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Date: Wed, 4 Mar 2009 21:26:45 +0100
From: Johannes Weiner <jw@...ix.com>
To: Daniel Walker <dwalker@...o99.com>
Cc: Johannes Weiner <hannes@...xchg.org>,
Chris Zankel <chris@...kel.net>, linux-kernel@...r.kernel.org,
John Stultz <johnstul@...ibm.com>
Subject: Re: [patch 3/3] xtensa: ccount clocksource
On Tue, Mar 03, 2009 at 01:36:04PM -0800, Daniel Walker wrote:
> On Tue, 2009-03-03 at 20:54 +0100, Johannes Weiner wrote:
> > On Tue, Mar 03, 2009 at 09:55:45AM -0800, Daniel Walker wrote:
> > > On Tue, 2009-03-03 at 16:30 +0100, Johannes Weiner wrote:
> > > > @ -29,6 +30,19 @@ unsigned long ccount_per_jiffy; /* per
> > > > unsigned long nsec_per_ccount; /* nsec per ccount increment
> > > > */
> > > > #endif
> > > >
> > > > +static cycle_t ccount_read(void)
> > > > +{
> > > > + return (cycle_t)get_ccount();
> > > > +}
> > > > +
> > > > +static struct clocksource ccount_clocksource = {
> > > > + .name = "ccount",
> > > > + .rating = 200,
> > > > + .read = ccount_read,
> > > > + .mask = CLOCKSOURCE_MASK(32),
> > > > + .mult = NSEC_PER_CCOUNT,
> > > > +};
> > >
> > > You don't want to use the shift field?
> >
> > Thanks for pointing it out.
> >
> > To make sure I understood this:
> >
> > If shift is 0, then a walltime adjustment would be done in 1/2^0
> > steps, meaning an adjustment of counting one nanosecond more or less
> > per ccount.
>
> The shift and mult are just used to convert your cycle counters current
> count into nanoseconds .. I'm not sure how much a zero shift would
> degrade the conversion to nanoseconds for your cycles counter tho. So it
> could be along the lines of what your suggesting above.
>
> > To give this a finer granularity and smooth out adjustments, the shift
> > should be a trade-off between too much adjustment and no adjustment
> > progress in a sane amount of time (and, of course, to stay within
> > bounds of the used type).
> >
> > Does that make sense?
>
> I don't really look at it in terms of walltime adjustments. The actual
> frequency of kernel time adjustments isn't defined in the clocksource
> structure AFAIK. From my experience you just want the clocksource to
> produce the most accurate nanosecond value your hardware can provide ,
> which would mean setting the shift as high as is safe for your hardware.
>
> I added John Stultz to the CC so he could comment further if he cares
> too.
>
> > I found a patch of yours that introduced clocksource_hz2shift() but it
> > seems it hasn't been merged (yet). Is it yet to get integrated?
>
> I've been meaning to update it, but haven't gotten around to it. If you
> know the input values you can run that function once just to produce a
> shift which you define statically in the clocksource structure. Then use
> that shift value with the other helper clocksource_hz2mult() to produce
> a mult value.
Ok, here is version 2. The shift now limits the CPU clock to a
minimum of 1 MHz, comment in the code. I think the trade-off should
be okay. John? Chris?
---
Subject: xtensa: ccount clocksource
From: Johannes Weiner <hannes@...xchg.org>
Switch to GENERIC_TIME by using the ccount register as a clock source.
Signed-off-by: Johannes Weiner <hannes@...xchg.org>
---
arch/xtensa/Kconfig | 3 +
arch/xtensa/kernel/time.c | 103 +++++++++++++---------------------------------
2 files changed, 33 insertions(+), 73 deletions(-)
--- a/arch/xtensa/Kconfig
+++ b/arch/xtensa/Kconfig
@@ -48,6 +48,9 @@ config HZ
int
default 100
+config GENERIC_TIME
+ def_bool y
+
source "init/Kconfig"
source "kernel/Kconfig.freezer"
--- a/arch/xtensa/kernel/time.c
+++ b/arch/xtensa/kernel/time.c
@@ -14,6 +14,7 @@
#include <linux/errno.h>
#include <linux/time.h>
+#include <linux/clocksource.h>
#include <linux/interrupt.h>
#include <linux/module.h>
#include <linux/init.h>
@@ -29,6 +30,26 @@ unsigned long ccount_per_jiffy; /* per
unsigned long nsec_per_ccount; /* nsec per ccount increment */
#endif
+static cycle_t ccount_read(void)
+{
+ return (cycle_t)get_ccount();
+}
+
+static struct clocksource ccount_clocksource = {
+ .name = "ccount",
+ .rating = 200,
+ .read = ccount_read,
+ .mask = CLOCKSOURCE_MASK(32),
+ /*
+ * With a shift of 22 the lower limit of the cpu clock is
+ * 1MHz, where NSEC_PER_CCOUNT is 1000 or a bit less than
+ * 2^10: Since we have 32 bits and the multiplicator can
+ * already take up as much as 10 bits, this leaves us with
+ * remaining upper 22 bits.
