[PATCH] btrfs: Use default subvolume as filesystem root

[Tom Rini]

On Sun, Aug 01, 2021 at 11:52:16PM +0300, Matwey V. Kornilov wrote:

> BTRFS volume consists of a number of subvolumes which can be mounted separately
> from each other. The top-level subvolume always exists even if no subvolumes
> were created manually. A subvolume can be denoted as the default subvolume i.e.
> the subvolume which is mounted by default.
> 
> The default "default subvolume" is the top-level one, but this is far from the
> common practices used in the wild. For instance, openSUSE provides an OS
> snapshot/rollback feature based on BTRFS. To achieve this, the actual OS root
> filesystem is located into a separate subvolume which is "default" but not
> "top-level". That means that the /boot/dtb/ directory is also located inside
> this default subvolume instead of top-level one.
> 
> However, the existing btrfs u-boot driver always uses the top-level subvolume
> as the filesystem root. This behaviour 1) is inconsistent with
> 
>     mount /dev/sda1 /target
> 
> command, which mount the default subvolume 2) leads to the issues when
> /boot/dtb cannot be found properly (see the reference).
> 
> This patch uses the default subvolume as the filesystem root to overcome
> mentioned issues.
> 
> Reference: https://bugzilla.suse.com/show_bug.cgi?id=1185656
> Signed-off-by: Matwey V. Kornilov <matwey.kornilov at gmail.com>
> Reviewed-by: Qu Wenruo <wqu at suse.com>

Applied to u-boot/master, thanks!

-- 
Tom
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