7.1. The barebox Porter’s Guide

While barebox puts much emphasis on portability, running on bare-metal means that there is always machine-specific glue that needs to be provided. This guide shows places where the glue needs to be applied and how to go about porting barebox to new hardware.

Note

This guide is written with mainly ARM and RISC-V barebox in mind. Other architectures may differ.

7.1.1. Introduction

Your usual barebox binary consists of two parts. A prebootloader doing the bare minimum initialization and then the proper barebox binary.

7.1.1.1. barebox proper

This is the main part of barebox and, like a multi-platform Linux kernel, is platform-agnostic: The program starts, registers its drivers and tries to match the drivers with the devices it discovers at runtime. It initializes file systems and common management facilities and finally starts an init process. barebox knows no privilege separation and the init process is built into barebox. The default init is the Hush shell, but can be overridden if required.

For such a platform-agnostic program to work, it must receive external input about what kind of devices are available: For example, is there a timer? At what address and how often does it tick? For most barebox architectures this hardware description is provided in the form of a flattened device tree (FDT). As part of barebox’ initialization procedure, it unflattens (parses) the device tree and starts probing (matching) the devices described within with the drivers that are being registered.

The device tree can also describe the RAM available in the system. As walking the device tree itself consumes RAM, barebox proper needs to be passed information about an initial memory region for use as stack and for dynamic allocations. When barebox has probed the memory banks, the whole memory will become available.

As result of this design, the same barebox proper binary can be reused for many different boards. Unlike Linux, which can expect a bootloader to pass it the device tree, barebox is the bootloader. For this reason, barebox proper is prefixed with what is called a prebootloader (PBL). The PBL handles the low-level details that need to happen before invoking barebox proper.

7.1.1.2. Prebootloader (PBL)

The prebootloader is a small chunk of code whose objective is to prepare the environment for barebox proper to execute. This means:

  • Setting up a stack

  • Determining a memory region for initial allocations

  • Provide the device tree

  • Jump to barebox proper

The prebootloader often runs from a constrained medium like a small (tens of KiB) on-chip SRAM or sometimes even directly from flash.

If the size constraints allow, the PBL will contain the barebox proper binary in compressed form. After ensuring any external DRAM can be addressed, it will unpack barebox proper there and call it with the necessary arguments: an initial memory region and the FDT.

If this is not feasible, the PBL will contain drivers to chain load barebox proper from the storage medium. As this is usually the same storage medium the PBL itself was loaded from, shortcuts can often be taken: e.g. a SD-Card could already be in the correct mode, so the PBL driver can just read the blocks without having to reinitialize the SD-card.

7.1.1.3. barebox images

In a typical build, the barebox build process generates multiple images (Multi Image Support). All enabled PBLs are each linked with the same barebox proper binary and then the resulting images are processed to be in the format expected by the loader.

The loader is often a BootROM, but maybe another first stage bootloader or a hardware debugger.

Let us now put these new concepts into practice. We will start by adding a new board for a platform, for which similar boards already exist. Then we’ll look at adding a new SoC, then a new SoC family and finally a new architecture.

7.1.2. Porting to a new board

Note

Parts of this guide are taken from this ELC-E 2020 talk: https://www.youtube.com/watch?v=Oj7lKbFtyM0

Chances are there’s already a supported board similar to yours, e.g. an evaluation kit from the vendor. Take a look at arch/$ARCH/boards/ and do likewise for you board. The main steps would be:

7.1.2.1. Entry point

The PBL’s entry point is the first of your code that’s run. What happens there depends on the previously running code. If a previous stage has already initialized the DRAM, the only thing you need to do is to set up a stack and call the common PBL code with a memory region and your device tree blob:

