This is Part 2 in the series: Linux on STM32MP135. See other articles.
In the previous article, we built a Linux kernel and manually copied it to an SD card. This works for a first test, but quickly becomes annoying. Here, we show how to use the STM32CubeProg to flash the SD card without removing it from the evaluation board.
Note: You may find the extensive explanations in the Bootlin article about flashing a similar chip helpful.
Finish the build process as per the previous
article, so as to have at least the
following files under buildroot/output/images/:
tf-a-stm32mp135f-dk.stm32fip.binu-boot-nodtb.binsdcard.imgGo to the ST website to download the STM32CubeProg. This unfortunately requires a registration and sign-up.
Get the Linux version, unpack in a new directory, and run the installer (just follow its verbose prompts):
$ cd cubeprog
$ unzip ../stm32cubeprg-lin-v2-20-0.zip
$ ./SetupSTM32CubeProgrammer-2.20.0.linux
Now plug in all three USB cables for the board. Set the DIP boot switches for serial boot (press in all the upper parts of the white rocker switches). Press the black reset button. If everything worked, you should be able to see the board under your USB devices:
jk@Lutien:/var/www/articles$ lsusb
...
Bus 001 Device 114: ID 0483:3753 STMicroelectronics STLINK-V3
Bus 001 Device 012: ID 0483:df11 STMicroelectronics STM Device in DFU Mode
...
The STLINK-V3 is what you can use to monitor the flashing progress via UART.
Simply open a serial monitor:
sudo picocom -b 115200 /dev/ttyACM0
Run the STM32CubeProg from the location that you installed it in to check that it is able to detect the board:
$ sudo ~/cube/bin/STM32_Programmer_CLI -l usb
-------------------------------------------------------------------
STM32CubeProgrammer v2.20.0
-------------------------------------------------------------------
===== DFU Interface =====
Total number of available STM32 device in DFU mode: 1
Device Index : USB1
USB Bus Number : 001
USB Address Number : 002
Product ID : USB download gadget@Device ID /0x501, @Revision ID /0x1003, @Name /STM32MP135F Rev.Y,
Serial number : 002800423232511538303631
Firmware version : 0x0110
Device ID : 0x0501
If that worked, it’s time to prepare the images for flashing. Go to
buildroot/output/images and create a file flash.tsv with the following
contents:
#Opt Id Name Type IP Offset Binary
- 0x01 fsbl1-boot Binary none 0x0 tf-a-stm32mp135f-dk.stm32
- 0x03 fip_boot Binary none 0x0 fip.bin
- 0x03 ssbl-boot Binary none 0x0 u-boot-nodtb.bin
P 0x10 sdcard RawImage mmc0 0x0 sdcard.img
Finally, run the flashing command itself:
sudo ~/cube/bin/STM32_Programmer_CLI -c port=usb1 -w flash.tsv
The STM32CubeProg will go through the sequence of files you wrote into
flash.tsv. First, the Arm Trusted Firmware (TF-A) gets written to the
memory and executed. It then does some secure magic behind the scenes and
accepts the next payload via the DFU protocol, the U-Boot. At last, U-Boot
itself is executed and it in turn accepts the last payload: the SD card
itself. Which was, after all, the only thing you wanted to transfer anyway
…
The tutorial above again presents the simplest method I have found so far, with a minimum of steps and prerequisites, to flash the SD card of the eval board without taking the card in and out. What’s the issue?
The STM32CubeProg comes in a 291M zip file, which gets installed as a 1.5G program. We use it to copy a disk image to the SD card. See the problem yet? Or let’s consider the on-board procedure: TF-A (4,212 files and 506,952 lines of code according to cloc) is used to run U-Boot (21,632 files and 3,419,116 lines of code), just so that a semi-standard USB DFU protocol can expose the SD card to write the image.
But why??? ChatGPT explains:
U-Boot became the standard since vendors upstreamed support there, and it offers cross-platform flashing via DFU/fastboot for factories and Windows users who can’t
ddraw disks. It also doubles as the hook for A/B updates, rollback, and secure boot. In practice, this forces developers into a complex boot stack, even though most boards could just boot Linux directly from SD/eMMC and use a tiny DFU mass-storage tool for recovery.
A more likely explanation is that the boot process has acquired an unnecessary reputation for being difficult, so that few want to mess with it. If there is a working solution, it will get incorporated into the software stack, no matter how baroque. The warning has been around for a long time:
Big building-blocks […] can lead to more compact code and shorter development time. […] Less clear, however, is how to assess the loss of control and insight when the pile of system-supplied code gets so big that one no longer knows what’s going on underneath.
