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, 7 insertions, 3 deletions

diff --git a/arch/powerpc/kernel/idle_power4.S b/arch/powerpc/kernel/idle_power4.S
index 6dad1c02496e..d85c7c938eed 100644
--- a/arch/powerpc/kernel/idle_power4.S
+++ b/arch/powerpc/kernel/idle_power4.S
@@ -35,12 +35,16 @@ BEGIN_FTR_SECTION
         DSSALL
         sync
 END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)
+        clrrdi  r9,r1,THREAD_SHIFT      /* current thread_info */
+        ld      r8,TI_LOCAL_FLAGS(r9)   /* set napping bit */
+        ori     r8,r8,_TLF_NAPPING      /* so when we take an exception */
+        std     r8,TI_LOCAL_FLAGS(r9)   /* it will return to our caller */
         mfmsr   r7
         ori     r7,r7,MSR_EE
         oris    r7,r7,MSR_POW@h
-        sync
+1:      sync
         isync
         mtmsrd  r7
         isync
-        sync
-        blr
+        b       1b
+