This is a minimal baremetal code to test system timer interrupt during a main loop on raspberry Pi4 and aarch64.

Many bare metal tutorials and code examples are:

• to complex to understand (many 32/64bit and Pi3/4 switches)
• do polling instead of interrupt for the timer
• uses Exceptions in a kind of bluescreen without saving registers
• did not use System Timer; they use the clockrate depending timer of a core
• did not explain or share a minimal example for GIC-400 (gicv2) on the Pi4

This minimum example has/is:

• Pi4 only
• AArch64 only
• minimum assembler code
• many in code comments
• minimum code tweaks and files

What this code do:

• hint: some firmware armstub8 code init System Timer to 1MHz before switching from EL3 to EL2
• startup.S use only core0 (subcores are idle)
• set 2 stacks: 0x80000 for c code, 0x40000 for exception asm/c code
• switch from EL2 to EL1 with an valid exception vector for IRQ
• kernel.c mask interrupts
• init the GIC-400 to forward all interrupts to core0 with prio A0
• init the GIC-400 core0 to react on prio lower than F0 (00 is the highest prio)
• say GIC-400 to react on SPI 99 (= system timer comparator 3)
• set comparator3 on system timer to an initial value
• say system timer to make IRQ if comparator3 is reached
• clear interrupt mask
• init uart and send "boo"
• no framebuffer output, just an output on UART 115200 on GPIO Pins 4 and 5
• NOT aux mini uart, not GPIO Pin 14,15
• running a loop and send sometimes "m" for signaling "main loop"
• if interrupt occours, a "i" is sended
• if interrupt is a timer interrupt a "t" is sended and comparator gets an new value

The code maybe not compatible with qemu (April 2020) or AEMv8-A Base Fixed Virtual Platform.

Take a look into Makefile and get/set your noeabi gcc crosscompile toolchain. Then cross your fingers and do make.

Take a look into config.txt for additional files you need.

Look into the someGraphics and someGraphics_pi1 Branch. There is a cool digital graphical clock and a good comparison between Pi1 and Pi4 interrupt handling.

This code is the best minimal mix from serveral different open source projects:

• minimal uart from raspbootin
• register save and restore from aarch64 disasm mini debug
• some inital boot code from raspi3-tutorial
• the gic-400 stuff and system timer from circle library
• some pi4 specific from Raspberry Pi4 osdev
• some other minimalistic tricks and UART3 from David Welch pi3 baremetal tutorial
• some hints about the system timer and exceptions from linux os explain on the Pi