[… As] libraries, interfaces, and tools become more complicated, they become less understood and less controllable. When everything works, rich programming environments can be very productive, but when they fail, there is little recourse.[1]
All these tool are intended to make our work easier, but as they are piled on
without any reasonable limit, the resulting mess is ironically far more
complicated than the problem they are solving. If the task at hand is to flash
an SD card image, why doesn’t the firmware expose the medium as a USB mass
storage device, so that standard tools like dd could be used to work with it?
The cynical answer suggests itself … They didn’t know better.
Those who do not understand Unix are condemned to reinvent it, poorly.[2]
Surely it cannot be too difficult to write a simple “bare-metal” program, which we could load to the board using the simple and well-documented UART protocol implemented in the ROM of the STM32MP1. The program would be very small and quick to load. The program would expose the available media as mass storage devices, and that’s it.
But … You may object, we need U-Boot anyways, otherwise how are we to load Linux? As we will explain in a future article, that is not so. U-Boot is entirely unnecessary for a large class of embedded Unix applications.
Unix was built on a handful of ideas that turned out to be both powerful and practical. The following discussion blends established Unix facts with interpretive commentary; it does not claim to describe any single historical Unix precisely.
The shell runs commands as programs. There’s no special class of built-ins; if you want a new command, you write a program. By default, programs read from standard input and write to standard output, unless redirected.
Most commands are small filters for text streams. They do one job, and they work together naturally. Connecting them with pipes lets you build bigger tools out of simpler ones.
Everything is a file: user data, programs, directories, and even devices. Directories form a tree; each entry points to an inode, which knows where the data blocks live. Devices show up as files too.
This means that I/O and storage use the same calls: open, close, read, write. That’s the interface for everything. Executables and data files are stored in the same way, reinforcing the idea that a single abstraction suffices.
The kernel is deliberately small. It multiplexes I/O and leaves the rest to user programs. Even init, the first process, is just a program: it opens terminals, prints the login message, and starts shells in a loop.
Processes come from the fork/exec pair. One process copies itself, then overlays the copy with another program. The idea is simple, and it works.
System calls are invoked by a trap instruction, wrapped in library functions so programs don’t depend directly on kernel details. Programs stay independent, and the operating system can change underneath.
Unix was small enough that one person could understand the whole thing. That made it easier to modify, port, and teach. The manuals were short, consistent, and focused on usage, not internals. A second volume provided tutorials and background for those who wanted more.
The guiding principle was: be general, but not too general; portable, but not too portable. If you try to solve every problem in advance, you get bloat. By keeping it modest, Unix was more useful—and paradoxically more general and portable—than larger systems.
Some parts were machine-specific, usually device drivers or bits of assembly. But not many. Most code was reusable, and the exceptions were small. An array of function pointers mapped device numbers to driver routines; that was about as complex as it got. For example, a character device[1] driver needs to expose the following functions:
extern struct cdevsw
{
int (*d_open)();
int (*d_close)();
int (*d_read)();
int (*d_write)();
int (*d_ioctl)();
int (*d_stop)();
struct tty *d_ttys;
} cdevsw[];
The 80/20 rule applied everywhere: make most of the system simple and portable, accept a little complexity when it really pays off. Code was meant to be 80% reusable, not 100%, which avoided the kind of rigidity seen in later systems.
Unix came with all its own sources and tools. It was self-hosting, and people could read, study, and change the code. The system included what you needed, and nothing more. No useless programs, no dead code, and very little irrelevant platform-specific clutter.
The philosophy was to write programs you would actually use, not ones meant to satisfy a standard or some hypothetical future need.
The enduring lesson of Unix is that simplicity beats complexity. Interfaces were orthogonal, text was the universal medium, and programs were small and self-contained. Each one did one thing, and did it well.
That philosophy proved more important than any single feature. It made Unix portable, teachable, and durable. It showed that you don’t need a committee or a grand design to build something powerful. You need clarity, restraint, and the discipline to write only what you need.
Unix also suggests how to go further. Small, portable, self-contained programs can approach the kind of stability that TeX achieved—systems so refined that they don’t need to change.
Portability itself can be modular. The Wollongong group[2] showed this by first porting Unix piece by piece to an Interdata 7/32, running it alongside the host system, and then replacing the host functions with assembly routines. That approach points toward kernels that are more modular, where pieces like fork and exec could be reused without bringing along a whole scheduler.
Device drivers can also be simplified. One idea is to treat them as user processes whose IDs match their device numbers. They would implement the usual open, read, and write interfaces, but otherwise behave like ordinary programs: start and stop freely, hold their own memory, receive signals. The kernel would not “manage” them, yet the familiar Unix file interface would still apply.
