[Buildroot] user question about ubi nand

Wed Feb 24 23:00:56 UTC 2016

On 02/24/16 04:34, Steve Calfee wrote:
> Ubi seems very well supported in buildroot and linux. The problem is
> figuring out how to use it.
> 
> I have created two nand partitions, one rootfs and one data. 

 That's already a bad start, because this way you reduce the wear levelling
opportunities. The best way to use UBI is to put everything in a single
partition, and create several UBI volumes within that partition. The canonical
flash layout is:

4   EB SPL (=pre-bootloader)
16  EB bootloader
8   EB bootloader environment (only relevant for U-Boot I guess)
the rest UBI
  2 volumes kernel
  2 volumes DTB
  2 volumes rootfs
  1 volume  rwdata
 (1 volume  per-device ro data is also often needed)

kernel, DTB and rootfs have two copies for upgrade, i.e. you write to a new
volume while keeping the currently running version alive.

> I would
> like the rootfs to be readonly and data to be r/w. After tons of
> searches I have configured the rootfs and stored the buildroot built
> image in it.
> 
> My question is how do I move r/w areas from rootfs into the r/w
> partition. Examples are /etc/ files /run files which start as /var/run
> and finally application files like mysql databases which are in
> /usr/share/.

 For this, there is no standard way because there are several ways to do it.

- Use some kind of overlayfs so that your readonly rootfs acts as if it's wriitable.

- Move the files that should be writeable to the rw partition and create
symlinks for them.

- Move the files that should be writeable to tmpfs and generate them at boot time.

- Configure packages to search for the writable files in the rw partition
instead of their usual location.

> I assume the post scripts can be used, but I cannot find
> examples. In post scripts how do I set permissions/user/group fields.

 For ownership and permissions, there is BR2_ROOTFS_DEVICE_TABLE. post-build
script doesn't help because you have to do that as root, and the post-build
script is executed as an ordinary user.

> How do I link to a mounted partition.
> 
> http://free-electrons.com/blog/creating-flashing-ubi-ubifs-images/ has
> lots of info. But say I want a custom ubi.cfg file, where do I put it
> or how do i menuconfig it? 

BR2_TARGET_ROOTFS_UBI_USE_CUSTOM_CONFIG

> Say I want 2 partitions, one ro and one rw
> I assume I need a special cfg file. 

 Buildroot will not help you to create the rw partition. In many systems, it is
created at boot time if it doesn't exist yet. That's also a simple way to do a
factory reset: just remove the rw volume. But you can also create it at build
time (using mkubifs) in the post-build script, and specify it in the custom ubi.cfg.

> Ubi asks for lots of magic
> numbers, how do I get them - such as erasesize, subpagesize, offsets

 On the device, you can use mtdinfo to get these. But unless you're using an
off-the-shelf board, you already had to specify these values in the device tree.

> etc? I assume defaults are good, but how can I know. Typical Linux
> stuff, however these thing are knowable, how do I find it out (via web
> searches etc). 

> How do I do set the config file in BR2_EXTERNAL?

 I don't see how this is related to ubi... Actually I don't understand the
question even.

> 
> All info I find is schizophrenic. Net searches say how to do things
> like this in uboot and linux. Sometimes in the same doc paragraph. It
> is never clear which instruction matches which of uboot and linux. For
> buildroot the best is stuff mainly done in the build, but something
> must be done on the target. What is the recommended procedure for mass
> building systems?

 You almost always need to do something at first boot, some kind of provisioning
e.g. to set MAC addresses. What I often see is that you build a specific
production image, flash that one in the factory (or put it on an SD card and
boot from that). This production image does some hardware tests, does
provisioning, and downloads the up-to-date application image to write to flash.
So it has a completely different configuration than the application image.

> 
> What if the nand and rootfs is bigger than ram? How do you update it in uboot?

 You don't want to do updates from the bootloader. You want to use swupdate,
though I'm not sure if it supports streaming upgrade. You also want to use a
recent kernel that support atomic volume rename, then you don't need any
involvement of the bootloader to do upgrades.

 Oh, and you probably want to hire someone like to figure all this out for you :-)

 Regards,
 Arnout

> 
> Steve
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> 

-- 
Arnout Vandecappelle                          arnout at mind be
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