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Unable to mount root FS, serial console problems?
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Bradgranath
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PostPosted: Fri Mar 27, 2020 10:37 pm    Post subject: Unable to mount root FS, serial console problems? Reply with quote

Got gentoo running on my laptop, moving on to the desktop. I’ve got a Dell Precision T7400 with a LSI Logic / Symbios Logic SAS1068 PCI-X Fusion-MPT SAS Hard disk controller. The liveDVD boots fine from unetbootin off a thumb drive, and I’ve built the kernel, and installed the tools and setup the bootloader. When I boot, grub is working fine. When I try to start the kernel I get a kernel panic that it’s
Code:
unable to mount the root fs, unknown block (8,36), yadda yadda. 
The console resolution is pretty small, so I’m only able to see the last few lines. I can tell the disk controller is working, since the kernel lists all the disks it can see, and it can definitely see one of the other disks that is in the machine on the last line before the terminal cuts it off.

No worries it’s cool, this ancient machine has a serial port! But I’ve misplaced my null modem adapter! Aha, I’ll just breadboard one with these little wires from an arduino project kit! Ok, raaaaaapidly approaching yak-shaving territory. Adding a console argument to the the kernel start line in grub gives no output. Rebooted back to the liveDVD, tried using screen to generate some output to test the connection, can’t get screen to... do anything?? I have no idea what it’s supposed to do when I issue
Code:
screen /dev/ttyS0
All I get is a whole bunch of blank lines.

Any ideas for testing ttyS0? Is there any other way to get a character device to output the kernel console? Could I do some framebuffer stuff to try to get a higher res console? Can I write a log to a USB stick? I’d reaaally like to read the rest of that error.
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NeddySeagoon
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PostPosted: Fri Mar 27, 2020 11:20 pm    Post subject: Reply with quote

Bradgranath,

Welcome to Gentoo.

Code:
unknown block (8,36)
is /dev/sdc4
The message means that the kernel cannot read /dev/sdc4. There are several reasons for that.

/dev/sdc4 could be swap, so there is no file system to read.
The filesystem driver may not be available to the kernel because is made as a module but needs to be built in.
Your drive ordering has changed between installing and booting. Its more common than you think.
The Fusion drivers on on their own kernel menu and are easy to miss.
You need the menu and your card inside the menu.
Code:
$ grep FUSION /usr/src/linux/.config
# CONFIG_FUSION is not set
is the wrong answer for you.

Code:
screen /dev/ttyS0
tells screen to connect to the first real serial port on the system running screen.
It makes no attempt to set 9600,8n1 or whatever you set on the kernel command line.
minicom will give you all the knobs to play with.

If you want to use screen, you probably need setserial to set up the listening serial port.
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NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.
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Bradgranath
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PostPosted: Sat Mar 28, 2020 12:15 am    Post subject: Reply with quote

This is all excellent information. I’m aware of how common the error is, but pretty much all the advice is “make sure ext4 and fusion are added to the kernel, make sure the partition is what you think it is and where you think it is” which I did. Same error.

I’ve been able to get grub to load a much higher resolution, and I have gfxpayload set to keep but somehow this has also made the console “quiet”? Quiet is not set (neither is console at the moment) what happened?
Edit:
The output of grep FUSION

Code:
CONFIG_FUSION=y
CONFIG_FUSION_SPI is not set
CONFIG_FUSION_SAS is not set
CONFIG_FUSION_MAX_SGE=128
CONFIG_FUSION_LOGGING is not set


My device has “SAS” in the name, but I am not using any SAS drives so I did not add this to the kernel because I thought I didn’t need it. Did I get that backward?


Last edited by Bradgranath on Sat Mar 28, 2020 12:29 am; edited 2 times in total
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Jaglover
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PostPosted: Sat Mar 28, 2020 12:18 am    Post subject: Reply with quote

Don't forget to turn on support for the partition table type you use.
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Bradgranath
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PostPosted: Sat Mar 28, 2020 12:37 am    Post subject: Reply with quote

Setting CONFIG_FUSION_SAS did the trick. Thank you!
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Bradgranath
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PostPosted: Wed Apr 01, 2020 1:26 am    Post subject: Reply with quote

The serial ports were disabled in the BIOS. Flipped em on. We’ll see if that has an effect on the serial console. EDIT: Yep, that did the trick. Thanks, minicom is exactly what I was looking for. I wasn’t root, and accidentally thought it wasn’t installed. My bodge cable works! It's booting... fine? Must be having videocard modeset issue now, as serial console indicates init has started, and the keyboard is still responsive to ctrlaltdel but there is no vga console now or maybe a getty thing. Kernel issues solved! How do I mark as such?
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