TouchBar appletbdrm the module reload functionality is broken, not the initial loading!

[xing0r]

TouchBar appletbdrm driver fails to bind on Linux 6.12.31+ - DRM API compatibility issue

Preliminary Note

I attempted to resolve this issue with assistance from Claude AI, including trying various workarounds and patches. When those attempts were unsuccessful (and in one case caused boot issues), Claude AI helped me compile and structure the following comprehensive bug report based on our debugging session and gathered system information.

System Information

Hardware:

• MacBook Pro 16,2 (2020 Intel 13" - 4 Thunderbolt Ports)
• Board: Mac-5F9802EFE386AA28 (Hardware revision 1.0)
• TouchBar Hardware:
  • 05ac:8302 - "Touch Bar Display" (Primary issue) - bcdDevice 2.01
  • 05ac:8102 - "Touch Bar Backlight" (Working) - bcdDevice 2.01
  • USB Version: 2.00
  • Manufacturer: Apple Inc.
• T2 Security Chip devices (all USB 2.00, bcdDevice 2.01):
  • 05ac:8233 - Apple T2 Controller
  • 05ac:8514 - FaceTime HD Camera (Built-in)
  • 05ac:8262 - Ambient Light Sensor
  • 05ac:8103 - Headset (Audio)
  • 05ac:027e - Apple Internal Keyboard/Trackpad
• Architecture: Intel x86-64 (Late 2020 generation)

Software:

• Distribution: Gentoo Linux
• Working Kernel: 6.12.21-gentoo-t2
• Broken Kernels:
  • 6.12.31-gentoo-t2
  • 6.15.3-vanilla-sources (same issue persists)
• appletbdrm module: Loads successfully but fails to bind
• tiny-dfr version: Latest

Problem Description

The appletbdrm kernel module loads successfully but fails to bind to the TouchBar USB device on Linux kernel 6.12.31, while it works perfectly on 6.12.21. This appears to be related to DRM subsystem API changes introduced between these kernel versions.

Symptoms

1. Module loads successfully:

   $ modinfo appletbdrm
   filename:       /lib/modules/6.12.31-gentoo-t2/kernel/drivers/gpu/drm/tiny/appletbdrm.ko
   license:        GPL
   description:    Apple Touch Bar DRM Driver
   author:         Kerem Karabay <kekrby@gmail.com>
   alias:          usb:v05ACp8302d*dc*dsc*dp*ic10isc*ip*in*

2. TouchBar hardware detected but bound to generic USB driver:

   $ ls /sys/bus/usb/devices/*/idVendor | xargs grep -l 05ac
   Apple Device: /sys/bus/usb/devices/5-6
   Vendor: 05ac
   Product: 8302
   Driver: ../../../../../../../../../bus/usb/drivers/usb  # Should be appletbdrm

3. No TouchBar DRM device created:

   $ ls -la /dev/dri/
   # Only shows Intel i915 card0, no TouchBar DRM device
   crw-rw----+  1 root video  226,   0 card0
   crw-rw-rw-   1 root render 226, 128 renderD128

4. tiny-dfr fails to find TouchBar:

   $ tiny-dfr --help
   Searching for cards in /dev/dri/
   Found card: "/dev/dri/card0"
   Trying to open card: "/dev/dri/card0"
   Failed to open card: "/dev/dri/card0" - Error: unknown system error: EBUSY: Device or resource busy
   thread 'main' panicked at src/main.rs:381:43:
   called `Result::unwrap()` on an `Err` value: No touchbar device found

Reproduction Steps

1. Boot Linux kernel 6.12.31-gentoo-t2
2. Verify appletbdrm module is loaded: lsmod | grep appletbdrm
3. Check TouchBar USB device: Find USB device 05ac:8302
4. Attempt manual binding:

   echo "5-6" > /sys/bus/usb/drivers/usb/unbind
   echo "05ac 8302" > /sys/bus/usb/drivers/appletbdrm/new_id
   echo "5-6" > /sys/bus/usb/drivers/appletbdrm/bind

   Result: bash: echo: write error: No such device

Detailed Logs

Detailed Error Logs

appletbdrm module loading and probe failure (Linux 6.12.31+):

$ modprobe appletbdrm
$ echo $?
0  # Module loads successfully

$ dmesg | tail -5
[ 1512.718129] bce-vhci: Not implemented: wait for pending output requests
[ 1512.719742] appletbdrm 5-6:2.1: [drm] *ERROR* Failed to send message (-110)
[ 1512.719750] appletbdrm 5-6:2.1: [drm] *ERROR* Failed to get display information
[ 1512.719755] appletbdrm 5-6:2.1: probe with driver appletbdrm failed with error -110
[ 1512.719844] usbcore: registered new interface driver appletbdrm

