DEBUG_BCM2835/DEBUG_BCM2836 sets the CONFIG_DEBUG_UART_VIRT and CONFIG_DEBUG_UART_PHYS, to the right one for that model

CONFIG_DEBUG_UART_VIRT / CONFIG_DEBUG_UART_PHYS can be manually set to any MMIO addr you want

CONFIG_SERIAL_AMBA_PL011 is required for ttyAMA0

CONFIG_DEBUG_UART_VIRT=0xf0201000 CONFIG_DEBUG_UART_PHYS=0x3f201000 is one example of a valid config

ret = fdt_open_into(v_fdt, v_fdt, 16 * 1024 * 1024);
patching the DTB to be 16mb in size, causes linux to crash during DTB parsing, so it can never print anything
expanding it to only 1mb in size is fine however


sched_clock: 56 bits at 0 Hz, resolution 0ns, wraps every 0ns
Division by zero in kernel.
pi2 and pi3 need the freq of the arm generic timer setup like:
__asm__ __volatile__ ("mcr p15, 0, %0, c14, c0, 0": :"r"(19200000));
https://github.com/raspberrypi/tools/blob/509f504e8f130ead31b85603016d94d5c854c10c/armstubs/armstub7.SL130-L135 is where i found that answer


(refer to BCM2835-ARM-Peripherals.pdf and https://github.com/hermanhermitage/videocoreiv/wiki/VideoCore-IV-Programmers-Manual#interrupts )
the hardware has a main array of 64 interrupt signals, divided into 2 banks
it also has an array of 8 arm peripheral interrupts

the 8 arm peripheral interrupts, 2 irq range flags (0-31, 32-63), and some select irq (7, 9, 10, 18, 19, 53, 54, 55, 56, 57, 62) are all exposed in a "basic pending" register
there are then 2 registers, with 1 bit each, to show the status of 0-31, and 32-63

linux maps the 8 arm-peripheral interrupts onto the 16-23 range
then it maps hw irq 0-63, onto the 24-87 range

irq 88 is involved in bcm2836_map() and arm local peripherals

that whole set of 64+8 is then duplicated, in FIQ mode
  arm peripherals again at 89-96
  hw 0-31 at 97-128
  hw 32-63 at 129-160

CONFIG_FRAMEBUFFER_CONSOLE renders the console text on a framebuffer

CONFIG_FB_SIMPLE enables adding a simple-framebuffer to the DTB
CONFIG_DRM_SIMPLEDRM doesnt work for unknown reasons
CONFIG_I2C_MUX_PINCTRL   allows muxing the i2c controllers
CONFIG_VIDEO_BCM2835_UNICAM  the CSI driver for the rpi
CONFIG_VIDEO_OV5647          the original rpi camera sensor


problem:
Run /nix/store/9c3jx4prcwabhps473p44vl2c4x9rxhm-nixos-system-nixos-20.09pre-git/init as init process
potentially unexpected fatal signal 4.
c2d ~ # kill -l
 1) SIGHUP       2) SIGINT       3) SIGQUIT      4) SIGILL       5) SIGTRAP

cause:
  the FPU wasnt enabled in the bootloader, and printf tried to use it
solution:
  __asm__ __volatile__ ("mcr p15, 0, %0, c1, c1, 2": :"r"(0x63ff));
  (on LK, its also trapped, use arm_fpu_set_enable(true); to re-enable)

problem:
  Warning: unable to open an initial console.
  (only visible with earlyprintk)
cause:
  /dev/console doesnt exist
solution:
  CONFIG_DEVTMPFS

problem:
  systemd[1]: Failed to mount tmpfs at /dev/shm: Invalid argument
solution:
  CONFIG_TMPFS

problem:
  systemd[1]: Failed to mount cgroup at /sys/fs/cgroup/systemd: No such file or directory
solution:
  CONFIG_CGROUPS

CONFIG_DRM_VC4 pi0-pi3 2d/3d drivers
CONFIG_DRM_V3D pi4 2d/3d drivers

problem:
  [   34.341449] usb 1-1: device descriptor read/64, error -110
cause:
  the usb PHY is not configured correctly
solution:
  https://github.com/librerpi/rpi-open-firmware/blob/61a62a148b07c981962fc177fdb7ebdd68e914bd/firmware/drivers/BCM2708UsbPhy.cc#L74-L106