Re: [PATCH] powerpc: Cell timebase bug workaround

On Mon, 2006-10-09 at 16:56 +1000, Stephen Rothwell wrote:

On Mon, 09 Oct 2006 15:10:22 +1000 Benjamin Herrenschmidt [off-list ref] wrote:

quoted

Index: linux-work/arch/powerpc/kernel/vdso64/gettimeofday.S
===================================================================

--- linux-work.orig/arch/powerpc/kernel/vdso64/gettimeofday.S	2006-10-06 13:47:54.000000000 +1000
+++ linux-work/arch/powerpc/kernel/vdso64/gettimeofday.S	2006-10-09 13:28:54.000000000 +1000

@@ -11,6 +11,8 @@
  * as published by the Free Software Foundation; either version
  * 2 of the License, or (at your option) any later version.
  */
+
+#include <linux/config.h>

You don't need that (in fact someone just remoed them all).

Oops, leftover of a previous patch. I'll remove it.

quoted

 #ifdef __ASSEMBLY__

-#define BEGIN_FTR_SECTION		98:
+#define BEGIN_FTR_SECTION_NESTED(label)	label:
+#define BEGIN_FTR_SECTION		BEGIN_FTR_SECTION_NESTED(98)

 #ifndef __powerpc64__
-#define END_FTR_SECTION(msk, val)		\
+#define END_FTR_SECTION_NESTED(msk, val, label) \
 99:						\
 	.section __ftr_fixup,"a";		\
 	.align 2;				\
 	.long msk;				\
 	.long val;				\
-	.long 98b;				\
+	.long label##b;				\
 	.long 99b;				\
 	.previous
 #else /* __powerpc64__ */
-#define END_FTR_SECTION(msk, val)		\
+#define END_FTR_SECTION_NESTED(msk, val, label) \
 99:						\
 	.section __ftr_fixup,"a";		\
 	.align 3;				\
 	.llong msk;				\
 	.llong val;				\
-	.llong 98b;				\
+	.llong label##b;				\
 	.llong 99b;	 			\
 	.previous
 #endif /* __powerpc64__ */

+#define END_FTR_SECTION(msk, val)		\
+	END_FTR_SECTION_NESTED(msk, val, 98)
+

The above should be in a separate patch.

Hrm... makes my life more complicated but I suppose I can split it.

quoted

+#ifdef CONFIG_PPC_CELL
+#define MFTB(dest)			\
+	mftb  dest;			\
+BEGIN_FTR_SECTION;			\
+	cmpwi dest,0;			\
+	bne+  97f;			\
+	mftb  dest;			\
+97:					\
+END_FTR_SECTION_IFSET(CPU_FTR_CELL_TB_BUG)

Why not branch back to the mftb if the lower word is zero (this would
also take care of the very unlikely "hitting the bug more than once" case
and the "normal" case would then not have to branch at all.  (And it uses
one less instruction.)

Because that would cause us to spinloop for the whole duration of a tb
tick in that case, which is pretty bad when the timebase is slow (the
slower I've seen so far is 14Mhz).

Ben.