#include <asm/barebox-arm.h>
#include <console.h>

ENTRY_FUNCTION_WITHSTACK(start_my_board, MY_STACK_TOP, r0, r1, r2)
{
      extern char __dtb_my_board_start[];
      void *fdt;

      relocate_to_current_adr();
      setup_c();

      pbl_set_putc(my_serial_putc, (void *)BASE_ADDR);

      barebox_arm_entry(0x80000000, SZ_256M, __dtb_my_board_start);
}

Lets look at this line by line:

ENTRY_FUNCTION_WITHSTACK(start_my_board, MY_STACK_TOP, r0, r1, r2)

The entry point is special: It needs to be located at the beginning of the image, it does not return and may run before a stack is set up. To make it possible to write this entry point in C, the macro places a machine code prologue that uses MY_STACK_TOP as the initial stack pointer. If the stack is already set up, you may pass 0 here.

Additionally, the macro passes along a number of registers, in case the Boot ROM has placed something interesting there.

extern char __dtb_my_board_start[];

When a device tree is built as part of the PBL, __dtb_*_start and __dtb_*_end will be defined for it by the build system; its name is determined by the name of the device tree source file. Declare the start variable, so you can pass along the address of the device tree.

relocate_to_current_adr();

Machine code contains a mixture of relative and absolute addressing. Because the PBL doesn’t know in advance which address it’s loaded to, the link address of global variables may not be correct. To correct them a runtime offset needs to be added, so they point at the correct location. This procedure is called relocation and is achieved by this function. Note that this is self-modifying code, so it’s not safe to call this when executing in-place from flash or ROM.

setup_c();

As a size optimization, zero-initialized variables of static storage duration are not written to the executable. Instead only the region where they should be located is described and at runtime that region is zeroed. This is what setup_c() does.

pbl_set_putc(my_serial_putc, (void *)BASE_ADDR);

Now that we have a C environment set up, lets set our first global variable. pbl_set_putc saves a pointer to a function (my_serial_putc) that is called by the pr_* functions to output a single character. This can be used for the early PBL console to output messages even before any drivers are initialized. The second parameter (UART register base address in this instance) is passed as a user parameter when the provided function is called.

barebox_arm_entry(...)

This will compute a new stack top from the supplied memory region, uncompress barebox proper and pass along its arguments.

Looking at other boards you might see some different patterns:

*_cpu_lowlevel_init();

Often some common initialization and quirk handling needs to be done at start. If a board similar to yours does this, you probably want to do likewise.

__naked

All functions called before stack is correctly initialized must be marked with this attribute. Otherwise, function prologue and epilogue may access the uninitialized stack. Note that even with __naked, the compiler may still spill excess local C variables used in a naked function to the stack before it was initialized. A naked function should thus preferably only contain inline assembly, set up a stack and jump directly after to a noinline non naked function where the stack is then normally usable. This pattern is often seen together with ENTRY_FUNCTION. Modern boards better avoid this footgun by using ENTRY_FUNCTION_WITHSTACK, which will take care to initialize the stack beforehand. If either a barebox assembly entry point, ENTRY_FUNCTION_WITHSTACK or earlier firmware has set up the stack, there is no reason to use __naked, just use ENTRY_FUNCTION_WITHSTACK with a zero stack top.

noinline

Compiler code inlining is oblivious to stack manipulation in inline assembly. If you want to ensure a new function has its own stack frame (e.g. after setting up the stack in a __naked function), you must jump to a __noreturn noinline function. This is already handled by ENTRY_FUNCTION_WITHSTACK.

arm_setup_stack

For 32-bit ARM, arm_setup_stack initializes the stack top when called from a naked C function, which allowed to write the entry point directly in C. Modern code should use ENTRY_FUNCTION_WITHSTACK instead. Note that in both cases the stack pointer will be decremented before pushing values. Avoid interleaving with C-code. See __naked above for more details.

