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Subject[tip:timers/core] clocksource: Add 'max_cycles' to ' struct clocksource'
Commit-ID:  fb82fe2fe8588745edd73aa3a6229facac5c1e15
Gitweb: http://git.kernel.org/tip/fb82fe2fe8588745edd73aa3a6229facac5c1e15
Author: John Stultz <john.stultz@linaro.org>
AuthorDate: Wed, 11 Mar 2015 21:16:31 -0700
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 12 Mar 2015 10:16:38 +0100

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>
---
include/linux/clocksource.h | 5 +++--
kernel/time/clocksource.c | 28 ++++++++++++++++------------
kernel/time/sched_clock.c | 2 +-
3 files changed, 20 insertions(+), 15 deletions(-)

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index 9c78d15..16d048c 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -56,6 +56,7 @@ struct module;
* @shift: cycle to nanosecond divisor (power of two)
* @max_idle_ns: max idle time permitted by the clocksource (nsecs)
* @maxadj: maximum adjustment value to mult (~11%)
+ * @max_cycles: maximum safe cycle value which won't overflow on multiplication
* @flags: flags describing special properties
* @archdata: arch-specific data
* @suspend: suspend function for the clocksource, if necessary
@@ -76,7 +77,7 @@ struct clocksource {
#ifdef CONFIG_ARCH_CLOCKSOURCE_DATA
struct arch_clocksource_data archdata;
#endif
-
+ u64 max_cycles;
const char *name;
struct list_head list;
int rating;
@@ -189,7 +190,7 @@ extern struct clocksource * __init clocksource_default_clock(void);
extern void clocksource_mark_unstable(struct clocksource *cs);

extern u64
-clocks_calc_max_nsecs(u32 mult, u32 shift, u32 maxadj, u64 mask);
+clocks_calc_max_nsecs(u32 mult, u32 shift, u32 maxadj, u64 mask, u64 *max_cycles);
extern void
clocks_calc_mult_shift(u32 *mult, u32 *shift, u32 from, u32 to, u32 minsec);

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index ace9576..fc2a9de 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 3b8ae45..ca3bc5c 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*/

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