Error Analysis:

• Error -110: ETIMEDOUT - USB communication timeout with TouchBar device
• Root cause: appletbdrm_get_information() function fails during initial TouchBar communication
• Probe failure: Driver binding succeeds but device initialization fails at USB protocol level
• bce-vhci warning: T2 controller shows "wait for pending output requests" - possible timing issue

Manual binding attempt:

$ echo "5-6" > /sys/bus/usb/drivers/usb/unbind          # Success
$ echo "05ac 8302" > /sys/bus/usb/drivers/appletbdrm/new_id  # Success  
$ echo "5-6" > /sys/bus/usb/drivers/appletbdrm/bind
bash: echo: Schreibfehler: Kein passendes Gerät gefunden.

tiny-dfr error output:

$ RUST_BACKTRACE=1 tiny-dfr --verbose
Searching for cards in /dev/dri/
Found card: "/dev/dri/by-path"
Found card: "/dev/dri/card0"
Trying to open card: "/dev/dri/card0"
Failed to open card: "/dev/dri/card0" - Error: unknown system error: EBUSY: Device or resource busy
Found card: "/dev/dri/renderD128"
thread 'main' panicked at src/main.rs:381:43:
called `Result::unwrap()` on an `Err` value: No touchbar device found
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
stack backtrace:
   0: rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::result::unwrap_failed
   3: tiny_dfr::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

dmesg output (no appletbdrm-specific errors shown):

• No error messages from appletbdrm during binding attempt
• No DRM device registration messages
• Only standard Intel i915 DRM operations visible

USB device information:

$ cat /sys/bus/usb/devices/5-6/uevent
MAJOR=189
MINOR=518
DEVNAME=bus/usb/005/007
DEVTYPE=usb_device
DRIVER=usb
PRODUCT=5ac/8302/201
TYPE=0/0/0
BUSNUM=005
DEVNUM=007

Root Cause Analysis

This issue appears to be identical to the known drm_open_helper crashes affecting multiple DRM drivers (NVIDIA, DisplayLink EVDI) on Linux 6.12, as documented in:

• [NVIDIA Issue #712](6.12: drm_open_helper RIP NVIDIA/open-gpu-kernel-modules#712)
• [DisplayLink EVDI Issue #495](drm_open_helper crash on linux kernel 6.12-rc4 DisplayLink/evdi#495)
• [Debian Bug #1089513](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1089513)

Suspected cause: DRM subsystem API changes in Linux 6.12 break out-of-tree DRM drivers, particularly around:

• drm_open_helper() function changes
• DRM file operations structure modifications
• Device registration/initialization sequence changes

Workaround

Temporary solution: Downgrade to Linux 6.12.21-gentoo-t2 where TouchBar functionality works perfectly.

Additional Testing

Upstream Kernel Testing:

• Linux 6.15.3 vanilla-sources: Tested independently - same issue occurs
• This confirms the problem is not distribution-specific (Gentoo vs upstream)
• Issue persists even in latest kernel versions that should include TouchBar improvements
• Problem affects both gentoo-sources and vanilla-sources trees

Additional Context

• TouchBar detection: ✅ Hardware properly detected via USB
• Module loading: ✅ appletbdrm loads without errors
• USB ID registration: ✅ new_id accepts the device ID
• Device binding: ❌ Fails at probe function level
• DRM registration: ❌ No TouchBar DRM device created

This suggests the issue occurs in the appletbdrm_probe() function, likely during DRM device initialization, due to API compatibility issues with the updated DRM subsystem in 6.12.31.

Expected Behavior

On Linux 6.12.21:

• TouchBar automatically binds to appletbdrm driver
• TouchBar DRM device appears in /dev/dri/
• tiny-dfr successfully controls TouchBar display
• Full TouchBar functionality available

Request

Please investigate DRM API compatibility for the appletbdrm driver with Linux 6.12.31+. This likely requires updating the driver to accommodate the new DRM subsystem API changes, similar to fixes already implemented for other DRM drivers.

Thank you for maintaining the excellent T2-Linux project! 🙏

[sharpenedblade]

Does the basic touchbar driver that you get without tiny-dfr work? 6.15.3 works for other people because some other distros have upgraded to it and nobody has complained about any breakage. Can you try booting into macos and then rebooting to linux and see if it works, the touchbar will break after an unclean shutdown. Also I don't know if loading the drm driver manually works, it should be automatically loaded by the tiny-dfr udev rules.

