Add minimal required CONFIG options

[saloniamatteo]

Greetings, after tinkering with Kernel options, since v4l2loopback was unable to build, I finally came up with a minimal list of required CONFIG options:

CONFIG_USB_VIDEO_CLASS=m
CONFIG_USB_VIDEO_CLASS_INPUT_EVDEV=y
CONFIG_V4L2_FWNODE=m
CONFIG_V4L_MEM2MEM_DRIVERS=y
CONFIG_V4L_PLATFORM_DRIVERS=y
CONFIG_V4L_TEST_DRIVERS=y
CONFIG_VIDEO=y
CONFIG_VIDEOBUF2_CORE=m
CONFIG_VIDEOBUF2_MEMOPS=m
CONFIG_VIDEOBUF2_V4L2=m
CONFIG_VIDEOBUF2_VMALLOC=m
CONFIG_VIDEO_DEV=m
CONFIG_VIDEO_MEM2MEM_DEINTERLACE=m
CONFIG_VIDEO_MUX=m
CONFIG_VIDEO_V4L2_I2C=y

(Perhaps even this list can be made smaller)

Please check if you have CONFIG_TRIM_UNUSED_KSYMS enabled: if it is, it WILL NOT allow you to compile v4l2loopback, unless you manually set the whitelist. Some (if not all) of the functions needed by v4l2loopback are:

__video_register_device()
ktime_get_ts64()
v4l2_ctrl_handler_free()
v4l2_ctrl_handler_init_class()
v4l2_ctrl_new_custom()
v4l2_ctrl_subscribe_event()
v4l2_device_register()
v4l2_event_queue()
v4l2_fill_pixfmt()
v4l2_fill_pixfmt_mp()
video_devdata()
video_device_alloc()
video_device_release()
video_ioctl2()
video_unregister_device()
vm_insert_page()

[umlaeute]

(Perhaps even this list can be made smaller)

on my system (linux-6.11.4, Debian kernel package), CONFIG_VIDEO_MEM2MEM_DEINTERLACE is not set, and CONFIG_VIDEO_MUX is not even present in the configuration (suggesting that there's no such option).

I don't think any of CONFIG_VIDEO_V4L2_I2C, CONFIG_USB_VIDEO_CLASS and CONFIG_USB_VIDEO_CLASS_INPUT_EVDEV are actually necessary (they seem to be hardware-specific), but haven't tested building a kernel without them.

[saloniamatteo]

USB_VIDEO_CLASS refers to the uvcvideo module, aka the webcam. EVDEV is not needed either, I guess. Same goes for VIDEO_V4L2_I2C.

I haven't tested VIDEO_MEM2MEM_DEINTERLACE nor VIDEO_MUX, so I'll have to when I have some free time.

Therefore, it is safe to say the rest of the config options are indeed needed, right?

[umlaeute]

btw, thanks for checking this out.

i think the best place would be make this a Q&A in the discussion section (and it probably should mention the kernel version this was tested against).

and yes, the rest of the config options look pretty sane to me :-)

[saloniamatteo]

No problem.

Opened discussion #604, closing this issue.

[umlaeute]

thanks. you probably should add your actual findings in a reply in #604 (so it can be marked as answer). I could do that, but then the attribution is wrong.

[saloniamatteo]

@umlaeute thanks, I'm now adding the replies and will continue to do further testing.