clocksource: Add 'max_cycles' to 'struct clocksource'

In order to facilitate clocksource validation, add a
'max_cycles' field to the clocksource structure which
will hold the maximum cycle value that can safely be
multiplied without potentially causing an overflow.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Cc: Dave Jones <davej@codemonkey.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1426133800-29329-4-git-send-email-john.stultz@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 17 insertions, 13 deletions

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index ace95763b3a6..fc2a9de43ca1 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -469,11 +469,13 @@ static u32 clocksource_max_adjustment(struct clocksource *cs)
  * @shift:      cycle to nanosecond divisor (power of two)
  * @maxadj:     maximum adjustment value to mult (~11%)
  * @mask:       bitmask for two's complement subtraction of non 64 bit counters
+ * @max_cyc:    maximum cycle value before potential overflow (does not include
+ *              any safety margin)
  *
  * NOTE: This function includes a safety margin of 50%, so that bad clock values
  * can be detected.
  */
-u64 clocks_calc_max_nsecs(u32 mult, u32 shift, u32 maxadj, u64 mask)
+u64 clocks_calc_max_nsecs(u32 mult, u32 shift, u32 maxadj, u64 mask, u64 *max_cyc)
 {
         u64 max_nsecs, max_cycles;
 
@@ -493,6 +495,10 @@ u64 clocks_calc_max_nsecs(u32 mult, u32 shift, u32 maxadj, u64 mask)
         max_cycles = min(max_cycles, mask);
         max_nsecs = clocksource_cyc2ns(max_cycles, mult - maxadj, shift);
 
+        /* return the max_cycles value as well if requested */
+        if (max_cyc)
+                *max_cyc = max_cycles;
+
         /* Return 50% of the actual maximum, so we can detect bad values */
         max_nsecs >>= 1;
 
@@ -500,17 +506,15 @@ u64 clocks_calc_max_nsecs(u32 mult, u32 shift, u32 maxadj, u64 mask)
 }
 
 /**
- * clocksource_max_deferment - Returns max time the clocksource should be deferred
- * @cs:         Pointer to clocksource
+ * clocksource_update_max_deferment - Updates the clocksource max_idle_ns & max_cycles
+ * @cs:         Pointer to clocksource to be updated
  *
  */
-static u64 clocksource_max_deferment(struct clocksource *cs)
+static inline void clocksource_update_max_deferment(struct clocksource *cs)
 {
-        u64 max_nsecs;
-
-        max_nsecs = clocks_calc_max_nsecs(cs->mult, cs->shift, cs->maxadj,
-                                          cs->mask);
-        return max_nsecs;
+        cs->max_idle_ns = clocks_calc_max_nsecs(cs->mult, cs->shift,
+                                                cs->maxadj, cs->mask,
+                                                &cs->max_cycles);
 }
 
 #ifndef CONFIG_ARCH_USES_GETTIMEOFFSET
@@ -684,7 +688,7 @@ void __clocksource_updatefreq_scale(struct clocksource *cs, u32 scale, u32 freq)
                 cs->maxadj = clocksource_max_adjustment(cs);
         }
 
-        cs->max_idle_ns = clocksource_max_deferment(cs);
+        clocksource_update_max_deferment(cs);
 }
 EXPORT_SYMBOL_GPL(__clocksource_updatefreq_scale);
 
@@ -730,8 +734,8 @@ int clocksource_register(struct clocksource *cs)
                 "Clocksource %s might overflow on 11%% adjustment\n",
                 cs->name);
 
-        /* calculate max idle time permitted for this clocksource */
-        cs->max_idle_ns = clocksource_max_deferment(cs);
+        /* Update max idle time permitted for this clocksource */
+        clocksource_update_max_deferment(cs);
 
         mutex_lock(&clocksource_mutex);
         clocksource_enqueue(cs);
diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 3b8ae45020c1..ca3bc5c7027c 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -126,7 +126,7 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
         new_mask = CLOCKSOURCE_MASK(bits);
 
         /* calculate how many nanosecs until we risk wrapping */
-        wrap = clocks_calc_max_nsecs(new_mult, new_shift, 0, new_mask);
+        wrap = clocks_calc_max_nsecs(new_mult, new_shift, 0, new_mask, NULL);
         new_wrap_kt = ns_to_ktime(wrap);
 
         /* update epoch for new counter and update epoch_ns from old counter*/