porting XNU to ARM (OMAP5 CPU)

Two weeks ago I have taken some time to look at the XNU port to the ARM architecture done by the developer who goes by the handle "winocm". Today I've decided to summarize my experience.

Here is a brief checklist if you want to start porting XNU to your board:

• Start from reading Wiki https://github.com/darwin-on-arm/wiki/wiki/Building-a-bootable-system
• Clone the DeviceTrees repository: https://github.com/darwin-on-arm/DeviceTrees . Basically, you can use slightly modified DTS files from Linux, but due to the fact that DTC compiler is unfinished, you'll have to rename and comment out some entries. For example, macros in included C headers are not expanded, so you'll have to hardcode values for stuff like A15 VGIC IRQs
• Get image3maker which is a tool to make images supported both by GenericBooter and Apple's iBoot bootloaders https://github.com/darwin-on-arm/image3maker
• Use the DTC from https://github.com/winocm/dtc-AppleDeviceTree to compile the abovementioned DTS files
• Take a look at init/main.c . You may need to add a hackish entry the way it's done for the "HD2" board to limit the amount of RAM available.
• I have built all the tools on OS X, but then found out that it's easier to use the prebuilt linux chroot image available at: https://github.com/stqism/xnu-chroot-x86_64

The most undocumented step is actually using the image3maker tool and building bootable images. You have to put the to the "images" directory in the GenericBooter-next source. As for the ramdisk, you may find some on github or unpack the iphone firmware, but I simply created an empty file, which is OK since I've not got that far in booting.

Building GenericBooter-next is straightforward, but you need to export the path to the toolchain, and possibly edit the Makefile to point to the correct CROSS_COMPILE prefix

rm images/Mach.*
../image3maker/image3maker  -f ../xnu/BUILD/obj/DEBUG_ARM_OMAP5432_UEVM/mach_kernel -t krnl -o images/Mach.img3
make clean
make

For the ramdisk, you should use the "rdsk" type, and "dtre" for the Device Tree (apparently there's also xmdt for xml device tree, but I did not try that);

Booting on the omap5 via fastboot (without u-boot):
./usbboot -f &
fastboot -c "-no-cache serial=0 -v -x cpus=1 maxmem=256 mem=256M console=ttyO2" -b 0x83ff8040 boot vmlinux.raw

Some notes about the current organization of the XNU kernel and what can be improved:

• All makefiles containing board identifiers should be sorted alphabetically, just for convenience when adding newer boards.
• Look into memory size limit. I had to limit the RAM to 256Mb in order to boot. If one enables the whole 2Gb available on the OMAP5432 uEVM board, the kernel fails to "steal pages" during the initialization (as can be seen in the screenshot).
• I have encountered an issue 

OMAP5 is an ARM Cortex-A15 core. Currently XNU port only has support for the A9 CPU, but if we're booting without SMP and L2 cache, the differences between these architectures are not a big problem. OMAP5 has a generic ARM GIC (Global Interrupt Controller), which is actually compatible to the GIC in the MSM8xxx CPUs, namely with APQ8060 in HP TouchPad, the support for which was added by winocm. UART is a generic 16550, compatible to the one in OMAP3 chip. Given all this, I have managed to get the kernel basically booting and printing some messages to the UART.

Unfortunately, I have not managed to bring up the timer yet. The reason is that I was booting the kernel via USB directly after OMAP5 internal loader, skipping u-boot and hardware initialization. Somehow the internal eMMC chip in my board does not allow me to overwrite the u-boot partition (although I did manage to do it once when I received the board)

I plan to look into it once again, now with hardware pre-initialized by u-boot, and write a detailed post. Looks like the TODO is the following:

• Mirror linux rootfs (relying on 3rd party github repos is dangerous)
• Bringing up OMAP5 Timer
• Building my own RAMDisk
• Enabling eMMC support
• Playing with IOKit