__dtb_z_my_board_start[];

Because the PBL normally doesn’t parse anything out of the device tree blob, boards can benefit from keeping the device tree blob compressed and only unpack it in barebox proper. Such compressed device trees are prefixed with __dtb_z_. It’s usually a good idea to use this.

imx6q_barebox_entry(...);

Sometimes it’s possible to query the memory controller for the size of RAM. If there are SoC-specific helpers to achieve this, you should use them.

get_runtime_offset()/global_variable_offset()

This functions return the difference between the link and load address. This is zero after relocation, but the function can be useful to pass along the correct address of a variable when relocation has not yet occurred. If you need to use this for anything more then passing along the FDT address, you should reconsider and probably rather call relocate_to_current_adr();.

*_start_image(...)/*_load_image(...)/*_xload_*(...)

If the SRAM couldn’t fit both PBL and the compressed barebox proper, PBL will need to chainload full barebox binary from the boot medium.

Repeating previous advice: The specifics about how different SoCs handle things can vary widely. You’re best served by mimicking a similar recently added board if one exists. If there’s none, continue reading the following sections.

7.1.2.2. Board code

If you need board-specific setup that’s not covered by any upstream device tree binding, you can write a driver that matches against your board’s /compatible:

static int my_board_probe(struct device *dev)
{
      /* Do some board-specific setup */
      return 0;
}

static const struct of_device_id my_board_of_match[] = {
      { .compatible = "my,cool-board" },
      { /* sentinel */ },
};

static struct driver my_board_driver = {
      .name = "board-mine",
      .probe = my_board_probe,
      .of_compatible = my_board_of_match,
};
device_platform_driver(my_board_driver);

Keep what you do here to a minimum. Many thing traditionally done here should rather happen in the respective drivers (e.g. PHY fixups).

7.1.2.3. Device-Tree

barebox regularly synchronizes its /dts/src directory with the upstream device trees in Linux. If your device tree happens to already be there you can just include it:

#include <arm/st/stm32mp157c-odyssey.dts>
#include "stm32mp151.dtsi"

/ {
     chosen {
             environment-emmc {
                     compatible = "barebox,environment";
                     device-path = &sdmmc2, "partname:barebox-environment";
             };
     };
};

&phy0 {
     reset-gpios = <&gpiog 0 GPIO_ACTIVE_LOW>;
};

Here, the upstream device tree is included, then a barebox-specific SoC device tree "stm32mp151.dtsi" customizes it. The device tree adds some barebox-specific info like the environment used for storing persistent data during development. If the upstream device tree lacks some info which are necessary for barebox there can be added here as well. Refer to Barebox devicetree handling and bindings for more information.

7.1.2.4. Boilerplate

A number of places need to be informed about the new board:

  • Either arch/$ARCH/Kconfig or arch/$ARCH/mach-$platform/Kconfig needs to define a Kconfig symbol for the new board

  • arch/$ARCH/boards/Makefile needs to be told which directory the board code resides in

  • arch/$ARCH/dts/Makefile needs to be told the name of the device tree to be built

  • images/Makefile.$platform needs to be told the name of the entry point(s) for the board

Example:

--- /dev/null
+++ b/arch/arm/boards/seeed-odyssey/Makefile
+lwl-y += lowlevel.o
+obj-y += board.o

--- a/arch/arm/mach-stm32mp/Kconfig
+++ b/arch/arm/mach-stm32mp/Kconfig
+config MACH_SEEED_ODYSSEY
+ select ARCH_STM32MP157
+ bool "Seeed Studio Odyssey"

--- a/arch/arm/boards/Makefile
+++ b/arch/arm/boards/Makefile
+obj-$(CONFIG_MACH_SEEED_ODYSSEY) += seeed-odyssey/

--- a/arch/arm/dts/Makefile
+++ b/arch/arm/dts/Makefile
+lwl-$(CONFIG_MACH_SEEED_ODYSSEY) += stm32mp157c-odyssey.dtb.o

--- a/images/Makefile.stm32mp
+++ b/images/Makefile.stm32mp
 $(obj)/%.stm32: $(obj)/% FORCE
 $(call if_changed,stm32_image)

