So I picked up a couple of Altera FPGA SoC boards a few weeks ago and immediately pulled up all of the documentation that I could find on the Cyclone V SoC. The Cyclone V SoC has an FPGA and a Hard Processor System (HPS) woven into a single processor package. The HPS is a dual core ARM Cortex A9. Building everything from scratch is the best way to figure out how the system works.
The boot sequence on a Cyclone V HPS works like this:
The On-chip ROM (for which source code is not provided) loads the preloader (1st stage bootloader). The preloader then loads U-boot. U-boot then loads the kernel and root file system.
There are two well thought out options for the preloader according to the Cyclone V boot guide. The two options are licensed differently depending on how you build the source code. One is licensed under a BSD license and the other under GPL v2 with U-Boot.
Building a pre-loader image for the DE0-Nano-SoC board was straightforward. Altera provides the bsp-editor utility for customizing the preloader configuration and generating the BSP HPS preloader source code, after which, make is used to build the sources using the Mentor ARM cross toolchain.
The preloader settings directory can be found on the DE0-Nano-SoC CD in the DE0_NANO_SOC_GHRD subdirectory.
The preloader load address can be set via the bsp-editor so that the on chip ROM either loads the preloader from an absolute zero address on the sdcard or from a fat partition with id equal to a2 on the sdcard. These are the options for booting from the sdcard.
After the sources are generated and the preloader image is built using the Makefile, U-boot must be compiled. An Altera port of U-Boot is available on github for the Cyclone V FPGA SoC. U-Boot is built using the Linaro ARM cross toolchain.
There's quite a bit you can do with the Cyclone V FPGA SoC boot configuration. FPGA images can be loaded from U-boot. The jumpers on the board can be configured to boot from the on-board serial flash (QSPI), bare metal applications can be loaded from the preloader, the FPGA can be configured from serial flash, and the list goes on. The HPS SoC Boot Guide for the Cyclone V SoC is a valuable reference and contains all of the boot configuration information.