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Hello! Thank you for your detailed description. You are correct in your understanding: on the Apalis iMX8, the standard way to load and run Cortex-M4 code is through U-Boot, as RemoteProc is not yet supported for this module (reference).
Why Linux Crashes When M4 is Active
The behavior you’re seeing—Linux crashing or rebooting when the M4 core is running—almost always points to resource (typically peripheral or pin) conflicts between Linux and the Cortex-M4. Even if it seems like Linux isn’t using a given UART or peripheral, device tree configuration is needed to truly “hand over” control and avoid overlap.
Solution: Device Tree Overlay
You must provide an appropriate Device Tree Overlay (DTO) so Linux does not claim the pins or peripherals that the M4 core requires. For the Apalis iMX8, Toradex provides recommended overlays. Here is the process:
1. Create and Apply the Device Tree Overlay
Suppose you are using Cortex-M4 Core 0 and require UART; you can use this sample overlay:
/dts-v1/;
/plugin/;
/* Disable UART1 Control Pins to use M4 Core 0 and 1 UART */
&iomuxc {
pinctrl-0 = <&pinctrl_cam1_gpios>, <&pinctrl_dap1_gpios>,
<&pinctrl_esai0_gpios>, <&pinctrl_fec2_gpios>,
<&pinctrl_gpio3>, <&pinctrl_gpio4>, <&pinctrl_gpio_keys>,
<&pinctrl_gpio_usbh_oc_n>,
<&pinctrl_lvds0_i2c0_gpio>, <&pinctrl_lvds1_i2c0_gpios>,
<&pinctrl_mipi_dsi_0_1_en>, <&pinctrl_mipi_dsi1_gpios>,
<&pinctrl_mlb_gpios>, <&pinctrl_qspi1a_gpios>,
<&pinctrl_sata1_act>, <&pinctrl_sim0_gpios>,
<&pinctrl_usdhc1_gpios>;
};
If you are using M4 Core 1 (and using its UART), use this overlay:
/dts-v1/;
/plugin/;
/* Disable PWM0 and PWM1 to use M4 Core 1 UART */
&lsio_pwm0 {
status = "disabled";
};
&lsio_pwm1 {
status = "disabled";
};
&lpuart2 {
status = "disabled";
};
You may need to tailor the overlay according to the peripherals your M4 firmware is using.
2. Use the Overlay with TorizonCore
- Place the compiled
.dtbo overlay file in /boot/overlays/.
- Add the line
fdt_overlays=your-overlay.dtbo to /boot/ostree/torizon.conf.
- Reboot the system so the overlay is applied.
3. Confirm Isolation
Confirm that the UART (or other resource) is NOT claimed/used by Linux (e.g., not visible in /dev/ or dmesg output referring to it). Linux must not touch peripherals that the M4 uses.
Additional Recommendations
- Make sure your M4 firmware runs in a persistent way (typically with a
while(1) at the end) so it stays alive after Linux boots. (See here)
- Be certain only the required pins/functions are disabled in Linux—not more, not less.
- Double-check the M4 binary start address and U-Boot commands as shown in the Loading Binaries Guide.
If you follow the above steps to properly isolate M4 and Linux resources, the board should no longer crash or reboot upon starting Linux after the M4 core is running.
Let me know if you need help creating the specific overlay for your setup or encounter any other issues!
Best regards,
Toradex AI Assistant.
