linux-kernel - Re: No /dev/root with devtmpfs?

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Message-ID: <CAPXgP12W8mTOX5-86sBCJtLACMJWo3ovAiH91VF4JoBq9kxnQA@mail.gmail.com>
Date:	Mon, 13 Feb 2012 20:30:37 +0100
From:	Kay Sievers <kay.sievers@...y.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Kirill Smelkov <kirr@....spb.ru>,
	Paul Parsons <lost.distance@...oo.com>,
	linux-kernel@...r.kernel.org
Subject: Re: No /dev/root with devtmpfs?

On Mon, Feb 13, 2012 at 19:48, H. Peter Anvin <hpa@...or.com> wrote:
> On 02/13/2012 12:25 AM, Kirill Smelkov wrote:
>>>
>>> The bootloader might pass the root partition to the kernel via the
>>> "root=" kernel parameter; these can be read at /proc/cmdline.
>>> For example, on an HP iPAQ hx4700: root=/dev/mtdblock2
>>
>> This does not work universally either, for example for
>>
>>     root=PARTUUID=00112233-4455-6677-8899-AABBCCDDEEFF
>>
>>
>> the kernel scans partitions and finds one for root by partition uuid
>> (see name_to_dev_t() in init/do_mounts.c).
>>
>> That's exactly my situation - I have universal flash image which can
>> bee booted via CF slot (on several boards, thus will have different
>> /dev/hd? or /dev/sd?) and via usb/cardreader (again different /dev/
>> entry), and root is mounted by partition id.
>>
>> The kernel has no problem finding root partition and mounting it. I just
>> needed a sane and robust way to know its choice.
>
> In this case it's not the kernel, but the initramfs which does this
> lookup.

root=PARTUUID= is in-kernel mount GTP support, which works without an initramfs.

> In that case the initramfs could/should create /dev/root.

We dropped all that in initramfs for the earlier in this thread
mentioned reasons.

/dev/root is not what we want, or what tools should expect to be there
today; it will not work with modern filesystems, which have no
single-device relationship.

Kay
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