linux-kernel - Re: [tip:x86/doc] x86/doc: mini-howto for using earlyprintk=dbgp

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Date:	Thu, 05 Mar 2009 07:36:06 -0800
From:	Daniel Walker <dwalker@...o99.com>
To:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	yinghai@...nel.org, stern@...land.harvard.edu,
	randy.dunlap@...cle.com, ebiederm@...ssion.com,
	akpm@...ux-foundation.org, gregkh@...e.de, tglx@...utronix.de,
	sarah.a.sharp@...el.com, mingo@...e.hu
Cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/doc] x86/doc: mini-howto for using earlyprintk=dbgp

On Thu, 2009-03-05 at 10:00 +0000, Yinghai Lu wrote:

> +
> +Mini-HOWTO for using the earlyprintk=dbgp boot option with a
> +USB2 Debug port key and a debug cable, on x86 systems.
> +
> +You need two computers, the 'USB debug key' special gadget and
> +and two USB cables, connected like this:
> +
> +  [host/target] <-------> [USB debug key] <-------> [client/console]
> +
> +1. There are three specific hardware requirements:
> +
> + a.) Host/target system needs to have USB debug port capability.
> +
> + You can check this capability by looking at a 'Debug port' bit in
> + the lspci -vvv output:

...

> + c.) Thirdly, you need a second client/console system with a regular USB port.

You might want to combine a.) and c.) since there is some disconnect
between the two. c.) doesn't explicitly say a second time that the
client/console needs the debug port capability ..

Or say

a.) You will need two USB ports. One on the client/console system and
one one the target system.

b.) The client/console and target USB ports must have the debug port
capability. You can check for this as follows,

c.) ...

I just felt the disconnect between the two a.) , and c.) left some
questions.

> +2. Software requirements:
> +
> + a.) On the host/target system:
> +
> +    You need to enable the following kernel config option:
> +
> +      CONFIG_EARLY_PRINTK_DBGP=y
> +
> +    And you need to add the boot command line: "earlyprintk=dbgp".
> +    (If you are using Grub, append it to the 'kernel' line in
> +     /etc/grub.conf)

Isn't the usual grub.conf in /boot/grub/grub.conf ? That's where mine
always seem to end up.

> +    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
> +    debugging crashes under Xorg, etc.
> +
> + b.) On the client/console system:
> +
> +    You should enable the following kernel config option:
> +
> +      CONFIG_USB_SERIAL_DEBUG=y
> +
> +    On the next bootup with the modified kernel you should
> +    get a /dev/ttyUSBx device(s).
> +
> +    Now this channel of kernel messages is ready to be used: start
> +    your favorite terminal emulator (minicom, etc.) and set
> +    it up to use /dev/ttyUSB0 - or use a raw 'cat /dev/ttyUSBx' to
> +    see the raw output.

You might want to switch the first /dev/ttyUSB0 to /dev/ttyUSBx .

> + c.) On Nvidia Southbridge based systems: the kernel will try to probe
> +     and find out which port has debug device connected.
> +
> +3. Testing that it works fine:
> +
> +   You can test the output by using earlyprintk=dbgp,keep and provoking
> +   kernel messages on the host/target system. You can provoke a harmless
> +   kernel message by for example doing:
> +
> +     echo h > /proc/sysrq-trigger
> +
> +   On the host/target system you should see this help line in "dmesg" output:
> +
> +     SysRq : HELP : loglevel(0-9) reBoot Crashdump terminate-all-tasks(E) memory-full-oom-kill(F) kill-all-tasks(I) saK show-backtrace-all-active-cpus(L) show-memory-usage(M) nice-all-RT-tasks(N) powerOff show-registers(P) show-all-timers(Q) unRaw Sync show-task-states(T) Unmount show-blocked-tasks(W) dump-ftrace-buffer(Z)
> +
> +   On the client/console system do:
> +
> +       cat /dev/ttyUSB0

Missing "x" here too.

Seems like a nice document. Thanks for writing it.

Daniel

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