not mistaken, this was long gone.
>>
>> Still, having a maximum limit is desirable as there is a hard limit
>> in terms of the memory.
>>
>>> 2. Has this limit been discussed previously, or is there any relevant
>>>    discussion that I may have missed?
>>
>> Probably, but likely very years ago. I can't recall last time we
>> discussed this particular limit.
>>
>>> If the current limit is no longer intentional and there's no historical
>>> context that I might've missed, I'd be happy to work on a patch series to
>>> address it, and I would appreciate any guidance on the preferred direction.
>>
>> What is the maximum limit your hardware has? Can you provide some
>> documentation related to it?
> 
> Assuming I understood your question correctly, because the workloads run
> on a server, theoretically, there is no fixed limit on the number of
> virtual video devices that could be used. As long as there is enough cpu
> power, memory, and i/o bandwidth available, it is beneficial to be able
> to add devices and, consequently, run more workloads.
> 
> That being said, to give some rough numbers, I think supporting at least
> 1000 to 2000 devices would likely cover most practical cases.
> 
> This is why I was wondering whether making this limit dynamically
> allocated or at least configurable might be beneficial, so, for example,
> mahcines with limited memory would not have to reserve an unnecessarily
> large array, while systems with more resources could scale accordingly.

It shouldn't be too hard to add a module option to set the max number of
devices. Minimum and default value would be 256, max 4096 (it can go higher: the
minor number has 20 bits, but I'd cap it to 4096 for now).

This module option is only available if CONFIG_VIDEO_FIXED_MINOR_RANGES is
not set, as that would cause problems (and perhaps it is time to retire
CONFIG_VIDEO_FIXED_MINOR_RANGES).

You're the first in my 20+ years of V4L experience who needed more than 256
devices, so this is not a common request :-)

Regards,

	Hans

> 
> For some additional context, the video streams are received over the
> network from various sources such as cameras and WebRTC. These streams
> are then routed to v4l2loopback devices, which are used by the workloads
> for processing.
> 
> Regards,
> -Ali
>