linux-kernel - boot device order troubleshooting without an initrd

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Date:	Mon, 19 Apr 2010 00:29:54 +0200
From:	Jan Kundrát <jkt@...too.org>
To:	linux-kernel@...r.kernel.org
Subject: boot device order troubleshooting without an initrd

Hi folks,
I'm looking for a way to pass a correct "root" parameter to the kernel
from the bootloader, independently on the number of attached disks. My
machine (VIA EPIA SN-1800) has four SATA ports and one CF card slot. The
CF slot is visible as an IDE device. The BIOS is configured for booting
from the CF card, and Grub2 has absolutely no problems booting the kernel.

The problem I'm facing is that if there are no SATA disks attached, my
CF card gets called /dev/sda, while if I attach two SATA drives, the CF
card gets called /dev/sdc. I can solve that "in userspace" without much
problem thanks to udev's names like
/dev/disk/by-id/scsi-SATA_ELITE_PRO_CF_..., but I have no idea about how
to pass the correct "root" argument to the kernel. If I use
"root=/dev/sda1", kernel will boot if and only if I don't have any SATA
devices attached; on the other hand, having two SATA drives attached
requires "root=/dev/sdc1". I've tried to use the by-id naming, but it is
apparently provided by udev much later in the "boot process", and the
document by gregkh [1] doesn't mention it, either.

I have disabled modules, and don't use initrd. Such a setup has worked
for me for a few years. Not having modules enabled makes me feel
"safer", and I have not ever needed an initrd, so far. If I was using
initrd, I'd probably be able to use the "root=LABEL=foo" stuff.

Relevant part of `lspci`:

00:0f.0 SATA controller: VIA Technologies, Inc. SATA RAID Controller
(rev 20)
00:0f.1 IDE interface: VIA Technologies, Inc.
VT82C586A/B/VT82C686/A/B/VT823x/A/C PIPC Bus Master IDE (rev 07)

I'm attaching full dmesg, as the output seems to be interleaved from
more drivers, and I'm afraid I'd skip something if I tried to include
only relevant parts. The kernel I'm using is 2.6.32-gentoo-r1.

What I'm looking for is an advice if it's possible to tell kernel to
"use the BIOS-provided HDD ordering". I suspect I'm likely going to be
told "you are supposed to use initrd"; if that is the case, I'd love to
see a technical argument about why.

Anyway, I'm looking for any input. I'd much appreciate if you could Cc
me on replies, as I'm not subscribed to the list.

Have fun,
-jkt

[1]
http://www.kernel.org/pub/linux/kernel/people/gregkh/lkn/lkn_pdf/ch09.pdf

-- 
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