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Date: Tue, 23 Aug 2016 01:33:11 +0200
From: Nicolai Stange <nicstange@...il.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: John Stultz <john.stultz@...aro.org>, linux-kernel@...r.kernel.org,
Nicolai Stange <nicstange@...il.com>
Subject: [RFC v4 13/22] clockevents: check a programmed delta's bounds in terms of cycles
Currently, clockevents_program_event()
- compares a given delta against ->min_delta_ns and ->max_delta_ns
- and transforms the given delta value from ns to device cycles via ->mult.
Future changes introducing NTP time awareness into the clockevents core
would possibly need to update ->mult, ->min_delta_ns and ->max_delta_ns
from a different CPU than clockevents_program_event() runs on: the
->*_delta_ns values depend on ->mult. In order to guarantee atomicity
between these updates, a seqlock would have to be introduced and acquired
for reading in clockevents_program_event().
In order to avoid this, do the bounds checking in terms of the always
invariant ->min_delta_ticks and ->max_delta_ticks values.
The transition from ->max_delta_ns to ->max_delta_ticks is straight
forward since they are 1:1.
For ->min_delta_ns, the situation is slightly different as it can get
larger than its initial value derived from ->min_delta_ticks for two
reason:
- ->min_delta_ns is enforced to be larger than 1us at initialization
- and it can grow over time with CONFIG_GENERIC_CLOCKEVENTS_MIN_ADJUST,
c.f. clockevents_increase_min_delta().
Introduce ->min_delta_ticks_adjusted always resembling the current value
of ->min_delta_ns. The original ->min_delta_ticks must be kept for
reconfiguration.
The invariant ->min_delta_ticks <= ->min_delta_ticks_adjusted will always
be guaranteed to hold. This will allow for non-atomic updates of ->mult
and ->min_delta_ticks_adjusted -- as long as we stay within a device's
allowed bounds, we don't care for small deviations.
Make clockevents_program_event() use ->min_delta_ticks_adjusted and
->max_delta_ticks for the bounds enforcement on the delta value.
Finally, slightly reorder the members of struct clock_event_device in order
to keep the now often used ones close together.
Also, since ->max_delta_ticks is being actively used now, remove the
assertion that it is "stored for reconfiguration" from its docstring.
Signed-off-by: Nicolai Stange <nicstange@...il.com>
---
include/linux/clockchips.h | 14 ++++++++------
kernel/time/clockevents.c | 15 ++++++++++++---
2 files changed, 20 insertions(+), 9 deletions(-)
diff --git a/include/linux/clockchips.h b/include/linux/clockchips.h
index a116926..8578e24 100644
--- a/include/linux/clockchips.h
+++ b/include/linux/clockchips.h
@@ -73,8 +73,8 @@ enum clock_event_state {
* @set_next_event: set next event function using a clocksource delta
* @set_next_ktime: set next event function using a direct ktime value
* @next_event: local storage for the next event in oneshot mode
- * @max_delta_ns: maximum delta value in ns
- * @min_delta_ns: minimum delta value in ns
+ * @max_delta_ticks: maximum delta value in ticks
+ * @min_delta_ticks_adjusted: minimum delta value, increased as needed
* @mult: nanosecond to cycles multiplier
* @shift: nanoseconds to cycles divisor (power of two)
* @state_use_accessors:current state of the device, assigned by the core code
@@ -87,7 +87,8 @@ enum clock_event_state {
* @tick_resume: resume clkevt device
* @broadcast: function to broadcast events
* @min_delta_ticks: minimum delta value in ticks stored for reconfiguration
- * @max_delta_ticks: maximum delta value in ticks stored for reconfiguration
+ * @max_delta_ns: maximum delta value in ns
+ * @min_delta_ns: minimum delta value in ns
* @name: ptr to clock event name
* @rating: variable to rate clock event devices
* @irq: IRQ number (only for non CPU local devices)
@@ -101,8 +102,8 @@ struct clock_event_device {
int (*set_next_event)(unsigned long evt, struct clock_event_device *);
int (*set_next_ktime)(ktime_t expires, struct clock_event_device *);
ktime_t next_event;
- u64 max_delta_ns;
- u64 min_delta_ns;
+ unsigned long max_delta_ticks;
+ unsigned long min_delta_ticks_adjusted;
u32 mult;
u32 shift;
enum clock_event_state state_use_accessors;
@@ -119,7 +120,8 @@ struct clock_event_device {
void (*suspend)(struct clock_event_device *);
void (*resume)(struct clock_event_device *);
unsigned long min_delta_ticks;
- unsigned long max_delta_ticks;
+ u64 max_delta_ns;
+ u64 min_delta_ns;
const char *name;
int rating;
diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index f352f54..7832050 100644
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -225,6 +225,11 @@ static int clockevents_increase_min_delta(struct clock_event_device *dev)
if (dev->min_delta_ns > MIN_DELTA_LIMIT)
dev->min_delta_ns = MIN_DELTA_LIMIT;
+ dev->min_delta_ticks_adjusted = (unsigned long)((dev->min_delta_ns *
+ dev->mult) >> dev->shift);
+ dev->min_delta_ticks_adjusted = max(dev->min_delta_ticks_adjusted,
+ dev->min_delta_ticks);
+
printk_deferred(KERN_WARNING
"CE: %s increased min_delta_ns to %llu nsec\n",
dev->name ? dev->name : "?",
@@ -332,10 +337,10 @@ int clockevents_program_event(struct clock_event_device *dev, ktime_t expires,
if (delta <= 0)
return force ? clockevents_program_min_delta(dev) : -ETIME;
- delta = min(delta, (int64_t) dev->max_delta_ns);
- delta = max(delta, (int64_t) dev->min_delta_ns);
-
clc = ((unsigned long long) delta * dev->mult) >> dev->shift;
+ clc = min_t(unsigned long, clc, dev->max_delta_ticks);
+ clc = max_t(unsigned long, clc, dev->min_delta_ticks_adjusted);
+
rc = dev->set_next_event((unsigned long) clc, dev);
return (rc && force) ? clockevents_program_min_delta(dev) : rc;
@@ -453,6 +458,10 @@ static void __clockevents_update_bounds(struct clock_event_device *dev)
*/
dev->min_delta_ns = cev_delta2ns(dev->min_delta_ticks, dev, false);
dev->max_delta_ns = cev_delta2ns(dev->max_delta_ticks, dev, true);
+ dev->min_delta_ticks_adjusted = (unsigned long)((dev->min_delta_ns *
+ dev->mult) >> dev->shift);
+ dev->min_delta_ticks_adjusted = max(dev->min_delta_ticks_adjusted,
+ dev->min_delta_ticks);
}
/**
--
2.9.2
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