kernel: ubifs: create use file system format 4 by default

Instead of creating an ubifs file system with format version 5 by
default on empty UBI volumes use the older format 4 by default. This
will make it possible to mount these file systems also on older kernel
versions.

When a user wants to do a sysupgrade from kernel 4.14 to kernel 4.9 the
old kernel has to read the file system created by the more recent kernel
which currently does not work for ubifs.

This fixes the problem by creating file systems which are compatible
with older kernel versions by default.
Kernel 4.14 will still be able to read and write UBI FS file system
version 5, it will just not be used when a ubifs partition is created
implicitly on an empty UBI volume.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

target/linux/generic/config-4.14

@@ -4982,6 +4982,7 @@ CONFIG_TTY=y
 # CONFIG_UACCESS_WITH_MEMCPY is not set
 # CONFIG_UBIFS_ATIME_SUPPORT is not set
 # CONFIG_UBIFS_FS_ENCRYPTION is not set
+CONFIG_UBIFS_FS_FORMAT4=y
 # CONFIG_UBIFS_FS_SECURITY is not set
 # CONFIG_UBSAN is not set
 # CONFIG_UCB1400_CORE is not set

target/linux/generic/pending-4.14/553-ubifs-Add-option-to-create-UBI-FS-version-4-on-empty.patch

@@ -0,0 +1,63 @@
+From 93c33e6a7f3b0aef99d02252e6232a3d8b80f2d5 Mon Sep 17 00:00:00 2001
+From: Hauke Mehrtens <hauke@hauke-m.de>
+Date: Sun, 21 Jan 2018 15:47:50 +0100
+Subject: ubifs: Add option to create UBI FS version 4 on empty UBI volume
+
+Instead of creating an ubifs file system with format version 5 by
+default on empty UBI volumes add a compile option to create an older ubi
+with file system format version 4 instated. This allows it to be mount
+as a volume on kernel versions < 4.10, which does not support format
+version 5.
+We saw that some people can not access their older data when they
+downgraded from kernel 4.14 to kernel 4.9 to prevent this this option
+would help.
+
+Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
+---
+ fs/ubifs/Kconfig | 13 +++++++++++++
+ fs/ubifs/sb.c    |  6 ++++++
+ 2 files changed, 19 insertions(+)
+
+--- a/fs/ubifs/Kconfig
++++ b/fs/ubifs/Kconfig
+@@ -74,3 +74,16 @@ config UBIFS_FS_SECURITY
+ 	  the extended attribute support in advance.
+ 
+ 	  If you are not using a security module, say N.
++
++config UBIFS_FS_FORMAT4
++	bool "Use file system format version 4 for new file systems"
++	depends on UBIFS_FS
++	help
++	  Instead of creating new file systems with the new ubifs file
++	  system version 5, use the old format version 4 for implicitly
++	  by the driver created file systems on an empty UBI volume. This
++	  makes it possible to  mount these file systems also with kernel
++	  versions before 4.10. The driver will still support file system
++	  format version 5 for ubifs file systems created with version 5.
++
++	  If you are unsure, say N.
+--- a/fs/ubifs/sb.c
++++ b/fs/ubifs/sb.c
+@@ -176,7 +176,9 @@ static int create_default_filesystem(str
+ 	tmp64 = (long long)max_buds * c->leb_size;
+ 	if (big_lpt)
+ 		sup_flags |= UBIFS_FLG_BIGLPT;
++#ifndef CONFIG_UBIFS_FS_FORMAT4
+ 	sup_flags |= UBIFS_FLG_DOUBLE_HASH;
++#endif
+ 
+ 	sup->ch.node_type  = UBIFS_SB_NODE;
+ 	sup->key_hash      = UBIFS_KEY_HASH_R5;
+@@ -192,7 +194,11 @@ static int create_default_filesystem(str
+ 	sup->jhead_cnt     = cpu_to_le32(DEFAULT_JHEADS_CNT);
+ 	sup->fanout        = cpu_to_le32(DEFAULT_FANOUT);
+ 	sup->lsave_cnt     = cpu_to_le32(c->lsave_cnt);
++#ifdef CONFIG_UBIFS_FS_FORMAT4
++	sup->fmt_version   = cpu_to_le32(4);
++#else
+ 	sup->fmt_version   = cpu_to_le32(UBIFS_FORMAT_VERSION);
++#endif
+ 	sup->time_gran     = cpu_to_le32(DEFAULT_TIME_GRAN);
+ 	if (c->mount_opts.override_compr)
+ 		sup->default_compr = cpu_to_le16(c->mount_opts.compr_type);