Originally Posted by Zem
I don't understand completely what these commands do, so I followed your instructions literally; mydump.txt does not contain the string "playing with the backlight".
I change a bit the procedure, could you try again (I edited my previous post)? The commands enable a tracer that logs any read or write to registers to the /sys/kernel/debug/tracing/trace_pipe file. But modules need to be reloaded after enabling the tracing mechanism. Writing to the /sys/kernel/debug/tracing/trace_marker file permits to add command to the trace file.