[POWERPC] Fix off-by-one error in setting decrementer on Book E/4xx (v2)

The decrementer in Book E and 4xx processors interrupts on the
transition from 1 to 0, rather than on the 0 to -1 transition as on
64-bit server and 32-bit "classic" (6xx/7xx/7xxx) processors.  At the
moment we subtract 1 from the count of how many decrementer ticks are
required before the next interrupt before putting it into the
decrementer, which is correct for server/classic processors, but could
possibly cause the interrupt to happen too early on Book E and 4xx if
the timebase/decrementer frequency is low.

This fixes the problem by making set_dec subtract 1 from the count for
server and classic processors, instead of having the callers subtract
1.  Since set_dec already had a bunch of ifdefs to handle different
processor types, there is no net increase in ugliness. :)

Note that calling set_dec(0) may not generate an interrupt on some
processors.  To make sure that decrementer_set_next_event always calls
set_dec with an interval of at least 1 tick, we set min_delta_ns of
the decrementer_clockevent to correspond to 2 ticks (2 rather than 1
to compensate for truncations in the conversions between ticks and
ns).

This also removes a redundant call to set the decrementer to
0x7fffffff - it was already set to that earlier in timer_interrupt.

Signed-off-by: Paul Mackerras <paulus@samba.org>

1 files changed, 13 insertions, 7 deletions

diff --git a/include/asm-powerpc/time.h b/include/asm-powerpc/time.h
index f05895522f7f..780f82642756 100644
--- a/include/asm-powerpc/time.h
+++ b/include/asm-powerpc/time.h
@@ -176,25 +176,31 @@ static inline unsigned int get_dec(void)
 #endif
 }
 
+/*
+ * Note: Book E and 4xx processors differ from other PowerPC processors
+ * in when the decrementer generates its interrupt: on the 1 to 0
+ * transition for Book E/4xx, but on the 0 to -1 transition for others.
+ */
 static inline void set_dec(int val)
 {
 #if defined(CONFIG_40x)
         mtspr(SPRN_PIT, val);
 #elif defined(CONFIG_8xx_CPU6)
-        set_dec_cpu6(val);
+        set_dec_cpu6(val - 1);
 #else
+#ifndef CONFIG_BOOKE
+        --val;
+#endif
 #ifdef CONFIG_PPC_ISERIES
-        int cur_dec;
-
         if (firmware_has_feature(FW_FEATURE_ISERIES) &&
                         get_lppaca()->shared_proc) {
                 get_lppaca()->virtual_decr = val;
-                cur_dec = get_dec();
-                if (cur_dec > val)
+                if (get_dec() > val)
                         HvCall_setVirtualDecr();
-        } else
+                return;
+        }
 #endif
-                mtspr(SPRN_DEC, val);
+        mtspr(SPRN_DEC, val);
 #endif /* not 40x or 8xx_CPU6 */
 }