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);
