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" <[email protected]> > To: [email protected] > Subject: Re: [gentoo-user] Initial console messages garbled > Date: Sun, 28 Jun 2020 11:03:12 -0500 > Reply-to: [email protected] > 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 <[email protected]> > > wrote: > > > > > > On Wed, 24 Jun 2020 13:31:30 -0400 > > > tedheadster <[email protected]> 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 <[email protected]> > To: [email protected] > Subject: Re: [gentoo-user] Initial console messages garbled > Date: Sun, 28 Jun 2020 10:51:26 -0400 > Reply-to: [email protected] > > On Thu, Jun 25, 2020 at 2:55 AM Robin Atwood <[email protected]> wrote: > > > > On Wed, 24 Jun 2020 13:31:30 -0400 > > tedheadster <[email protected]> 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=@/,[email protected] > > 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 --
