Our SoCs have a very specific boot process. First it executes a tiny on chip rom (BROM) which then checks the buttons for FEL mode and then starts checking the various storage options for a valid boot signature at the right location.
NAND and SD-card
There is no real difference between NAND and an SD-card apart from the fact that directly attached flash use the Sunxi NAND controller directly while SD-Cards come with a standard interface and an embedded controller. The sunxi nand controller is harder to implement than the sunxi sd-card controller, and the sample code provided by allwinner is rather large (and shared between U-boot and the kernel).
- "Bootable SD card" article contains more informations about SD card boot process and explains how to make a bootable SD card.
- "NAND" article contains more informations about NAND boot process
- "Installing to NAND" article contains informations to make a bootable NAND.
That's also possible to boot system on other devices (SATA, USB, network using TFTP/NFS,...) but U-boot standing on NAND or SD card is needed as first boot step.
Determining boot source
On boards that have a choice of different boot media, it is sometimes useful to determine which device the bootloader was started from. In U-Boot, this can be done by inspecting the byte at address 0x28 (tested on S3):
=> md.b 0x28 1 00000028: 03
In this case, the value 03 indicates that we booted from SPI NOR. The byte reads 0 when booting from SD card.
The kernel can be loaded using TFTP, that is supported by U-boot and system can be booted using NFS. In network boot process, each of this choice are not mandatory for the other one.
- "How_to_boot_the_A10_or_A20_over_the_network" explains how to configure U-boot for TFTP boot and NFS for system network boot process.
In this case, initial Ramdisk is needed.
- Category:Devices with SATA port contains list of devices with SATA port.
Most devices with Allwinner SoC have USB ports. U-boot also support USB Boot but U-boot needs to be built specifically for booting over USB
- FEL/USBBoot explains how to prepare U-boot for USB boot.
A10 Boot overview
While the Allwinner series of SoC's are quite open, there is an unmodifiable ROM called BROM or Boot ROM that is in charge of booting the SoC. The BROM will try to load the SPL from U-Boot, which in turn loads the kernel.
It should be noted, that if using Allwinner bootloaders (especially true when booting from nand storage), the order is slightly different. BROM loads boot0 as its SPL and that chainloads boot1. These all reside in unaccessable (not easily anyway) nand flash, before the partition table. boot1 loads boot.axf from the first fat partition which in turn chainloads (in our case) u-boot and then the kernel. It is in theory possible to directly boot the kernel, or some other OS. Also boot.axf has the ability to display images on the framebuffer.