SYSFS: Allow boot time switching between deprecated and modern sysfs layout

I have some systems which need legacy sysfs due to old tools that are
making assumptions that a directory can never be a symlink to another
directory, and it's a big hazzle to compile separate kernels for them.

This patch turns CONFIG_SYSFS_DEPRECATED into a run time option
that can be switched on/off the kernel command line. This way
the same binary can be used in both cases with just a option
on the command line.

The old CONFIG_SYSFS_DEPRECATED_V2 option is still there to set
the default. I kept the weird name to not break existing
config files.

Also the compat code can be still completely disabled by undefining
CONFIG_SYSFS_DEPRECATED_SWITCH -- just the optimizer takes
care of this now instead of lots of ifdefs. This makes the code
look nicer.

v2: This is an updated version on top of Kay's patch to only
handle the block devices. I tested it on my old systems
and that seems to work.

Cc: axboe@kernel.dk
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 22 insertions, 4 deletions

diff --git a/init/Kconfig b/init/Kconfig
index 137609f33ebc..d742b6fca8d2 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -660,8 +660,12 @@ config SYSFS_DEPRECATED
         depends on SYSFS
         default n
         help
-          This option switches the layout of the "block" class devices, to not
-          show up in /sys/class/block/, but only in /sys/block/.
+          This option adds code that switches the layout of the "block" class
+          devices, to not show up in /sys/class/block/, but only in
+          /sys/block/.
+
+          This switch is only active when the sysfs.deprecated=1 boot option is
+          passed or the SYSFS_DEPRECATED_V2 option is set.
 
           This option allows new kernels to run on old distributions and tools,
           which might get confused by /sys/class/block/. Since 2007/2008 all
@@ -672,8 +676,22 @@ config SYSFS_DEPRECATED
           option enabled.
 
           Only if you are using a new kernel on an old distribution, you might
-          need to say Y here. Never say Y, if the original kernel, that came
-          with your distribution, has not set this option.
+          need to say Y here.
+
+config SYSFS_DEPRECATED_V2
+        bool "enabled deprecated sysfs features by default"
+        default n
+        depends on SYSFS
+        depends on SYSFS_DEPRECATED
+        help
+          Enable deprecated sysfs by default.
+
+          See the CONFIG_SYSFS_DEPRECATED option for more details about this
+          option.
+
+          Only if you are using a new kernel on an old distribution, you might
+          need to say Y here. Even then, odds are you would not need it
+          enabled, you can always pass the boot option if absolutely necessary.
 
 config RELAY
         bool "Kernel->user space relay support (formerly relayfs)"