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

[Mauro Carvalho Chehab <mchehab+huawei@kernel.org>]

On Sun, 2 Aug 2026 08:24:29 +0330
Ali Nasrolahi <a.nasrolahi01@gmail.com> wrote:

> Hi everyone,
> 
> 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?

Thanks,
Mauro