Repartitioning a flash chip / Resizing Bootloader

For Developers
1

I own a Xiaomi Mi router 4a (100M, International). I want to put on it a custom build of the U-Boot bootloader. So far I figured out how to build U-Boot and added it to the Openwrt build process.

The original bootloader seems to be some a modified version of U-Boot 1.1.3 (Indicated by the string I get on the serial console while booting). The version I compiled is v2024.10 (this is the version currently supported by most devices with U-Boot support in the Openwrt project).

The problem arises when I compare binary sizes. The original bootloader fits inside a 128KiB partition, while my version seems to take up ~300KiB. I tried to minimize the binary size with no luck (I tried disabling drivers and software I don't need in the U-Boot menuconfig).

UPDATE: I disabled absolutely everythig I found and U-Boot still takes up ~200KiB. This indicates, that I must change partition sizes in order to get the new bootloader onto my device.

The next idea I had was to resize the boot partition, since I have enough free space on the rootfs_data.

My questions are:

  • Is there a simple configuration for U-Boot that builds a minimal image? (EDIT: pointless question, new versions are just larger)
  • What are the steps required to resize the boot partition? (This means that I would need to move partitions following the boot partition)
  • What partitions get remade when installing a new version of OpenWrt?

P.S.: I am using a SPI NOR Flash chip

Some information on the partitions availible:

# cat /proc/mtd 
dev:    size   erasesize  name
mtd0: 00020000 00010000 "bootloader"
mtd1: 00010000 00010000 "config"
mtd2: 00010000 00010000 "factory"
mtd3: 00010000 00010000 "crash"
mtd4: 00010000 00010000 "cfg_bak"
mtd5: 00200000 00010000 "overlay"
mtd6: 00da0000 00010000 "firmware"
mtd7: 00231eb4 00010000 "kernel"
mtd8: 00b6e14c 00010000 "rootfs"
mtd9: 004d0000 00010000 "rootfs_data"

device-tree describing partitions:

&spi0 {
	status = "okay";

	flash0: flash@0 {
		compatible = "jedec,spi-nor";
		reg = <0>;
		spi-max-frequency = <10000000>;

		partitions: partitions {
			compatible = "fixed-partitions";
			#address-cells = <1>;
			#size-cells = <1>;

			partition@0 {
				label = "bootloader";
				reg = <0x0 0x20000>;
				read-only;
			};

			partition@20000 {
				label = "config";
				reg = <0x20000 0x10000>;
				read-only;
			};

			partition@30000 {
				label = "factory";
				reg = <0x30000 0x10000>;
				read-only;

				nvmem-layout {
					compatible = "fixed-layout";
					#address-cells = <1>;
					#size-cells = <1>;

					eeprom_factory_0: eeprom@0 {
						reg = <0x0 0x400>;
					};

					eeprom_factory_8000: eeprom@8000 {
						reg = <0x8000 0x200>;
					};

					macaddr_factory_4: macaddr@4 {
						compatible = "mac-base";
						reg = <0x4 0x6>;
						#nvmem-cell-cells = <1>;
					};

					macaddr_factory_28: macaddr@28 {
						reg = <0x28 0x6>;
					};
				};
			};

			partition@40000 {
				label = "crash";
				reg = <0x40000 0x10000>;
				read-only;
			};

			partition@50000 {
				label = "cfg_bak";
				reg = <0x50000 0x10000>;
				read-only;
			};

			partition@60000 {
				label = "overlay";
				reg = <0x60000 0x100000>;
				read-only;
			};

			partition@160000 {
				label = "firmware";
				reg = <0x160000 0xea0000>;
				compatible = "denx,uimage";
			};
		};
	};
};
2

You are pretty much on your own, just keep them aligned to 64kB eraseblock boundary.

3

If You could go below 192 KiB then not much would be required to change, or even nothing, if the U-Boot environment would be compiled in. Then writing simply to first 2 partitions would suffice. One caveat is You'll need to change (or delete) where uboot-tools package look for U-Boot environment.
Btw. there seem to be a lot of space wasted by crash, cfg_bak and overlay partitions, You could repurpose them if they are not used.

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