EHCI debugging on the BeagleBone Black

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EHCI debugging

  1. Find USB port on the target that supports EHCI debug

  2. Initial setup of BBB to act as EHCI debug dongle
  3. Patch BBB’s g_dbgp module (optional, but highly recommended)
  4. Configure libreboot with EHCI debug
    1. Selecting HCD Index and USB Debug port
  5. How to get the debug logs
  6. Eneble EHCI Debug on the target’s kernel (optional, recommended)
  7. References

NOTE: this documentation may be outdated, and discusses configuring EHCI debug on the default Debian system that the BBB sometimes comes with. If you want an easier time, just use BBB ScrewDriver which comes pre-configured.

If your computer does not boot after installing libreboot, it is very useful to get debug logs from it, from the payload (grub) and/or the kernel (if gets to there). All of them stream debug logs on the available serial (RS-232) by default. However, most of todays laptops lack RS-232 port. The other option is to stream the logs to USB EHCI debug port.

This section explains step-by-step how to setup BBB as a “USB EHCI debug dongle” and configure libreboot and the linux kernel to stream logs to it (TODO: grub).

I will refer to three computers:

Find USB port on the target that supports EHCI debug

Not all USB controllers support EHCI debug (see: EHCI Debug Port ). Even more, if a USB controller supports EHCI debug, it is available only on a single port that might or might not be exposed externally.

  1. Plug the usb stick in the first available usb port
  2. Run the script, you will get output similar to following:
  3. The buses the support debug are Bus 3 (0000:00:1a.0) on Port 1 and Bus 4 (0000:00:1d.0) on port 2. Your usb stick is plugged on Bus 1, Port 3
  4. Repeat the steps, plugging the USB stick in the next available port
  5. Go through all available ports and remember(write down) those for which bus/port of the usb stick matches one of the bus/port that support debug (bold).

Remember (write down) for each port (external plug) you found that supports debug: PCI device id, the bus id, the port number, and the physical location of the usb plug.

If you do not find a match, you can not get debug over EHCI. Sorry.

1 The guys from coreboot were talking about including the script in coreboot distribution (check the status).

Initial setup of BBB to act as EHCI debug dongle

BBB must be powered with a barrel power connector since the mini-B USB plug will be used for the EHCI debug stream. So you will need:

(On BBB) The linux kernel includes module (g_dbgp that enables one of the usb ports on a computer to behave as EHCI debug dongle. Make sure you have this module available on your BBB (Debian 7.8 that comes with BBB should have it), if not, you should compile it yourself (see next section):

ls /lib/modules/3.8.13-bone70/kernel/drivers/usb/gadget/g_dbgp.ko

Unload all other g_* modules:

# lsmod
# rmmod g_multi

Then load g_dbgp :

# modprobe g_dbgp
# lsmod # should show that g_dbgp is loaded, and no other g_*

Plug the mini-B side of the USB cable in your BBB and the A side in your target. Then one of the usb devices on your target (with lsusb ) should be:

Bus 001 Device 024: ID 0525:c0de Netchip Technology, Inc.

If you see the device on the target, you are good to continue to the next step.

Patch BBB’s g_dbgp module (optional, but highly recommended)

For the reasons why you need this, see: EHCI Gadget Debug.
Make sure that you have cross compiling environment for arm-linux-gnueabihf setup on your host.

$ cd $work_dir
$ git clone
$ cd kernel
$ git checkout $mav (see above)
$ ./
$ wget\;a=blob_plain\;f=bin/am335x-pm-firmware.bin\;hb=HEAD -O kernel/firmware/am335x-pm-firmware.bin
$ cp configs/beaglebone kernel/arch/arm/configs/beaglebone_defconfig
git apply ../usbdebug-gadget/v3.8-debug-gadget/0001-usb-dbgp-gadget-Fix-re-connecting-after-USB-disconne.patch
git apply ../usbdebug-gadget/v3.8-debug-gadget/0002-usb-serial-gadget-no-TTY-hangup-on-USB-disconnect-WI.patch
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- beaglebone_defconfig -j4@
$ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- -j4 (is it possoble to build only the gadget modules)
$ mkdir ../tmp && make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- INSTALL_MOD_PATH=../tmp modules_install

Configure libreboot with EHCI debug

Libreboot(coreboot) should be configured with debug turned on and to push debug messages to the EHCI debug port.
If you’ve downloaded the binary distribution, you can check if it is properly configured in the following way:

