Handle unaligned read at fs layer?

[Qu Wenruo]

Hi U-boot fs guys,

With my previous rework on U-boot btrfs, and my own btrfs-fuse project,
it turns out that, although U-boot implements its fs with a very fuse
like interface, there are still some quality-of-life features missing.

One of the most obvious one is making each fs to handle unaligned read.

This is especially obvious when comparing btrfs-fuse btrfs_file_read()
against U-boot btrfs_file_read().

They share almost the same main loop, but btrfs_file_read() in U-boot
have two extra handling, for unaligned leading sector and unaligned
tailing sector.


So I just did a quick check on the other fses, it turns out that we have
a very different behaviors:

- FAT
- Ext4
   Have the handling for both tailing and ending block.

- Btrfs
   My code, doing unaligned leading/tailing block manually.
   (the tailing part looks unnecessary though)

- Ubifs
   Explicitly reject read with non-aligned offset.

- Squshfs
   Explicitly reject read with non-zero offset.

- Erofs
   Have the handling for tailing unaligned part, but doesn't seem to
   handle unaligned start part.

Thus if we have a proper common routine for unaligned read routine (get
the file length, read the first and the last unaligned block, then let
the .read callback to handle the aligned range), then all fs drivers
would have a much better life.

Mind me to do such small but quality-of-life improve for all involved fses?
(This will need a new call back to query the blocksize of each fs though)

Thanks,
Qu