[RFC patch 05/15] get_cycles() : MIPS HAVE_GET_CYCLES_32

[Mathieu Desnoyers]

partly reverts commit efb9ca08b5a2374b29938cdcab417ce4feb14b54. Selects
HAVE_GET_CYCLES_32 only on CPUs where it is safe to use it.

Currently consider the "_WORKAROUND" cases for 4000 and 4400 to be unsafe, but
should probably add other sub-architecture to the blacklist.

Do not define HAVE_GET_CYCLES because MIPS does not provide 64-bit tsc (only
32-bits).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
CC: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
CC: David Miller <davem@xxxxxxxxxxxxx>
CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
CC: linux-arch@xxxxxxxxxxxxxxx
---
 arch/mips/Kconfig        |    4 ++++
 include/asm-mips/timex.h |   25 +++++++++++++++++++++++++
 2 files changed, 29 insertions(+)

Index: linux-2.6-lttng/include/asm-mips/timex.h
===================================================================
--- linux-2.6-lttng.orig/include/asm-mips/timex.h	2008-10-16 12:25:47.000000000 -0400
+++ linux-2.6-lttng/include/asm-mips/timex.h	2008-10-16 12:34:18.000000000 -0400
@@ -29,14 +29,39 @@
  * which isn't an evil thing.
  *
  * We know that all SMP capable CPUs have cycle counters.
+ *
+ * Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
+ * HAVE_GET_CYCLES makes sure that this case is handled properly :
+ *
+ * Ralf Baechle <ralf@xxxxxxxxxxxxxx> :
+ * This avoids us executing an mfc0 c0_count instruction on processors which
+ * don't have but also on certain R4000 and R4400 versions where reading from
+ * the count register just in the very moment when its value equals c0_compare
+ * will result in the timer interrupt getting lost.
  */
 
 typedef unsigned int cycles_t;
 
+#ifdef HAVE_GET_CYCLES_32
+static inline cycles_t get_cycles(void)
+{
+	return read_c0_count();
+}
+
+static inline void get_cycles_barrier(void)
+{
+}
+
+static inline cycles_t get_cycles_rate(void)
+{
+	return CLOCK_TICK_RATE;
+}
+#else
 static inline cycles_t get_cycles(void)
 {
 	return 0;
 }
+#endif
 
 #endif /* __KERNEL__ */
 
Index: linux-2.6-lttng/arch/mips/Kconfig
===================================================================
--- linux-2.6-lttng.orig/arch/mips/Kconfig	2008-10-16 12:25:47.000000000 -0400
+++ linux-2.6-lttng/arch/mips/Kconfig	2008-10-16 12:34:01.000000000 -0400
@@ -1581,6 +1581,10 @@ config CPU_R4000_WORKAROUNDS
 config CPU_R4400_WORKAROUNDS
 	bool
 
+config HAVE_GET_CYCLES_32
+	def_bool y
+	depends on !CPU_R4400_WORKAROUNDS
+
 #
 # Use the generic interrupt handling code in kernel/irq/:
 #

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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