cpu hotplug ppc64 bug

On Power4 and earlier hardware there is no need to clear the CPPR (see RPAp 479 section 18.5.4.7.2 for what little info there is on the CPPR) when stopping a cpu. On hardware that uses Power5 an undocumented change has been made that requires the CPPR to be cleared if an isolate is to be done on the stopped cpu. So the following patch lets cpu hotplug work on the recent hardware.

I sent this patch to the ppc64-dev list back in mid April and Suse picked it up then for SLES9 so it has been well tested for several months.

# This is a BitKeeper generated patch for the following project:
# Project Name: Linux kernel tree
# This patch format is intended for GNU patch command version 2.5 or higher.
# This patch includes the following deltas:
# ChangeSet 1.1883 -> 1.1884
# include/asm-ppc64/xics.h 1.5 -> 1.6
# arch/ppc64/kernel/xics.c 1.45 -> 1.46
# arch/ppc64/kernel/smp.c 1.72 -> 1.73
#
# The following is the BitKeeper ChangeSet Log
# --------------------------------------------
# 04/07/26 [email protected] 1.1884
# Power5 has a requirement to clear the CPPR before doing a stop_self or isolates will fail later on. Clearing the CPPR is OK on Power4 so just

# added it to the path.
# --------------------------------------------
#
diff -Nru a/arch/ppc64/kernel/smp.c b/arch/ppc64/kernel/smp.c
--- a/arch/ppc64/kernel/smp.c	Mon Jul 26 18:26:45 2004
+++ b/arch/ppc64/kernel/smp.c	Mon Jul 26 18:26:45 2004
@@ -300,6 +300,10 @@
 void cpu_die(void)
 {
 	local_irq_disable();
+	/* Some hardware requires clearing the CPPR, while other hardware does not
+	 * it is safe either way
+	 */
+	pSeriesLP_cppr_info(0, 0);
 	rtas_stop_self();
 	/* Should never get here... */
 	BUG();
diff -Nru a/arch/ppc64/kernel/xics.c b/arch/ppc64/kernel/xics.c
--- a/arch/ppc64/kernel/xics.c	Mon Jul 26 18:26:45 2004
+++ b/arch/ppc64/kernel/xics.c	Mon Jul 26 18:26:45 2004
@@ -190,7 +190,7 @@
 		      val64); 
 }

-static void pSeriesLP_cppr_info(int n_cpu, u8 value) +void pSeriesLP_cppr_info(int n_cpu, u8 value)
{
unsigned long lpar_rc;

diff -Nru a/include/asm-ppc64/xics.h b/include/asm-ppc64/xics.h
@@ -19,6 +19,9 @@
 void xics_setup_cpu(void);
 void xics_cause_IPI(int cpu);
 
+/* first argument is ignored for now*/
+void pSeriesLP_cppr_info(int n_cpu, u8 value);
+
 struct xics_ipi_struct {
 	volatile unsigned long value;
 } ____cacheline_aligned;

I think that function pointers are a good idea too. But I'm not sure why that should hold up this patch. As it stands now all ppc64 platforms that do hotplug happen to be pSeries. I don't know of any other ppc64 platforms that are not pSeries who plan on doing this in the next few years. The function is already pSeries specific anyway with the rtas_stop_self call.

Meanwhile if we do not put this patch in all Power5 machines will be unable to do cpu hotplug with a mainline kernel.