The same lesson holds today. Artificial intelligence can sometimes repair or adapt programs automatically, but only if the systems are small and self-contained. Large, tangled software offers no foothold. Unix worked because it avoided dead code, avoided over-abstraction, and made each interface simple enough to understand and replace.
Finally, Unix showed that the way forward can’t be too innovative. If “the way” is too radical, no one will follow it.[3] The genius of Unix was that it was just radical enough.
From version 7 Unix, found in
/usr/sys/h/conf.h. ↩︎
Juris Reinfelds: The First Port of Unix. Department of Computing Science, The University of Wollongong. See also Richard Miller: The First Unix Port. Miller Research Ltd. (Both documents undated. Why don’t people date all their documents!?) ↩︎
Still looking for the source of this quote … ↩︎
This is Part 1 in the series: Linux on STM32MP135. See other articles.
Wouldn’t it be great to have a single board computer that runs just Linux? That is, no external libraries, tools, driver modules—just the kernel binary, and whichever programs you choose to run under it. After all, Linux is just a C program, so how hard can it be to compile and run it? Read on to get started exploring the wild world of embedded Linux.
In this tutorial, we show how to get a basic “minimal” Linux installed on the STM32MP135 evaluation board with a minimum of steps or obscure scripts. For detailed explanations, refer to the excellent writeup from Bootlin.
Get a copy of Buildroot:
$ git clone https://gitlab.com/buildroot.org/buildroot.git
$ cd buildroot
As of this writing, the latest commit in this repository is
$ git rev-parse HEAD
bbb0164de08f761a3399c961700db44befff5c70
Find the default configuration appropriate for this board:
$ make list-defconfigs | grep stm32mp135
This shows that stm32mp135f_dk_defconfig is available. Install it by calling
make on it:
$ make stm32mp135f_dk_defconfig
Let’s enable the USB mode for Arm Trusted Firmware (TF-A) so that we will be able to use USB flashing in the future. Open
$ make menuconfig
Navigate under Bootloaders ---> ARM Trusted Firmware (ATF) and add the
following at the end of the “Additional ATF build variables” string:
STM32MP_USB_PROGRAMMER=1
Then select “OK”, and “Esc” your way out of the menuconfig. Make sure to say “Yes” when asked whether to save the new configuration.
Run the build, and collect logs into a file:
$ time make >log.txt 2>&1
On my dual-core i5-7300U laptop, this took about an hour and a half.
Watch the build from another terminal:
$ tail -f log.txt
Copy the generated image to an SD card (assumed to be at /dev/sdb):
$ sudo dd if=output/images/sdcard.img of=dev/sdb bs=1M
Time to run it on the evaluation board! Set it up as follows:
CN12, labelled PWR_IN)
to a powered USB hubCN10, left of the screen) to a desktop computer,
which will enumerate as a serial port (/dev/ttyACM0 on my computer).
Set the DIP switches to boot from the SD card as shown in the image below. In
this orientation, press in on the upper side of the rockers of BOOT0 and
BOOT2, and on the lower side for BOOT1.
Press the black reset button and if everything went right, you should see the kernel boot messages displayed on the serial monitor, until the login prompt gets displayed. Done!
Welcome to Buildroot
buildroot login: root
# uname -a
Linux buildroot 6.12.22 #1 SMP PREEMPT Wed Sep 3 20:23:46 PDT 2025 armv7l GNU/Linux
This is, to my knowledge, the fastest way to get started with embedded Linux on
“real” hardware in terms of number of steps. However, it does not result in the
most minimal distribution. Besides the Linux kernel, the default configuration
pulls in a number of large programs, such as U-Boot, ATF, OP-TEE, and more.
(Examine buildroot/dl) after building to see all the stuff that got downloaded
in the build process.)
For someone used to bare-metal embedded work, the size and complexity of this auxiliary software is utterly baffling. A bootloader with more lines of code than my whole project? Several different compilers and interpreters (C, Rust, Perl, Python) needed for a “Hello, world!?”
In my mind I hold an ideal of the “pure” essence of Unix: there is one kernel,
which at the end of its boot process invokes one user-space program (init),
which then does anything it wants to. I believe in the simplicity of the Linux
boot process, as outlined by Russell King in the 2002 masterpiece, “Booting ARM
Linux”:
r2It is time we regain the pristine uncomplicated state that used to exist before everything became bundled and containerized and wrapped inside countless layers of abstraction that are supposed to make things simpler, but in the end serve only to confuse everyone.
In the next articles, we will take this “primordial mess” and cut it down to size. Stay tuned!