nohz: Fix old dynticks idle Kconfig backward compatibility

In order to enforce backward compatibility with older
config files, we want the new dynticks-idle Kconfig entry
to default its value to the one of the old CONFIG_NO_HZ symbol
if present.

Namely we want:

	config NO_HZ # old obsolete dynticks idle symbol
		bool

	config NO_HZ_IDLE # new dynticks idle symbol
		default NO_HZ

However Kconfig prevents this to work if the old symbol
is not visible. And this is currently the case because
NO_HZ lacks a title in order to show it in make oldconfig
and alike.

To fix this, bring a minimal title and help text to the
obsolete Kconfig entry that explains its purpose. This
makes the "defaulting" to work.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 8 insertions, 4 deletions

diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index 27cc404ea187..cbe64be17d1f 100644
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -75,10 +75,6 @@ config NO_HZ_COMMON
         depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS
         select TICK_ONESHOT
 
-# Kept around for compatibility, maps to NO_HZ_IDLE
-config NO_HZ
-        bool
-
 choice
         prompt "Timer tick handling"
         default NO_HZ_IDLE if NO_HZ
@@ -130,6 +126,14 @@ config NO_HZ_EXTENDED
 
 endchoice
 
+config NO_HZ
+        bool "Old Idle dynticks config"
+        depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS
+        help
+          This is the old config entry that enables dynticks idle.
+          We keep it around for a little while to enforce backward
+          compatibility with older config files.
+
 config HIGH_RES_TIMERS
         bool "High Resolution Timer Support"
         depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS