[gentoo-user] Initial console messages garbled [REPOST]

[Robin Atwood]

Since as Dale and others have pointed out, I posted my first message on
this subject by using an existing message as a template which doesn't
work because it gets added to the existing thread. So here goes
again. (I have also just noticed some replies that I had missed for the
same reason: lesson learned! Rich's response looks very useful.)

I posted about this on the Gentoo forums (see [1]) but didn't get any
response so I'll try my luck here! The post explains everything but.
briefly, normally when I boot a kernel I initially get some messages
in a large clunky font, and then the frame-buffer module loads and
everything is hi-res and fills the entire monitor. On my new box the
first messages are video-static until the frame-buffer takes over. I
have an error in a new root partition I have built and need to read
those messages! What of all the numerous parameters in various places
should I try tweaking?

Thanks
Robin

1. http://forums.gentoo.org/viewtopic-t-1114668-highlight-.html

> From: "Sid Spry" <sid@××××.us>
> To: gentoo-user@l.g.o
> Subject: Re: [gentoo-user] Initial console messages garbled
> Date: Sun, 28 Jun 2020 11:03:12 -0500
> Reply-to: gentoo-user@l.g.o
> User-Agent:
> Cyrus-JMAP/3.3.0-dev0-543-gda70334-fm-20200618.004-gda703345
> X-Mailer: MessagingEngine.com Webmail Interface
>
> > On Thu, Jun 25, 2020 at 2:55 AM Robin Atwood <robin@×××××.org>
> > wrote:
> > >
> > > On Wed, 24 Jun 2020 13:31:30 -0400
> > > tedheadster <tedheadster@×××××.com> wrote:
> > >
> > > > Robin,
> > > >  are you comfortable just going with a bare-bones console and
> > > > build a new kernel where you _disable_ CONFIG_FB? That might do
> > > > it.
> > > >
> > > > Alternately, you can hook up a serial cable to another computer
> > > > and set "console=ttyS0,115200n8".
> > >
> > > I will try that, if it works it will at least give me a chance to
> > > look at the error messages.
> > >
> > > I don't think I have any serial cables!
> > >
> >
>
> Well, you may need to buy a cable one way or another if your issue is
> very early in the boot process, typically qualified as "before the
> console comes up." Netconsole may help you though.
>
> Most motherboards used to have potentially unpopulated serial port
> headers on them. Those seen to be disappearing.
>
> The replacement is
> https://www.kernel.org/doc/html/v5.4/driver-api/usb/usb3-debug-port.html.
> Most (all?) desktop xHCI controllers support a device mode that is
> essentially a very high speed CBC ACM serial port. This is especially
> useful for debugging laptops.
>
> You do need to either make or buy the special A to A cable.


> From: Rich Freeman <rich0@g.o>
> To: gentoo-user@l.g.o
> Subject: Re: [gentoo-user] Initial console messages garbled
> Date: Sun, 28 Jun 2020 10:51:26 -0400
> Reply-to: gentoo-user@l.g.o
>
> On Thu, Jun 25, 2020 at 2:55 AM Robin Atwood <robin@×××××.org> wrote:
> >
> > On Wed, 24 Jun 2020 13:31:30 -0400
> > tedheadster <tedheadster@×××××.com> wrote:
> >
> > > Robin,
> > >  are you comfortable just going with a bare-bones console and
> > > build a new kernel where you _disable_ CONFIG_FB? That might do
> > > it.
> > >
> > > Alternately, you can hook up a serial cable to another computer
> > > and set "console=ttyS0,115200n8".
> >
> > I will try that, if it works it will at least give me a chance to
> > look at the error messages.
> >
> > I don't think I have any serial cables!
> >
>
> While serial consoles are one solution, I'd take a look at network
> consoles.  They're FAR easier to manage on commodity hardware.  All
> you really need is another host on the network that can run netcat.
>
> I stick this in my /etc/grub/default - or otherwise get it onto the
> command line in the bootloader:
> netconsole=@/,6666@192.168.1.1
>
> That tells the kernel to send all console output over UDP to
> 192.168.1.1:6666.  If you have multiple interfaces/etc you might need
> to expand that command line a bit.  I have no idea how it comes up
> with the sending IP - if you care about that you can specify it.  I'm
> guessing it doesn't run DHCP - but this is just plain UDP so it is
> one-way and there is no need for acks to get back to the sender.
>
> On the destination host I run:
> nc -u -l -p 6666
>
> (nc is provided by the netcat package - a very basic tool that should
> be available everywhere - probably on non-linux operating systems
> also)
>
> Start up the reception part before you try booting the host you're
> troubleshooting, because it is just going to send packets blind into
> the ether and if nothing is listening they're gone.  Obviously
> netconsole is a very simple implementation so that it can run during
> early boot.  It is very good for capturing panics/etc.
>
> I don't know how it compares with serial console in terms of how early
> it starts.  I think it does capture stuff very early in boot though -
> both systems require a degree of hardware initialization before they
> can work, but both are also very simple.
>
> This does need to be enabled in the kernel.
>
> You can also enable this on a running kernel but of course that does
> no good for issues during boot.  Full docs are at:
> https://www.kernel.org/doc/Documentation/networking/netconsole.txt


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