linux-kernel - Re: fastboot/async and initramfs: How am I supposed to know when?devices are finished initializing?

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Date:	Sun, 29 Mar 2009 21:03:34 +0100
From:	Alexander Clouter <alex@...riz.org.uk>
To:	linux-kernel@...r.kernel.org
Subject:  Re: fastboot/async and initramfs: How am I supposed to know when?devices are finished initializing?

Thomas Bächler <thomas@...hlinux.org> wrote:
> 
> Kay Sievers schrieb:
>> In initramfs, you have to wait until the device shows up, not for a
>> random module to initialize, or a bus to be scanned -- that can never
>> work correctly, it's pure luck, that your logic was always slower than
>> the kernel.
>> 
>> You need a block device -- so you should just wait for the block
>> device, instead of making assumptions about initialization of drivers
>> or buses. :)
> 
> I am planning to do that but there is one problem: Usually, the user 
> specifies a root device, or a device that I want to open with 
> cryptsetup, or anything similar and I can wait until it shows up.
> 
> For lvm however, the user does not specify a specific block device that 
> I can wait for, instead lvm scans all available block devices. Now, how 
> do I know that the block device that contains my physical volume is 
> already there? Maybe I have several hard drives, and several volume 
> groups, so should I now call vgchange -ay again and again until the 
> right volume group shows up?
> 
If the user is specifying the boot device, could you not 'teach' them to 
LABEL the boot device and live off something slightly more fancy (but 
akin to) the following:

----
while true
do
  mount LABEL=root /whatever

  if [ $? -eq 0 ]; then
    break
  fi

  # do some md and lvm magic dance here

  sleep 1
done
----

Your initramfs could do extra steps everytime the loop...erm...loops.

Now of course the downside is LABELing mounted devices, XFS for example, 
is not an easy thing to do... :-/

Cheers

-- 
Alexander Clouter
.sigmonster says: Am I ranting?  I hope so.  My ranting gets raves.

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