Re: [PATCH v2 0/3] media: Two oopses and a hang when unbinding a streaming sensor

[Nicola Fiorillo <nicfio@gmail.com>]

Hi Sakari,

On Sat, Sep 12, 2026 at 01:23:42PM +0300, Sakari Ailus wrote:
> Thanks for the patchset. These are known issues and unfortunately unbinding
> drivers while streaming isn't supported on MC-enabled drivers currently.
> This is a MC/V4L2 framework limitation and cannot be meaningfully worked
> around in drivers.

Understood, and I won't argue the point for 1/3 and 3/3: both are IPU6
driver changes for exactly the unsupported scenario you describe. I'll
drop them.

Could I ask you to look at 2/3 on its own, though? I believe it is a
different bug, and the cover letter framed it badly -- that is my fault.

2/3 is not a driver change and does not involve streaming at all. It is a
NULL dereference in subdev_open() in the framework itself, caused by the
ordering inside v4l2_device_unregister_subdev():

	sd->v4l2_dev = NULL;
	/* ... */
	media_device_unregister_entity(&sd->entity);
	/* clears sd->entity.graph_obj.mdev via media_gobj_destroy() */
	/* ... */
	video_unregister_device(sd->devnode);	/* the node goes away last */

Because the device node is removed last, there is a window in which
/dev/v4l-subdevN can still be opened while both sd->v4l2_dev and
sd->entity.graph_obj.mdev are already NULL. subdev_open() then runs

	if (sd->v4l2_dev->mdev && sd->entity.graph_obj.mdev->dev)

on those pointers and oopses.

No capture has to be running for this, and it is not specific to IPU6:
any sub-device with a device node can hit it on any removal path --
unbind, module unload, or a hot-removed PCI/USB device. Unbinding during
streaming is simply how I ran into it.

If you would rather have it resent on its own, with a cover letter that
doesn't mention unbinding a streaming sensor, I'm happy to do that
instead.

Thanks for taking the time to look.

-- 
Nicola Fiorillo