Re: [PATCH 1/2] ubi: mount partitions specified in device tree

Am 19.06.2016 um 01:20 schrieb Daniel Golle:

In MBR there used to be an 'active' flag stored for each partition.
Maybe it'd be nice to introduce something similar to mark UBI volumes
to be the default rootfs.
Currently we solve this issue by convention: If a volume is named
'rootfs' it is automatically mounted during boot. Depending on the
filesystem in use a ubiblock device has to be created as well.
This is mostly just the continuation of the existing naming convention
of mtd partitions, a patch OpenWrt is carrying around for a long
while already.
To support the same on UBI, another set of patches was made.

Sorry, I still have troubles to understand your use case.
Both of you seem to hate the kernel command line for reasons
I don't fully understand so far.

I agree that there should be a way to pass this through the of_node
of the mtd partition which is defined in the device tree.
Selecting to-be-ubi-attached mtd partitions in device-tree would
replace patch:

https://git.lede-project.org/?p=source.git;a=blob;f=target/linux/generic/patches-4.4/490-ubi-auto-attach-mtd-device-named-ubi-or-data-on-boot.patch

What is the need of this? Use use the kernel command line to tell UBI from which MTD to attach.

To auto-select the rootfs, we currently carry another bunch of patches

https://git.lede-project.org/?p=source.git;a=blob;f=target/linux/generic/patches-4.4/491-ubi-auto-create-ubiblock-device-for-rootfs.patch

Same question here.

https://git.lede-project.org/?p=source.git;a=blob;f=target/linux/generic/patches-4.4/492-try-auto-mounting-ubi0-rootfs-in-init-do_mounts.c.patch

Ditto.

https://git.lede-project.org/?p=source.git;a=blob;f=target/linux/generic/patches-4.4/493-ubi-set-ROOT_DEV-to-ubiblock-rootfs-if-unset.patch

Ditto.

This is more or less filesystem-agnostic and works fine as long as
there is only one volume called 'rootfs' and this volume is always
used as rootfs.

Dual-boot setups will need some way to pass the active rootfs volume to
the kernel. While I agree that this is possible by appending or
prepending to the cmdline string passed to the kernel, this either
limits the users' ability to manually specify the rootfs using the
cmdline or becomes a more complex task to only append/prepend the
cmdline in case the user-defined string doesn't already contain
relevant parameters...

Sorry, but this is just a tooling problem and not to be addressed in the kernel.
There is also the possibility to use an initramfs (either as file or embedded in the kernel)
if the mount/attach logic becomes *really* complicated...

Thus it'd be nicer to flag the default rootfs volume via the device-
tree.

As I said, as far I'm informed device tree is for configuring Linux, it describes
the hardware. We also don't have LVM, DM or iSCSI bindings in DT. ;)
Maybe device tree folks will tell more...

Thanks,
//richard
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