05-18-2020, 12:00 AM
(05-16-2020, 10:34 PM)Der Geist der Maschine Wrote: Sysrqs are triggered over procfs or keyboard combinations. [..]
How have you traced invocations of sysrqs?
SYSRQs can also be issued over the UART. And the UART on the PBP is also being used as the headphone jack for audio.
There is significant (~3Vpp) Crosstalk between the audio and UART lines. This is a hardware issue.
Here's a video of my PBP playing audio at full volume (and already distorting a little bit):
https://www.youtube.com/watch?v=MPKHM1J_Uxk
This causes erroneous inputs on the UART /dev/ttyS2. Normally ttyS2 has a getty and the kernel will respond to SYSRQs on this terminal.
These inputs will cause all sorts of behaviour, including hangs, crashes and stuttering because the kernel will stop everything it does to parse the SYSRQs.
If you want to reproduce this, set everything in alsamixer and pavucontrol to max volume, play a loud rock song and make sure to stop serial-getty@ttyS2 (or the equivalent for your OS, otherwise the getty will fetch all your tty input before your cat can).
