Hi, during work on a driver I encountered a behaviour I can’t explain and would like to hear the opinion of the experts here. We work with the rasperry PI 3 (in it’s compute module incarnation, but that shouldn’t make a difference for this discussion). It is hooked up to a microcontroller via SPI, and because the uC needs to signal that it is ready to receive data, we strung an IRQ line to a GPIO on the pi. In the driver I register an ISR for that GPIO which signals a wait_queue_head_t with wake_up. The ISR is allocated with IRQF_NO_THREAD because this is on our critical path and should be as fast as possible. The user space thread (our actual realtime thread) invokes an ioctl that will wait_event_interruptible for this event and then initiates the SPI communication. After a few minutes of operation, the system freezes. Without any discernible output, no ooops, no nothing. Just stops. Further investigation revealed that this only happens if we actually wait for the event. Not just by running the ISR. In order to report this problem, I reduced the setup to a simpler one that can be created with a simple RPI3: - allocate one IRQ gpio 12. - allocate a trigger gpio 13. - short circuit the two with a cable. - load the kernel module. - run a test program that triggers and waits in a different thread. After a few seconds, the system freezes. You can find the kernel module here: https://gist.github.com/dir-ableton/7005fa10fd4bdcf65cbc21ab22f5a572 The test program: https://gist.github.com/dir-ableton/6330988cbde504bd6f024f5513571ab6 Kernel version a42048c6eee58b1b8d252e30224b7d065615c3fd from https://github.com/raspberrypi/linux/tree/rpi-4.14.y-rt which is a 4.14.71 with PREEMPT_RT patch applied as of two days ago. I’ve also run this test with the corresponding vanilla kernel 12d78096b1669a08d440f7ebaddf5d925e52fe79 https://github.com/raspberrypi/linux/tree/rpi-4.14.y This did not freeze after running a full night. I’m probably doing something stupid here, by not disabling IRQs or similar stuff. I took the example for a gpio ISR from a linux driver development book (german, “Linux Treiber Entwickeln”). Maybe the example is not complete? Any hints on what is going on here, or how to debug the problem further are much appreciated. Cheers, Diez B. Roggisch