 STM32MP1_OPTS = -a 0xc0100000 -e 0xc0100000 -v1

+pblb-$(CONFIG_MACH_SEEED_ODYSSEY) += start_stm32mp157c_seeed_odyssey
+FILE_barebox-stm32mp157c-seeed-odyssey.img = start_stm32mp157c_seeed_odyssey.pblb.stm32
+OPTS_start_stm32mp157c_seeed_odyssey.pblb.stm32 = $(STM32MP1_OPTS)
+image-$(CONFIG_MACH_SEEED_ODYSSEY) += barebox-stm32mp157c-seeed-odyssey.img

7.1.3. Porting to a new SoC

So, barebox supports the SoC’s family, but not this particular SoC. For example, the new fancy network controller is lacking support.

Note

If your new SoC requires early boot drivers, like e.g. memory controller setup. Refer to the next section.

Often drivers can be ported from other projects. Candidates are the Linux kernel, the bootloader maintained by the vendor or other projects like Das U-Boot, Zephyr or EDK.

Porting from Linux is often straight-forward, because barebox imports many facilities from Linux. A key difference is that barebox does not utilize interrupts, so kernel code employing them needs to be modified into polling for status change instead. In this case, porting from U-Boot may be easier if a driver already exists. Usually, ported drivers will be a mixture of both if they’re not written from scratch.

Drivers should probe from device tree and use the same bindings like the Linux kernel. If there’s no upstream binding, the barebox binding should be documented and prefixed with barebox,.

Considerations when writing Linux drivers also apply to barebox:

  • Avoid use of #ifdef HARDWARE. Multi-image code should detect at runtime what hardware it is, preferably through the device tree

  • Don’t use __weak symbols for ad-hoc plugging in of code. They make code harder to reason about and clash with multi-image.

  • Write drivers so they can be instantiated more than once

  • Modularize. Describe inter-driver dependency in the device tree

Miscellaneous Linux porting advice:

  • Branches dependent on system_state: Take the SYSTEM_BOOTING branch

  • usleep and co.: use [mud]elay

  • jiffies: use get_time_ns()

  • time_before: use !is_timeout()

  • clk_hw_register_fixed_rate_with_accuracy: use clk_hw_register_fixed_rate without accuracy

  • CLK_SET_RATE_GATE can be ignored

  • clk_prepare: is for the non-atomic code preparing for clk enablement. Merge it into clk_enable

7.1.4. Porting to a new SoC family

Extending support to a new SoC family can involve a number of things:

7.1.4.1. New header format

Your loader may require a specific header or format. If the header is meant to be executable, it should be written in assembly. If the C compiler for that platform supports __attribute__((naked)), it can be written in inline assembly inside such a naked function. See for example __barebox_arm_head for ARM32 or __barebox_riscv_header for RISC-V.

For platforms, without naked function support, inline assembly may not be used and the entry point should be written in a dedicated assembly file. This is the case with ARM64, see for example __barebox_arm64_head and the ENTRY_PROC macro.

Another way, which is often used for non-executable headers with extra meta-information like a checksum, is adding a new tool to scripts/ and have it run as part the image build process. images/ contains various examples.

7.1.4.2. Memory controller setup

If you’ve an external DRAM controller, you will need to configure it. This may involve enabling clocks and PLLs. This should all happen in the PBL entry point.

7.1.4.3. Chainloading

If the whole barebox image couldn’t be loaded initially due to size constraints, the prebootloader must arrange for chainloading the full barebox image.

One good way to go about it is to check whether the program counter is in DRAM or SRAM. If in DRAM, we can assume that the image was loaded in full and we can just go into the common PBL entry and extract barebox proper. If in SRAM, we’ll need to load the remainder from the boot medium.

This loading requires the PBL to have a driver for the boot medium as well as its prerequisites like clocks, resets or pin multiplexers.

