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, 9 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 3d854c9ae53a..67fa3fdc128f 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2365,6 +2365,15 @@ and is between 256 and 4096 characters. It is defined in the file
 
         switches=       [HW,M68k]
 
+        sysfs.deprecated=0|1 [KNL]
+                        Enable/disable old style sysfs layout for old udev
+                        on older distributions. When this option is enabled
+                        very new udev will not work anymore. When this option
+                        is disabled (or CONFIG_SYSFS_DEPRECATED not compiled)
+                        in older udev will not work anymore.
+                        Default depends on CONFIG_SYSFS_DEPRECATED_V2 set in
+                        the kernel configuration.
+
         sysrq_always_enabled
                         [KNL]
                         Ignore sysrq setting - this boot parameter will