[PATCH 5/5] powerpc: Cell timebase bug workaround
Benjamin Herrenschmidt
benh at kernel.crashing.org
Fri Oct 13 18:04:36 EST 2006
The Cell CPU timebase has an errata. 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. Simply reading the timebase a second time if the low order bits
are 0 is enough (the second read will always miss that window).
Note that there is still a potential issue if the process gets
interrupted between the 2 reads for long enough that the second reads
pricely hits that same window on the next 32 bits wrap. However, this is
so extremely unlikely to happen that we choose not to do anything
against it (the 32 bits wrap happens every few seconds or so, in fact
every few minutes on IBM cell blades).
Signed-off-by: Benjamin Herrenschmidt <benh at kernel.crashing.org>
---
This is the "old" patch adapted to apply on top of the previous changes.
I'll post a new version of the patch using a different technique for the
workaround asap. (should have been today but testing & debugging the
previous patches of this serie took more time than expected).
Index: linux-cell/arch/powerpc/kernel/vdso64/gettimeofday.S
===================================================================
--- linux-cell.orig/arch/powerpc/kernel/vdso64/gettimeofday.S 2006-10-13 16:00:25.000000000 +1000
+++ linux-cell/arch/powerpc/kernel/vdso64/gettimeofday.S 2006-10-13 16:59:41.000000000 +1000
@@ -229,8 +229,10 @@ V_FUNCTION_BEGIN(__do_get_xsec)
xor r0,r8,r8 /* create dependency */
add r3,r3,r0
- /* Get TB & offset it */
- mftb r7
+ /* Get TB & offset it. We use the MFTB macro which will generate
+ * workaround code for Cell.
+ */
+ MFTB(r7)
ld r9,CFG_TB_ORIG_STAMP(r3)
subf r7,r9,r7
Index: linux-cell/include/asm-powerpc/cputable.h
===================================================================
--- linux-cell.orig/include/asm-powerpc/cputable.h 2006-10-13 16:58:13.000000000 +1000
+++ linux-cell/include/asm-powerpc/cputable.h 2006-10-13 16:59:41.000000000 +1000
@@ -143,6 +143,7 @@ extern struct cpu_spec *identify_cpu(uns
#define CPU_FTR_CI_LARGE_PAGE LONG_ASM_CONST(0x0000100000000000)
#define CPU_FTR_PAUSE_ZERO LONG_ASM_CONST(0x0000200000000000)
#define CPU_FTR_PURR LONG_ASM_CONST(0x0000400000000000)
+#define CPU_FTR_CELL_TB_BUG LONG_ASM_CONST(0x0000800000000000)
#ifndef __ASSEMBLY__
@@ -331,7 +332,7 @@ extern struct cpu_spec *identify_cpu(uns
#define CPU_FTRS_CELL (CPU_FTR_SPLIT_ID_CACHE | CPU_FTR_USE_TB | \
CPU_FTR_HPTE_TABLE | CPU_FTR_PPCAS_ARCH_V2 | CPU_FTR_CTRL | \
CPU_FTR_ALTIVEC_COMP | CPU_FTR_MMCRA | CPU_FTR_SMT | \
- CPU_FTR_PAUSE_ZERO | CPU_FTR_CI_LARGE_PAGE)
+ CPU_FTR_PAUSE_ZERO | CPU_FTR_CI_LARGE_PAGE | CPU_FTR_CELL_TB_BUG)
#define CPU_FTRS_PA6T (CPU_FTR_SPLIT_ID_CACHE | CPU_FTR_USE_TB | \
CPU_FTR_HPTE_TABLE | CPU_FTR_PPCAS_ARCH_V2 | \
CPU_FTR_ALTIVEC_COMP | CPU_FTR_CI_LARGE_PAGE | \
Index: linux-cell/include/asm-powerpc/ppc_asm.h
===================================================================
--- linux-cell.orig/include/asm-powerpc/ppc_asm.h 2006-10-13 16:00:25.000000000 +1000
+++ linux-cell/include/asm-powerpc/ppc_asm.h 2006-10-13 16:59:41.000000000 +1000
@@ -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 +274,18 @@ END_FTR_SECTION_IFSET(CPU_FTR_601)
#define ISYNC_601
#endif
+#ifdef CONFIG_PPC_CELL
+#define MFTB(dest) \
+ mftb dest; \
+BEGIN_FTR_SECTION_NESTED(96); \
+ cmpwi dest,0; \
