powerpc: Use correct sequence for putting CPU into nap mode

We weren't using the recommended sequence for putting the CPU into
nap mode.  When I changed the idle loop, for some reason 7447A cpus
started hanging when we put them into nap mode.  Changing to the
recommended sequence fixes that.

The complexity here is that the recommended sequence is a loop that
keeps putting the cpu back into nap mode.  Clearly we need some way
to break out of the loop when an interrupt (external interrupt,
decrementer, performance monitor) occurs.  Here we use a bit in
the thread_info struct to indicate that we need this, and the exception
entry code notices this and arranges for the exception to return
to the value in the link register, thus breaking out of the loop.
We use a new `local_flags' field in the thread_info which we can
alter without needing to use an atomic update sequence.

The PPC970 has the same recommended sequence, so we do the same thing
there too.

This also fixes a bug in the kernel stack overflow handling code on
32-bit, since it was causing a value that we needed in a register to
get trashed.

Signed-off-by: Paul Mackerras <paulus@samba.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 2cdc35ce8045..65f67f986156 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -366,6 +366,7 @@ config PPC_PMAC64
         select U3_DART
         select MPIC_BROKEN_U3
         select GENERIC_TBSYNC
+        select PPC_970_NAP
         default y
 
 config PPC_PREP
@@ -383,6 +384,7 @@ config PPC_MAPLE
         select MPIC_BROKEN_U3
         select GENERIC_TBSYNC
         select PPC_UDBG_16550
+        select PPC_970_NAP
         default n
         help
           This option enables support for the Maple 970FX Evaluation Board.
@@ -457,6 +459,10 @@ config PPC_MPC106
         bool
         default n
 
+config PPC_970_NAP
+        bool
+        default n
+
 source "drivers/cpufreq/Kconfig"
 
 config CPU_FREQ_PMAC