[POWERPC] Cell timebase bug workaround

The Cell CPU timebase has an erratum. When reading the entire 64 bits
of the timebase with one mftb instruction, there is a handful of cycles
window during which one might read a value with the low order 32 bits
already reset to 0x00000000 but the high order bits not yet incremeted
by one. This fixes it by reading the timebase again until the low order
32 bits is no longer 0. That might introduce occasional latencies if
hitting mftb just at the wrong time, but no more than 70ns on a cell
blade, and that was considered acceptable.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>

1 files changed, 14 insertions, 4 deletions

diff --git a/include/asm-powerpc/ppc_asm.h b/include/asm-powerpc/ppc_asm.h
index a940cfe040da..fa083d8e4663 100644
--- a/include/asm-powerpc/ppc_asm.h
+++ b/include/asm-powerpc/ppc_asm.h
@@ -30,9 +30,9 @@ BEGIN_FTR_SECTION;							\
         mfspr   ra,SPRN_PURR;           /* get processor util. reg */   \
 END_FTR_SECTION_IFSET(CPU_FTR_PURR);                                    \
 BEGIN_FTR_SECTION;                                                      \
-        mftb    ra;                     /* or get TB if no PURR */      \
+        MFTB(ra);                       /* or get TB if no PURR */      \
 END_FTR_SECTION_IFCLR(CPU_FTR_PURR);                                    \
-        ld      rb,PACA_STARTPURR(r13);                         \
+        ld      rb,PACA_STARTPURR(r13);                                 \
         std     ra,PACA_STARTPURR(r13);                                 \
         subf    rb,rb,ra;               /* subtract start value */      \
         ld      ra,PACA_USER_TIME(r13);                                 \
@@ -45,9 +45,9 @@ BEGIN_FTR_SECTION;							\
         mfspr   ra,SPRN_PURR;           /* get processor util. reg */   \
 END_FTR_SECTION_IFSET(CPU_FTR_PURR);                                    \
 BEGIN_FTR_SECTION;                                                      \
-        mftb    ra;                     /* or get TB if no PURR */      \
+        MFTB(ra);                       /* or get TB if no PURR */      \
 END_FTR_SECTION_IFCLR(CPU_FTR_PURR);                                    \
-        ld      rb,PACA_STARTPURR(r13);                         \
+        ld      rb,PACA_STARTPURR(r13);                                 \
         std     ra,PACA_STARTPURR(r13);                                 \
         subf    rb,rb,ra;               /* subtract start value */      \
         ld      ra,PACA_SYSTEM_TIME(r13);                               \
@@ -274,6 +274,16 @@ END_FTR_SECTION_IFSET(CPU_FTR_601)
 #define ISYNC_601
 #endif
 
+#ifdef CONFIG_PPC_CELL
+#define MFTB(dest)                      \
+90:     mftb  dest;                     \
+BEGIN_FTR_SECTION_NESTED(96);           \
+        cmpwi dest,0;                   \
+        beq-  90b;                      \
+END_FTR_SECTION_NESTED(CPU_FTR_CELL_TB_BUG, CPU_FTR_CELL_TB_BUG, 96)
+#else
+#define MFTB(dest)                      mftb dest
+#endif
 
 #ifndef CONFIG_SMP
 #define TLBSYNC