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Subject[PATCH 10/10] ehci-dbgp,documentation: Documentation updates for ehci-dbgp
Add missing information about requirements of using the EHCI usb debug
controller as well as to mention you can use a debug controller other
than the first one in the system.

Signed-off-by: Jason Wessel <>
Cc: Greg KH <>
Cc: Ingo Molnar <>
Cc: Andrew Morton <>
Cc: Yinghai Lu <>
Cc: "Eric W. Biederman" <>
Cc: Alan Stern <>
Cc: Sarah Sharp <>
Cc: Randy Dunlap <>
Documentation/kernel-parameters.txt | 2 +-
Documentation/x86/earlyprintk.txt | 39 +++++++++++++++++++++++++++++++++-
2 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index dd1a6d4..e709fc0 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -670,7 +670,7 @@ and is between 256 and 4096 characters. It is defined in the file
earlyprintk= [X86,SH,BLACKFIN]
- earlyprintk=dbgp
+ earlyprintk=dbgp[debugController#]

Append ",keep" to not disable it when the real console
takes over.
diff --git a/Documentation/x86/earlyprintk.txt b/Documentation/x86/earlyprintk.txt
index 607b1a0..f19802c 100644
--- a/Documentation/x86/earlyprintk.txt
+++ b/Documentation/x86/earlyprintk.txt
@@ -7,7 +7,7 @@ and two USB cables, connected like this:

[host/target] <-------> [USB debug key] <-------> [client/console]

-1. There are three specific hardware requirements:
+1. There are a number of specific hardware requirements:

a.) Host/target system needs to have USB debug port capability.

@@ -42,7 +42,35 @@ and two USB cables, connected like this:
This is a small blue plastic connector with two USB connections,
it draws power from its USB connections.

- c.) Thirdly, you need a second client/console system with a regular USB port.
+ c.) You need a second client/console system with a high speed USB 2.0
+ port.
+ d.) The Netchip device must be plugged directly into the physical
+ debug port on the "host/target" system. You cannot use a USB hub in
+ between the physical debug port and the "host/target" system.
+ The EHCI debug controller is bound to a specific physical USB
+ port and the Netchip device will only work as an early printk
+ device in this port. The EHCI host controllers are electrically
+ wired such that the EHCI debug controller is hooked up to the
+ first physical and there is no way to change this via software.
+ You can find the physical port through experimentation by trying
+ each physical port on the system and rebooting. Or you can try
+ and use lsusb or look at the kernel info messages emitted by the
+ usb stack when you plug a usb device into various ports on the
+ "host/target" system.
+ Some hardware vendors do not expose the usb debug port with a
+ physical connector and if you find such a device send a complaint
+ to the hardware vendor, because there is no reason not to wire
+ this port into one of the physically accessible ports.
+ e.) It is also important to note, that many versions of the Netchip
+ device require the "client/console" system to be plugged into the
+ right and side of the device (with the product logo facing up and
+ readable left to right). The reason being is that the 5 volt
+ power supply is taken from only one side of the device and it
+ must be the side that does not get rebooted.

2. Software requirements:

@@ -56,6 +84,13 @@ and two USB cables, connected like this:
(If you are using Grub, append it to the 'kernel' line in

+ On systems with more than one EHCI debug controller you must
+ specify the correct EHCI debug controller number. The ordering
+ comes from the PCI bus enumeration of the EHCI controllers. The
+ default with no number argument is "0" the first EHCI debug
+ controller. To use the second EHCI debug controller, you would
+ use the command line: "earlyprintk=dbgp1"
NOTE: normally earlyprintk console gets turned off once the
regular console is alive - use "earlyprintk=dbgp,keep" to keep
this channel open beyond early bootup. This can be useful for

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