Kernel 3.8.13-rt9 hangs up during boot (BBB), how to get debug messages

I successfully boot the 3.8.13 kernel with some modifications in .config (namely, I am disabling all the ethernet drivers like cpsw). However, when I try to boot the 3.8.13-rt9 kernel (with preempt_rt patch), it hangs up after “Uncompressing Linux… done, booting the kernel.”

  • all four LEDS (D2 to D5) are lit up immediately after and are kept lit until the system restarts after a minute or so
  • there is no output on the serial console
  • I don’t think that there is a problem with specifying the wrong tty port in bootargs in the uEnv.txt since I kept the same settings as for regular 3.8.13 kernel
  • I am booting the zImage from the rfs partition of the SD card, u-boot is in the boot partition of the sd card; the actual rfs used is in the eMMC - this configuration works with regular 3.8.13 kernel

My main question is how to get any debug messages when the kernel is stuck so early. I set CONFIG_DEBUG_LL=y, CONFIG_DEBUG_LL_UART_NONE=n, CONFIG_EARLYPRINTK=y etc., but there is still no output and no log to kern.log
Did anyone else experience the situation when all LEDs lit up during boot?

It seems something goes wrong very early in the boot process, like some uboot parameters are set wrong. But I did not change anything compared to the setup I use for booting the 3.8.13 kernel without preempt_rt. This is my uEnv.txt, for example:

#kernel_file=/boot/vmlinuz-3.8.13/zImage
#dtb_file=/boot/dtbs-3.8.13/am335x-boneblack.dtb
kernel_file=/boot/vmlinuz-3.8.13-rt9/zImage
dtb_file=/boot/dtbs-3.8.13-rt9/am335x-boneblack.dtb

loadzimage=load mmc 0:2 ${loadaddr} ${kernel_file}
loadfdt=load mmc 0:2 ${fdtaddr} ${dtb_file}

console=ttyO0,115200n8
mmcroot=/dev/mmcblk1p2 ro fixrtc
mmcrootfstype=ext4 rootwait

mmcargs=setenv bootargs root=${mmcroot} rootfstype=${mmcrootfstype} console=${console}

#zImage:
uenvcmd=run loadzimage; run loadfdt; run mmcargs; bootz ${loadaddr} - ${fdtaddr}

Could the problem be that I am using the kernel from here: GitHub - beagleboard/kernel at 3.8-rt
Perhaps someone could suggest a more recently tested version with preempt_rt patch? Perhaps something from the RobertCNelson repo: https://github.com/beagleboard/linux/tree/3.8