+ bne+ 90f; \
+ mftb dest; \
+90: \
+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
Index: linux-cell/include/asm-powerpc/reg.h
===================================================================
--- linux-cell.orig/include/asm-powerpc/reg.h 2006-10-13 16:00:25.000000000 +1000
+++ linux-cell/include/asm-powerpc/reg.h 2006-10-13 16:59:41.000000000 +1000
@@ -617,11 +617,34 @@
asm volatile("mfspr %0," __stringify(rn) \
: "=r" (rval)); rval;})
#define mtspr(rn, v) asm volatile("mtspr " __stringify(rn) ",%0" : : "r" (v))
-
+#ifdef CONFIG_PPC_CELL
+#define mftb() ({unsigned long rval; \
+ asm volatile( \
+ " mftb %0;\n" \
+ "97: cmpwi %0,0;\n" \
+ " bne+ 99f;\n" \
+ " mftb %0;\n" \
+ "99:\n" \
+ ".section __ftr_fixup,\"a\"\n" \
+ ".align 3\n" \
+ "98:\n" \
+ " .llong %1\n" \
+ " .llong %1\n" \
+ " .llong 97b-98b\n" \
+ " .llong 99b-98b\n" \
+ ".previous" \
+ : "=r" (rval) : "i" (CPU_FTR_CELL_TB_BUG)); rval;})
+#else
#define mftb() ({unsigned long rval; \
asm volatile("mftb %0" : "=r" (rval)); rval;})
+#endif
+
+#ifndef __powerpc64__
#define mftbl() ({unsigned long rval; \
asm volatile("mftbl %0" : "=r" (rval)); rval;})
+#define mftbu() ({unsigned long rval; \
+ asm volatile("mftbu %0" : "=r" (rval)); rval;})
+#endif /* __powerpc64__ */
#define mttbl(v) asm volatile("mttbl %0":: "r"(v))
#define mttbu(v) asm volatile("mttbu %0":: "r"(v))
Index: linux-cell/include/asm-powerpc/time.h
===================================================================
--- linux-cell.orig/include/asm-powerpc/time.h 2006-10-13 16:00:25.000000000 +1000
+++ linux-cell/include/asm-powerpc/time.h 2006-10-13 16:59:41.000000000 +1000
@@ -85,26 +85,28 @@ struct div_result {
/* On ppc64 this gets us the whole timebase; on ppc32 just the lower half */
static inline unsigned long get_tbl(void)
{
- unsigned long tbl;
-
#if defined(CONFIG_403GCX)
+ unsigned long tbl;
asm volatile("mfspr %0, 0x3dd" : "=r" (tbl));
+ return tbl;
+#elif defined(CONFIG_PPC32)
+ return mftbl();
#else
- asm volatile("mftb %0" : "=r" (tbl));
+ return mftb();
#endif
- return tbl;
}
static inline unsigned int get_tbu(void)
{
+#ifdef CONFIG_403GCX
unsigned int tbu;
-
-#if defined(CONFIG_403GCX)
asm volatile("mfspr %0, 0x3dc" : "=r" (tbu));
+ return tbu;
+#elif defined(CONFIG_PPC32)
+ return mftbu();
#else
- asm volatile("mftbu %0" : "=r" (tbu));
+ return mftb();
#endif
- return tbu;
}
static inline unsigned int get_rtcl(void)
Index: linux-cell/include/asm-powerpc/timex.h
===================================================================
--- linux-cell.orig/include/asm-powerpc/timex.h 2006-10-13 16:58:13.000000000 +1000
+++ linux-cell/include/asm-powerpc/timex.h 2006-10-13 16:59:41.000000000 +1000
@@ -8,6 +8,7 @@
*/
#include <asm/cputable.h>
+#include <asm/reg.h>
#define CLOCK_TICK_RATE 1024000 /* Underlying HZ */
@@ -15,13 +16,11 @@ typedef unsigned long cycles_t;
static inline cycles_t get_cycles(void)
{
- cycles_t ret;
-
#ifdef __powerpc64__
-
- __asm__ __volatile__("mftb %0" : "=r" (ret) : );
-
+ return mftb();
#else
+ cycles_t ret;
+
/*
* For the "cycle" counter we use the timebase lower half.
* Currently only used on SMP.
@@ -41,9 +40,8 @@ static inline cycles_t get_cycles(void)
" .long 99b-98b\n"
".previous"
: "=r" (ret) : "i" (CPU_FTR_601));
-#endif
-
return ret;
+#endif
}
#endif /* __KERNEL__ */
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