Static UBI volumes and ubiupdatevol ?

[Ricard Wanderlof]

On Fri, 17 Dec 2010, Artem Bityutskiy wrote:

>>>> With a system based on mtd+jffs2 it is actually possible to reflash the
>>>> partition with the [read only] root file system on it without unmounting,
>>>> as long as no accesses are made to the physical flash during the rewrite
>>>> or afterwards. In other words, immediately after reflashing the system
>>>> must be rebooted.
>>>
>>> Hmm, I presume you can do this with UBIFS, but you need to make sure all
>>> the backgound stuff like moving and eraseing eraseblocs is done before
>>> doing this by doing fsync() on your /dev/ubiX_Y device (UBI volume
>>> character device), see the 'vol_cdev_fsync()' function. Ah, and of
>>> course you need to sync UBIFS before this.
>>>
>>> Then if you can guarantee that no one is reading writing to it, you can
>>> change the flash.
>>
>> It would be nice to use ubiupdatevol to do this, but it won't let me do
>> that if the volume is mounted (naturally). I don't know if this is
>> something that is enforced by ubiupdatevol or ubi. I would assume the
>> latter but I haven't checked the code.
>>
>> Of course I could always write my own "ubiupdatevol" to deal with this but
>> it's always nice to use existing and supported tools.
>
> This is UBI - it does not allow updating if someone keeps the volume
> open. When UBI is mounted, it keeps the volume open. It closes the
> volume when unmounted.

Yes, it make definitely makes sense to have it work this way to avoid 
problems.

> It looks error-prone to allow updating while there are users, from UBI
> POW, I think.

I agree. It just happens to be the way our firmware update process works 
currently ... :-)

> And I cannot think of a nice way of doing this.
>
> Why can't you just
> 1. lazily unmount UBIFS
> 2. find all users of UBIFS and kill them
> 3. this will make UBIFS finally unmount
> 4. you safely update the UBI volume

The problem is that in this case it is our root file system which is to be 
reflashed, so we can't just unmount it. I have an idea about using 
pivot_root though, in a similar manner to the way the root file system is 
switched from initrd to disk during an ordinary Linux boot with initrd.

Something like
1. Close down everything.
2. Create a tmpfs and mount it.
3. Copy all necessary binaries and libraries to the tmpfs.
4. Use pivot_root to switch root file systems.
5. Unmount the original (UBIFS) root file system.
6. Rewrite the root file system using ubiupdatevol.
7. Reboot.

Right now I'm stumped at how to restart init in order for it to relase its 
stdin,stdout and stderr, which still refer to the (old) /dev/console, but 
there must be a solution somehow as the same situation must occur with an 
ordinary (initrd) kernel booting.

/Ricard
-- 
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30