[xing0r]

Thanks for the quick response! Let me address your points:

Basic TouchBar driver testing:
Yes, I tested 6.15.3 with the native appletbdrm driver, and the result is identical:

[ 1512.719742] appletbdrm 5-6:2.1: [drm] *ERROR* Failed to send message (-110)
[ 1512.719750] appletbdrm 5-6:2.1: [drm] *ERROR* Failed to get display information  
[ 1512.719755] appletbdrm 5-6:2.1: probe with driver appletbdrm failed with error -110
[ 1512.719844] usbcore: registered new interface driver appletbdrm

macOS reboot test:
Unfortunately, I can't test this - I have a Gentoo-only setup (no macOS dual-boot). The MacBook runs pure Linux.

Driver loading:
You're absolutely right - the driver loads automatically via udev rules in normal operation. The manual loading is purely for debugging and capturing these error logs.

Regression timeline:
I can confirm this is a clear regression:

• ✅ 6.12.21-gentoo-t2: TouchBar works perfectly
• ❌ 6.12.31-gentoo-t2: Same -110 (ETIMEDOUT) error occurs consistently
• ❌ 6.15.3 vanilla-sources: Identical failure

(Note: Gentoo sources jumped directly from 6.12.21 to 6.12.31, so I can't pinpoint the exact kernel version where the regression was introduced)

Hardware specificity question:
This might be key - do the people reporting 6.15.3 working also have a MacBook Pro 16,2 (2020 Intel 13")? This model is quite rare, and I'm wondering if this is hardware-revision specific. My TouchBar shows bcdDevice 2.01 which might have different timing requirements that were broken in the 6.12.31+ USB subsystem changes.

Additional question: Which specific kernel versions are these other distros using? Are they running 6.15.3 with distribution-specific patches, or are they using different kernel series altogether? This could help identify whether the issue is related to vanilla kernel vs. distribution-patched kernels, or specific kernel configurations.

The fact that other distros upgraded without complaints makes me suspect this could be MacBook Pro 16,2 specific, or perhaps related to the specific TouchBar hardware revision (bcdDevice 2.01).

[sharpenedblade]

Is that an LLM? Can you respond by yourself, if I wanted an LLM's response I can ask it myself.

Yes, I tested 6.15.3 with the native appletbdrm driver, and the result is identical:

By basic touchbar I meant the one you get without tiny-dfr. This will not load appletbdrm at all, so I think you might not have actually removed tiny-dfr when you tried this. Can you completely uninstall tiny-dfr, reboot, and check if there are the fn or media keys on your touchbar.

Unfortunately, I can't test this - I have a Gentoo-only setup (no macOS dual-boot). The MacBook runs pure Linux.

You can boot into online recovery if you don't have macos installed.

This might be key - do the people reporting 6.15.3 working also have a MacBook Pro 16,2 (2020 Intel 13")? This model is quite rare, and I'm wondering if this is hardware-revision specific. My TouchBar shows bcdDevice 2.01 which might have different timing requirements that were broken in the 6.12.31+ USB subsystem changes.

This has not been an issue in the past, and is very unlikely to be an issue now. If something breaks it will break for everyone. And there have been a lot of kernel versions since 6.12 on other distros, none of them had any touchbar regressions (that weren't immediately fixed).

There is a known bug where an unclean reboot breaks the t2 which causes the touchbar to stop working, the only known fix is to reboot to macos. Its possible that after you tested 6.12, there was a kernel panic or you force rebooted, which would make 6.15 appear broken. Can you test 6.12 again and check if it is working now, without any changes other than a single reboot to the new kernel.

[xing0r]

Without tiny-dfr the touchbar also remains black with 6.15.3.

grep -i apple /usr/src/linux-6.15.3/.config
CONFIG_APPLE_PROPERTIES=y
CONFIG_KEYBOARD_APPLESPI=m
CONFIG_MOUSE_APPLETOUCH=m
CONFIG_SENSORS_APPLESMC=m
CONFIG_DRM_APPLETBDRM=m
CONFIG_BACKLIGHT_APPLE=m
CONFIG_HID_APPLE=m
CONFIG_HID_APPLEIR=m
CONFIG_HID_APPLETB_BL=m
CONFIG_HID_APPLETB_KBD=m
CONFIG_USB_APPLEDISPLAY=m
CONFIG_APPLE_MFI_FASTCHARGE=m
CONFIG_APPLE_BCE=m
CONFIG_APPLE_GMUX=m

I have not applied patches 1013 - 1017 and 1010 - 1011 to kernel 6.15.3. I have applied all the remaining patches. Even after booting the recovery console it does not work. I have current kernels 6.6.74, 6.12.21, 6.12.31 and 6.15.3 on my system.
The touchbar works with 6.6.74 and 6.12.21.
6.12.31 and 6.15.3 no longer work.

restart in recovery console then boot with 6.15.3 without tiny-dfr - touchbar black no function

reboot to 6.12.31 with tiny-dfr - touchbar black no function

reboot to 6.12.21 with tiny-dfr - touchbar is on see F-keys

[sharpenedblade]

Its possible that this could be a gentoo bug, but its hard to know exactly when the bug was introduced since bisecting the full kernel will take a long time. Can you test kernel 6.15.3 with all of the patches on the main branch, we only test our patches as a whole so removing some will break things.