./cbfstool/i686/cbfstool $img_path extract -n config -f ./my_config
CONFIG_USBDEBUG=y (Generic Drivers -> USB 2.0 EHCI debug dongle support)
CONFIG_USBDEBUG_IN_ROMSTAGE=y (Generic Drivers -> Enable early (pre-RAM) usbdebug)
CONFIG_USBDEBUG_HCD_INDEX=<HCD Index of usb controller - see below> (Generic Drivers -> Index for EHCI controller to use with usbdebug)
CONFIG_USBDEBUG_DEFAULT_PORT=<USB Debug port - see below> (Generic Drivers -> Default USB port to use as Debug Port)

The following three are behind radio button in the menu. Only the first one2 should be = y

USBDEBUG_DONGLE_STD=y                       (Generic Drivers -> Type of dongle (Net20DC or compatible) -> Net20DC or compatible)
CONFIG_USBDEBUG_DONGLE_BEAGLEBONE=n         (Generic Drivers -> Type of dongle (Net20DC or compatible) -> BeagleBone)
CONFIG_USBDEBUG_DONGLE_BEAGLEBONE_BLACK=n   (Generic Drivers -> Type of dongle (Net20DC or compatible) -> BeagleBone Black)

2 The g_dbgp module on BeagleBone Black (Rev. C) reports it self as Net20DC, the other options are for older BB(B) - ver1. This is documented on John Lewis’s blog (also tested/verified).

CONFIG_CONSOLE_USB=y (Console -> USB dongle console output)

Also Debugging -–> Output verbose XYZ ) (FIXME somebody verify these):

CONFIG_DEBUG_CBFS=y (Output verbose CBFS debug messages )
CONFIG_HAVE_DEBUG_RAM_SETUP=y (??? What/where is this)
CONFIG_DEBUG_RAM_SETUP=y (Output verbose RAM init debug messages)
CONFIG_DEBUG_SMI=y      (Output verbose SMI debug messages)
CONFIG_DEBUG_ACPI=y     (Output verbose ACPI debug messages )
CONFIG_DEBUG_USBDEBUG=y (Output verbose USB 2.0 EHCI debug dongle messages)

If some of the above mentioned configuration options are not as specified, you have to configure and compile libreboot yourself. Please refer to the doc(FIXME: link about compiling libreboot.

Selecting HCD Index and USB Debug port

This applies (and works) only if the USB controller that supports debug (found in the first section) is from Intel.
If the PCI ID of the port you found in the first section is 0000:00:1a.0 or 0000:00:1d.0 , you are ok. Otherwise you have to try without guarantee that will work.

If the externally exposed port is on a bus with PCI ID == 0000:00:1a.0 then for CONFIG_USBDEBUG_HCD_INDEX choose 2, otherwise choose 0 .

For CONFIG_USBDEBUG_DEFAULT_PORT choose the port from the first section that correspond to the PCI ID

The above is based on the implementation of coreboot/src/southbridge/intel/common/usb_debug.c : pci_ehci_dbg_dev() .
This is enough as it applies for the supported GM45/G45 Thinkpads. coreboot support some other contollers too, but they are irellevent for libreboot (for now).

How to get the debug logs

stty -icrnl -inlcr -F /dev/ttyGS0
cat /dev/ttyGS0

Note that this is not permanent on BBB, if you reboot it, you have to rmmod g_* and modprobe g_dbgp

Eneble EHCI Debug on the target’s kernel (optional, recommended)

You have to know how to compile kernel for your target.

  1. Check if early debugging is already enabled: grep CONFIG_EARLY_PRINTK_DBGP /boot/config-
  2. If enabled, you do not have to compile the kernel (skip this step). Otherwise, prepare kernel source for your distribution and select (Kernel hacking -> Early printk via EHCI debug port). Compile and install the new kernel.
  3. Edit your grub configuration and add following to the kenel parameters2021: earlyprintk=dbgp,keep. Also, try: earlyprintk=dbgp,keep where N is the debug port id if the first does not work.


10 EHCI Debug Port

11 coreboot EHCI debug gadget demonstration

12 EHCI Gadget Debug

13 Ehci-debug-gadget-patches.tar.gz

14 Compiling the BeagleBone Black Kernel



17 Building BBB Kernel


19 Exploring USB at the Hardware/Software Interface




  1. grub does not send messages to EHCI debug. Investigate.
  2. The section “Configure libreboot with EHCI debug” can be skipped/simplified if a common configuration works for all relevant targets is selected as defualt
  3. Patch and compule g_dbgp on BBB instead cross-compile
  4. Find a simple way to send debug messages from targets userland

Copyright © 2015 Alex David
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License Version 1.3 or any later version published by the Free Software Foundation with no Invariant Sections, no Front Cover Texts, and no Back Cover Texts. A copy of this license is found in ../fdl-1.3.html

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