The Mystery of XF86AudioPlay: Investigating and Resolving Abnormal Audio Key Events on Linux

Repeated Unexplained Errors on the Emacs Status Bar Using a Linux desktop environment over a long period, one can occasionally encounter inexplicable phenomena. A blog post published by developer Michael Prokop meticulously documents his journey in tracking down one such “mysterious bug.” The issue was straightforward: the message <XF86AudioPlay> is undefined kept appearing on the Emacs status bar every 2–3 seconds. XF86AudioPlay is the X11 key symbol name assigned to the media play key on keyboards. This message is displayed by Emacs when it receives a key event but doesn’t have a corresponding key binding defined for it. What made the situation bizarre was that this issue only occurred within Emacs. There were no related entries in the system logs, nor were there any issues with other applications. However, the messages persisted.

Narrowing Down the “Culprit” Using Sway Key Bindings Prokop set out to identify the root cause without rebooting his system. He started by modifying the configuration of his Wayland-based window manager, Sway. He bound the XF86AudioPlay key event to a command that toggles play and pause on a music player: ```

bindsym XF86AudioPlay exec playerctl play-pause ``` Immediately after reloading Sway, the anomaly became apparent. The music player started toggling between play and pause every 2–3 seconds. This confirmed that it wasn’t a bug in Emacs but rather that the XF86AudioPlay key event was indeed being sent to the system at regular intervals.

The USB Keyboard Was Not the Culprit The next suspect was a physical keyboard malfunction. If the key were physically stuck in a pressed state, the same event would repeatedly fire. However, even after disconnecting the USB keyboard from the system, the issue persisted. It was clear that the keyboard was not the source of the problem. So, which device was sending these key events? This is where the libinput-tools came into play.

Identifying the Culprit Device with libinput Prokop ran the libinput debug-events command to monitor input device events in real-time. The output revealed the following: ```

event12 KEYBOARD_KEY +0.000s KEY_PLAYPAUSE (164) pressed event12 KEYBOARD_KEY +0.000s KEY_PLAYPAUSE (164) released event12 KEYBOARD_KEY +2.887s KEY_PLAYPAUSE (164) pressed event12 KEYBOARD_KEY +2.887s KEY_PLAYPAUSE (164) released event12 KEYBOARD_KEY +5.773s KEY_PLAYPAUSE (164) pressed event12 KEYBOARD_KEY +5.774s KEY_PLAYPAUSE (164) released ``` It was confirmed that KEY_PLAYPAUSE events were being sent at intervals of approximately 2.887 seconds from a device labeled event12. Keycode 164 corresponds to KEY_PLAYPAUSE, interpreted by the system as the XF86AudioPlay key symbol.

Exposing the Culprit with udevadm To uncover the identity of event12, Prokop used the udevadm info command. This revealed the following device path: ```

/devices/pci0000:00/0000:00:1f.3/skl_hda_dsp_generic/sound/card0/input17/event12 ``` It turned out to be an input device associated with the Intel audio subsystem on the PCI bus, specifically the sound card labeled skl_hda_dsp_generic. Further details obtained via udevadm info -a identified the device name as “sof-hda-dsp Headphone.” Sound Open Firmware (SOF) is an open-source audio DSP framework developed by Intel. Combined with High Definition Audio (HDA), “sof-hda-dsp” is a common audio driver configuration in modern Intel-based laptops. Additional confirmation via the evtest command also identified event12 as the “sof-hda-dsp Headphone” device.

The Culprit: Intel’s Audio Driver The root cause of the issue was ultimately found to be false KEY_PLAYPAUSE events being sent by Intel’s audio subsystem. The sof-hda-dsp Headphone device was sending media play key events to the system at regular intervals, completely unrelated to actual headphone use. Such issues have been reported before in certain combinations of SOF drivers and specific hardware. While the root cause might involve mechanisms for detecting headphone connection status or power-saving recovery processes in the DSP, Prokop’s article does not delve into these details.

Key Takeaways from the Investigation This case highlights effective methods for investigating inexplicable input events on Linux systems: 1. Use key binding settings in window managers like Sway or i3 to visually or functionally confirm that the problematic key event is occurring. 2. Use the debug-events command in libinput-tools to identify which input device is sending the events. 3. Use udevadm and evtest to pinpoint the specific device responsible. Cases where non-traditional input devices, like built-in audio subsystems, send key events can easily be overlooked. However, as demonstrated here, diligently checking libinput’s event logs can be the key to resolving such issues effectively.

The “Invisible Inputs” of Linux Desktops In Linux desktop environments, various hardware devices are capable of sending input events via the udev and libinput systems. These aren’t limited to keyboards and mice but also include power buttons, sleep buttons, audio controls, and even function keys on laptops. This case highlights an example where one of these input events was being sent periodically without any user intervention. Applications sensitive to key bindings, like Emacs, are more likely to expose such hidden events. Conversely, without the status bar error in Emacs, this issue might have gone unnoticed. This scenario is a reminder for desktop Linux users of the value of understanding tools like libinput and udevadm. These tools can be crucial for troubleshooting and resolving unexpected behavior in the Linux input subsystem.