[AdityaGarg8]

On Ubuntu the latest 6.15.3 kernel is working properly with tiny-dfr. So definitely not an API breakage. Besides, the driver was upstreamed taking the latest DRM API in mind.

[AdityaGarg8]

1010 - 1011 Patches are needed for touchbar work in drm mode. Who is asking you to not apply them?

[AdityaGarg8]

You are using 6.12 kernel, so get patches from here:

https://github.com/t2linux/linux-t2-patches/tree/6.12

[xing0r]

I have rebuilt the 6.15.3 kernel several times. Once with all patches. Once without 1013-1017 because I assumed that it belonged to appletbdrm and I didn't want to overwrite the native driver. In the end then without 1010 - 1011 and without 1013 - 1017 because I thought I had found these 2 options natively in the kernel, but with all attempts always came:

[ 1512.719742] appletbdrm 5-6:2.1: [drm] ERROR Failed to send message (-110)
[ 1512.719750] appletbdrm 5-6:2.1: [drm] ERROR Failed to get display information
[ 1512.719755] appletbdrm 5-6:2.1: probe with driver appletbdrm failed with error -110
[ 1512.719844] usbcore: registered new interface driver appletbdrm

I have patched the 6.12.31 kernel with all patches from the 6.12 brunch as well as the 6.12.21 which works.

I checked out the 6.15.3 patches like this:
git clone https://github.com/t2linux/linux-t2-patches.git

I checked out the 6.12 patches like this:
git clone -b 6.12 https://github.com/t2linux/linux-t2-patches.git

[AdityaGarg8]

The native driver was upstreamed by us so you can trust the patches right?

[xing0r]

I have now reloaded kernel 6.15.3, repatched it with all patches, and coincidentally disabled one of my scripts. This script used to unload and reload appletbdrm, hid_appletb_kbd, hid_appletb_bl, and apple_bce during boot.

I had been working on a workaround for suspend back then, since unloading often went wrong during suspend. After that, I always had to boot twice until everything worked again, which is why I had this script during boot that unloaded and reloaded the modules - then everything worked directly again after a crash.
When I disable this script, I can now boot 6.15.3 and the TouchBar runs as usual with tiny-dfr. So either something was changed in the patches since kernel 6.12.21, or in the kernel itself, because I can no longer unload and reload appletbdrm - that always worked before.
This also means that even though the TouchBar works again, when I use suspend, I have to unload appletbdrm, hid_appletb_bl, hid_appletb_bl before suspend and reload them after suspend as described in the wiki - that was the only way I could get the TouchBar back after suspend. But this no longer works. When I unload the drivers and then reload them, I get this already mentioned error:

[ 1790.336574] appletbdrm 5-6:2.1: [drm] ERROR Failed to send message (-110)
[ 1790.336582] appletbdrm 5-6:2.1: [drm] ERROR Failed to get display information
[ 1790.336588] appletbdrm 5-6:2.1: probe with driver appletbdrm failed with error -110
[ 1790.336685] usbcore: registered new interface driver appletbdrm

best regards

[xing0r]

Even after a restart, the error persists. I really have to start the recovery console every time to get it working again as sharpenedblade described.
But now it's like this: When I have booted 6.15.3 and unloaded appletbdrm, then I get this error:

[ 1790.336574] appletbdrm 5-6:2.1: [drm] ERROR Failed to send message (-110)
[ 1790.336582] appletbdrm 5-6:2.1: [drm] ERROR Failed to get display information
[ 1790.336588] appletbdrm 5-6:2.1: probe with driver appletbdrm failed with error -110
[ 1790.336685] usbcore: registered new interface driver appletbdrm

Even after a reboot, the error persists. When I now reactivate my script during boot that unloads and reloads appletbdrm, hid_appletb_kbd, hid_appletb_bl, and apple_bce, and boot kernel 6.12.21, the TouchBar works immediately after booting without problems. Now I can remove the script again and load 6.15.3 and the TouchBar works again too.

I compiled 6.12.21 on May 19th and for that I applied the 6.12 patches from the state of May 19th. Either something was changed in the patches or in the kernel that leads to this behavior where the modules can no longer be unloaded or reloaded.

best regards

[AdityaGarg8]

Having that script at boot time is a very very bad idea. Force unloading Apple bce should be done only when needed, and not unnecessarily.

You can git checkout 70c5ad133fffa21d052317d3da0c7170221c9c67 on 6.12 patches if you want to test the patches before May 19.

[AdityaGarg8]

You should check for differences in you kernel config between the working and non working versions.