[PATCH V10] Add support for BTRFS raid5/6 to GRUB

[Goffredo Baroncelli <kreijack@libero.it>]

Hi All,

the aim of this patches set is to provide support for a BTRFS raid5/6
filesystem in GRUB.

The first patch, implements the basic support for raid5/6. I.e this works when
all the disks are present.

The next 5 patches, are preparatory ones.

The 7th patch implements the raid5 recovery for btrfs (i.e. handling the
disappearing of 1 disk).
The 8th patch makes the code for handling the raid6 recovery more generic.
The last one implements the raid6 recovery for btrfs (i.e. handling the
disappearing up to two disks).

I tested the code in grub-emu, and it works both with all the disks,
and with some disks missing. I checked the crc32 calculated from grub and
from linux and these matched. Finally I checked if the support for md raid6
still works properly, and it does (with all drives and with up to 2 drives
missing)

Comments are welcome.

Changelog
v1: initial support for btrfs raid5/6. No recovery allowed
v2: full support for btrfs raid5/6. Recovery allowed
v3: some minor cleanup suggested by Daniel Kiper; reusing the
    original raid6 recovery code of grub
v4: Several spell fix; better description of the RAID layout
    in btrfs, and the variables which describes the stripe
    positioning; split the patch #5 in two (#5 and #6)
v5: Several spell fix; improved code comment in patch #1, small
    clean up in the code
v6: Small cleanup; improved the wording in the RAID6 layout
    description; in the function raid6_recover_read_buffer() avoid
    a unnecessary memcpy in case of invalid data;
v7: - patch 2,3,5,6,8 received an Review-by Daniel, and were unchanged from
    the last time (only minor cleanup in the commit description requested by
    Daniel)
    - patch 7 received some small update rearranging a for(), and some
    bracket around if()
    - patch 4, received an update message which explains better why NULL
    is stored in data->devices_attached[]
    - patch 9, received a blank line to separate better a code line from
    a previous comment. A description of 'parities_pos' was added
    - patch 1, received a major update about the variable meaning description
    in the comment. However I suspect that we need some further review to reach
    a fully agreement about this text. NB: the update are relate only to
    comments
v8: - patch 2,5,6,8 received an Review-by Daniel, and were unchanged from
    the last time (only minor cleanup in the commit description requested by
    Daniel)
    - patch 1 received some adjustement to the variables description due to
      the different terminology between BTRFS and other RAID implementatio.
      Added a description for the "nparities" variable.
    - patch 3 removed some unnecessary curly brackets (change request by Daniel)
    - patch 4 received an improved commit description about why and how
      the function find_device() is changed
    - patch 7 received an update which transforms a i = 0; while(i..) i++; in
      for( i = 0..... ; i++);
    - patch 9 received an update to the comment
v9: - patch 1: update comments
    - patch 4: update commit messages
    - patch 7: added a comment about accessing an array of structs
      after another structs; changed if(err == GRUB_ERR_NONE) in if(!err)
      changed if(err != GRUB_ERR_NONE) in if(err)

v10:- patch 1: update comments (replace might with can)
    - patch 4: add a Signed off by Daniel
    - patch 7: drop an empty line; changed some text in grub_dprintf;
      reversed the logic of an if (if(!is_raid56) A else B -> if(is_raid5) B
      else A); add a space between the function name and the '('
    - patch 9: update the wording in the comment; s/raid6/RAID 6/ in 
      grub_dprintf()

BR
G.Baroncelli

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