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authorUwe Kleine-König <u.kleine-koenig@pengutronix.de>2014-02-03 11:31:19 +0100
committerUwe Kleine-König <u.kleine-koenig@pengutronix.de>2014-02-18 21:23:19 +0100
commitfb3174e4ad2427c6ad90c67093d6ca97f13e8672 (patch)
treecfa185f249c166f8302513a6b57432bfa11b53c8 /arch/arm/mach-ixp4xx
parent79f08d9ed217318b4f325b7fcf8f26dcd0e41689 (diff)
ARM: ixp4xx: fix timer latch calculation
In commit f0402f9b4711 ("ARM: ixp4xx: stop using <mach/timex.h>") I didn't intend to implement a functional change, but as Olof noticed I failed---at least a bit. Before this commit the following was used to determine the latch value used: #define IXP4XX_TIMER_FREQ 66666000 #define CLOCK_TICK_RATE \ (((IXP4XX_TIMER_FREQ / HZ & ~IXP4XX_OST_RELOAD_MASK) + 1) * HZ) #define LATCH ((CLOCK_TICK_RATE + HZ/2) / HZ) The complicated calculation was done "b/c the timer register ignores the bottom 2 bits of the LATCH value." With HZ=100 CLOCK_TICK_RATE used to calculate to 66666100 and so LATCH to 666661. In ixp4xx_set_mode the term LATCH & ~IXP4XX_OST_RELOAD_MASK was used to write to the relevant register (with IXP4XX_OST_RELOAD_MASK being 3) and so effectively 666660 was used. In commit f0402f9b4711 I translated that to: #define IXP4XX_TIMER_FREQ 66666000 #define IXP4XX_LATCH DIV_ROUND_CLOSEST(IXP4XX_TIMER_FREQ, HZ) which results in the same register writes, but still doesn't bear in mind that the two least significant bits cannot be specified (which is relevant only when HZ or IXP4XX_TIMER_FREQ are changed). Instead of reverting back to the old approach use a more obvious and also more correct way to calculate LATCH. (Regarding the more correct claim: With IXP4XX_TIMER_FREQ == 66665999, the old code resulted in LATCH = 666657 corresponding to a cycle time of 0.009999940149400597 seconds (error: -6.0e-8 s) while the new approach results in LATCH = 666660 and so a cycle time of 0.010000000150001503 seconds (error: 1.5e-10 s).) Fixes: f0402f9b4711 ("ARM: ixp4xx: stop using <mach/timex.h>") Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Diffstat (limited to 'arch/arm/mach-ixp4xx')
-rw-r--r--arch/arm/mach-ixp4xx/common.c10
1 files changed, 9 insertions, 1 deletions
diff --git a/arch/arm/mach-ixp4xx/common.c b/arch/arm/mach-ixp4xx/common.c
index 20b62aa30086..37e6cdac1642 100644
--- a/arch/arm/mach-ixp4xx/common.c
+++ b/arch/arm/mach-ixp4xx/common.c
@@ -45,7 +45,15 @@
#include <asm/mach/time.h>
#define IXP4XX_TIMER_FREQ 66666000
-#define IXP4XX_LATCH DIV_ROUND_CLOSEST(IXP4XX_TIMER_FREQ, HZ)
+
+/*
+ * The timer register doesn't allow to specify the two least significant bits of
+ * the timeout value and assumes them being zero. So make sure IXP4XX_LATCH is
+ * the best value with the two least significant bits unset.
+ */
+#define IXP4XX_LATCH DIV_ROUND_CLOSEST(IXP4XX_TIMER_FREQ, \
+ (IXP4XX_OST_RELOAD_MASK + 1) * HZ) * \
+ (IXP4XX_OST_RELOAD_MASK + 1)
static void __init ixp4xx_clocksource_init(void);
static void __init ixp4xx_clockevent_init(void);