Thanks you both for your replies!
TL;DR: Buildroot was not including bcm2712-rpi-5-b.dtb in the boot partition image, it only had bcm2712-rpi-cm5-cm5io.dtb and bcm2712-rpi-cm5l-cm5io.dtb. With the right .dtb included, all that was needed to get console out on GPIO14/15 was enable_uart=1.
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I tried aBUGSworstnightmare's additions to config.txt and cmdline.txt to no avail. I was still suspicious of my last clue where the status light was not indicating a missing kernel on the buildroot image if I deliberately misnamed it. The mass-storage-gadget64 image that rpiboot uses to always produces console output so to see if I was just missing needed device tree stuff I copied over all the .dtb files from / and the /overlay folder but left my buildroot kernel image, config.txt, and cmdline.txt (with aBUGSworstnightmare's additions). I did get a console. Through process of elimination I determined that the minimum contents of the boot partition are:
With the right .dtb files in place, misnaming the kernel file has the expected effect of producing the error code of 7 short flashes of the status LED.
Final config.txt:Final cmdline.txt:
Thank you again! This is one of those issues that will probably seem obvious as I learn more but I greatly appreciate your efforts to assist.
TL;DR: Buildroot was not including bcm2712-rpi-5-b.dtb in the boot partition image, it only had bcm2712-rpi-cm5-cm5io.dtb and bcm2712-rpi-cm5l-cm5io.dtb. With the right .dtb included, all that was needed to get console out on GPIO14/15 was enable_uart=1.
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I tried aBUGSworstnightmare's additions to config.txt and cmdline.txt to no avail. I was still suspicious of my last clue where the status light was not indicating a missing kernel on the buildroot image if I deliberately misnamed it. The mass-storage-gadget64 image that rpiboot uses to always produces console output so to see if I was just missing needed device tree stuff I copied over all the .dtb files from / and the /overlay folder but left my buildroot kernel image, config.txt, and cmdline.txt (with aBUGSworstnightmare's additions). I did get a console. Through process of elimination I determined that the minimum contents of the boot partition are:
- bcm2712-rpi-5-b.dtb - without it, no console, probably not even booting
- bcm2712-rpi-cm5-cm5io.dtb - without it, kernel can't find eMMC
- kernel image, named Image in buildroot defconfig
- config.txt - see below
- cmdline.txt - see below
- overlay folder
- uart_2ndstage=1 - no effect on GPIO14/15 console output
- enable_rp1_uart=1 - produces "RP1_UART 0000001c00030000" line before kernel output on GPIO14/15 console output
- dtparam=uart0_console=on - no effect on GPIO14/15 console output
- enable_uart=1 - produces output, login prompt on GPIO14/15 console
With the right .dtb files in place, misnaming the kernel file has the expected effect of producing the error code of 7 short flashes of the status LED.
Final config.txt:
Code:
# Please note that this is only a sample, we recommend you to change it to fit# your needs.# You should override this file using BR2_PACKAGE_RPI_FIRMWARE_CONFIG_FILE.# See http://buildroot.org/manual.html#rootfs-custom# and http://elinux.org/RPiconfig for a description of config.txt syntaxkernel=Image# To use an external initramfs file#initramfs rootfs.cpio.gz# Disable overscan assuming the display supports displaying the full resolution# If the text shown on the screen disappears off the edge, comment this outdisable_overscan=1# enable RTCdtparam=i2c_vc=ondtoverlay=i2c-rtc,pcf85063a,i2c_csi_dsi# enable dwc2 USB controller (USB 2.0)dtoverlay=dwc2,dr_mode=host# Enable UART0 for serial console on ttyAMA0enable_uart=1Code:
root=/dev/mmcblk0p2 rootwait console=tty1 console=ttyAMA0,115200Thank you again! This is one of those issues that will probably seem obvious as I learn more but I greatly appreciate your efforts to assist.
Statistics: Posted by clicknett — Fri Aug 29, 2025 6:37 pm