Examples for this are the i.MX xload functions. Some BootROMs boot from a FAT file system. There is vfat support in the PBL. Refer to the sama5d2 board support for an example.

7.1.4.4. Core drivers

barebox contains some stop-gap alternatives that can be used before dedicated drivers are available:

  • Clocksource: barebox often needs to delay for a specific time. CLOCKSOURCE_DUMMY_RATE can be used as a stop-gap solution during initial bring up.

  • Console driver: serial output is very useful for debugging. Stop-gap solution can be DEBUG_LL console

7.1.5. Porting to a new architecture

7.1.5.1. Makefile

arch/$ARCH/Makefile defines how barebox is built for the architecture. Among other things, it configures which compiler and linker flags to use and which directories Kbuild should descend into.

7.1.5.2. Kconfig

arch/$ARCH/Kconfig defines the architecture’s main Kconfig symbol, the supported subarchitectures as well as other architecture specific options. New architectures should select OFTREE and OFDEVICE as well as HAVE_PBL_IMAGE and HAVE_PBL_MULTI_IMAGES.

7.1.5.3. Header files

Your architecture needs to implement following headers:

  • <asm/bitops.h> Defines optimized bit operations if available

  • <asm/bitsperlong.h> sizeof(long) Should be the size of your pointer

  • <asm/byteorder.h> If the compiler defines a macro to indicate endianness, use it here.

  • <asm/elf.h> If using ELF relocation entries

  • <asm/dma.h> Only if HAS_DMA is selected by the architecture.

  • <asm/io.h> Defines I/O memory and port accessors

  • <asm/mmu.h>

  • <asm/string.h>

  • <asm/swab.h>

  • <asm/types.h>

  • <asm/unaligned.h> Defines accessors for unaligned access

  • <asm/setjmp.h> Must define setjmp, longjmp and initjmp. setjmp and longjmp can be taken out of libc. As barebox does no floating point operations, saving/restoring these registers can be dropped. initjmp is like setjmp, but only needs to store 2 values in the jmpbuf: new stack top and address longjmp should branch to

Most of these headers can be implemented by referring to the respective <asm-generic/*.h> versions.

7.1.5.4. Relocation

Because there might be no single memory region that works for all images in a multi-image build, barebox needs to be relocatable. This can be done by implementing three functions:

  • get_runtime_offset(): This function should return the difference between the link and load address. One easy way to implement this is to force the link address to 0 and to determine the load address of the barebox _text section.

  • relocate_to_current_adr(): This function walks through the relocation entries and fixes them up by the runtime offset. After this is done get_runtime_offset() should return 0 as _text should also be fixed up by it.

  • relocate_to_adr(): This function copies the running barebox to a new location in RAM, then does relocate_to_current_adr() and resumes execution at the new location. This can be omitted if barebox won’t initially execute out of ROM.

  • relocate_to_adr_full(): This function does what relocate_to_adr() does and in addition moves the piggy data (the usually compressed barebox appended to the prebootloader).

Of course, for these functions to work. The linker script needs to ensure that the ELF relocation records are included in the final image and define start and end markers so code can iterate over them.

To ease debugging, even when relocation has no yet happened, barebox supports DEBUG_LL, which acts similarly to the PBL console, but does not require relocation. This is incompatible with multi-image, so this should only be considered while debugging.

7.1.5.5. Linker scripts

You’ll need two linker scripts, one for barebox proper and the other for the PBL. Refer to the ARM and/or RISC-V linker scripts for an example.

7.1.5.6. Generic DT image

It’s a good idea to have the architecture generate an image that looks like and can be booted just like a Linux kernel. This allows easy testing with QEMU or booting from barebox or other bootloaders. Refer to BOARD_GENERIC_DT for examples. If not possible, the (sub-)architecture making use of the image should register_image_handler that can chain-boot the format from a running barebox. This allows for quick debugging iterations.