+ */
+ .shift = 22,
+};
+
static irqreturn_t timer_interrupt(int irq, void *dev_id);
static struct irqaction timer_irqaction = {
.handler = timer_interrupt,
@@ -38,9 +59,11 @@ static struct irqaction timer_irqaction
void __init time_init(void)
{
- /* The platform must provide a function to calibrate the processor
- * speed for the CALIBRATE.
- */
+ xtime.tv_nsec = 0;
+ xtime.tv_sec = read_persistent_clock();
+
+ set_normalized_timespec(&wall_to_monotonic,
+ -xtime.tv_sec, -xtime.tv_nsec);
#ifdef CONFIG_XTENSA_CALIBRATE_CCOUNT
printk("Calibrating CPU frequency ");
@@ -48,12 +71,10 @@ void __init time_init(void)
printk("%d.%02d MHz\n", (int)ccount_per_jiffy/(1000000/HZ),
(int)(ccount_per_jiffy/(10000/HZ))%100);
#endif
-
- xtime.tv_nsec = 0;
- xtime.tv_sec = read_persistent_clock();
-
- set_normalized_timespec(&wall_to_monotonic,
- -xtime.tv_sec, -xtime.tv_nsec);
+ ccount_clocksource.mult =
+ clocksource_hz2mult(CCOUNT_PER_JIFFY * HZ,
+ ccount_clocksource.shift);
+ clocksource_register(&ccount_clocksource);
/* Initialize the linux timer interrupt. */
@@ -61,69 +82,6 @@ void __init time_init(void)
set_linux_timer(get_ccount() + CCOUNT_PER_JIFFY);
}
-
-int do_settimeofday(struct timespec *tv)
-{
- time_t wtm_sec, sec = tv->tv_sec;
- long wtm_nsec, nsec = tv->tv_nsec;
- unsigned long delta;
-
- if ((unsigned long)tv->tv_nsec >= NSEC_PER_SEC)
- return -EINVAL;
-
- write_seqlock_irq(&xtime_lock);
-
- /* This is revolting. We need to set "xtime" correctly. However, the
- * value in this location is the value at the most recent update of
- * wall time. Discover what correction gettimeofday() would have
- * made, and then undo it!
- */
-
- delta = CCOUNT_PER_JIFFY;
- delta += get_ccount() - get_linux_timer();
- nsec -= delta * NSEC_PER_CCOUNT;
-
- wtm_sec = wall_to_monotonic.tv_sec + (xtime.tv_sec - sec);
- wtm_nsec = wall_to_monotonic.tv_nsec + (xtime.tv_nsec - nsec);
-
- set_normalized_timespec(&xtime, sec, nsec);
- set_normalized_timespec(&wall_to_monotonic, wtm_sec, wtm_nsec);
-
- ntp_clear();
- write_sequnlock_irq(&xtime_lock);
- return 0;
-}
-
-EXPORT_SYMBOL(do_settimeofday);
-
-
-void do_gettimeofday(struct timeval *tv)
-{
- unsigned long flags;
- unsigned long volatile sec, usec, delta, seq;
-
- do {
- seq = read_seqbegin_irqsave(&xtime_lock, flags);
-
- sec = xtime.tv_sec;
- usec = (xtime.tv_nsec / NSEC_PER_USEC);
-
- delta = get_linux_timer() - get_ccount();
-
- } while (read_seqretry_irqrestore(&xtime_lock, seq, flags));
-
- usec += (((unsigned long) CCOUNT_PER_JIFFY - delta)
- * (unsigned long) NSEC_PER_CCOUNT) / NSEC_PER_USEC;
-
- for (; usec >= 1000000; sec++, usec -= 1000000)
- ;
-
- tv->tv_sec = sec;
- tv->tv_usec = usec;
-}
-
-EXPORT_SYMBOL(do_gettimeofday);
-
/*
* The timer interrupt is called HZ times per second.
*/
@@ -177,4 +135,3 @@ void __cpuinit calibrate_delay(void)
(loops_per_jiffy/(10000/HZ)) % 100);
}
#endif
-
--
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