linux-kernel - Re: [PATCH] x86/x86_64_defconfig: Enable the serial console

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Message-ID: <a165781d-d1c8-04e2-0b60-8d776a21a625@collabora.com>
Date:   Mon, 12 Oct 2020 16:10:45 +0100
From:   Guillaume Tucker <guillaume.tucker@...labora.com>
To:     Willy Tarreau <w@....eu>, Borislav Petkov <bp@...en8.de>
Cc:     Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Collabora Kernel ML <kernel@...labora.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Diego Elio Pettenò <flameeyes@...meeyes.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        kernelci@...ups.io
Subject: Re: [PATCH] x86/x86_64_defconfig: Enable the serial console

On 12/10/2020 15:40, Willy Tarreau wrote:
> On Mon, Oct 12, 2020 at 04:32:12PM +0200, Borislav Petkov wrote:
>> On Mon, Oct 12, 2020 at 11:22:10AM +0100, Guillaume Tucker wrote:
>>> However, it was found while adding some x86 Chromebooks[1] to
>>> KernelCI that x86_64_defconfig lacked some basic things for
>>> anyone to be able to boot a kernel with a serial console enabled
>>> on those.
>>
>> Hold on, those are laptops, right? How come they do have serial console?
>> Because laptops don't have serial console - that has been the eternal
>> problem with debugging kernels on laptops.

Yes the link you pointed at is a prerequisite to enable serial
console in the firmware (Coreboot/Depthcharge).

> Well, to be precise, they don't have *anymore*. I used to exclusively
> select laptops having a serial port given that I was using it daily with
> routers, until I had to resign when I abandonned my good old NC8000 :-/

You can get serial console on recent enough Chromebooks with a
debug interface such as SuzyQable:

  https://www.sparkfun.com/products/14746

It's not a USB Type-C adapter, it has a debug interface which
works with Chromebooks that support Case-Closed Debugging.
Anyone can do that without modifying the Chromebook, and with a
bit of patience to go through the documentation[1]...

The KernelCI sample results from my previous email were run using
just that: off-the-shelf Chromebooks + SuzyQ + rebuilt firmware
for interactive console and tftp boot + kernel with the config
options in Enric's patch.

Thanks,
Guillaume


[1] https://chromium.googlesource.com/chromiumos/platform/ec/+/cr50_stab/docs/case_closed_debugging_cr50.md