Subject: [PATCH 1/2] powerpc/timer - large decrementer support

From: Oliver O'Halloran <[email-masked]>
Date: Tue, 31 May 2016 17:16:55 +1000

POWER ISA v3 adds large decrementer (LD) mode of operation which increases
the size of the decrementer register from 32 bits to an implementation
defined with of up to 64 bits.

This patch adds support for the LD on processors with the CPU_FTR_ARCH_300
cpu feature flag set. For CPUs with this feature LD mode is enabled when
when the ibm,dec-bits devicetree property is supplied for the boot CPU. The
decrementer value is a signed quantity (with negative values indicating a
pending exception) and this property is required to find the maximum
positive decrementer value. If this property is not supplied then the
traditional decrementer width of 32 bits is assumed and LD mode is disabled.

This patch was based on initial work by Jack Miller.

Signed-off-by: Oliver O'Halloran <[email-masked]>
Cc: Michael Neuling <[email-masked]>
Cc: Balbir Singh <[email-masked]>
Cc: Jack Miller <[email-masked]>
---
 arch/powerpc/include/asm/reg.h  |  1 +
 arch/powerpc/include/asm/time.h |  6 +--
 arch/powerpc/kernel/time.c      | 94 +++++++++++++++++++++++++++++++++++++----
 3 files changed, 90 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/include/asm/reg.h b/arch/powerpc/include/asm/reg.h
index c1e82e968506..2793f3f03f9b 100644
--- a/arch/powerpc/include/asm/reg.h
+++ b/arch/powerpc/include/asm/reg.h
@@ -332,6 +332,7 @@
 #define   LPCR_AIL_0	0x00000000	/* MMU off exception offset 0x0 */
 #define   LPCR_AIL_3	0x01800000	/* MMU on exception offset 0xc00...4xxx */
 #define   LPCR_ONL	0x00040000	/* online - PURR/SPURR count */
+#define   LPCR_LD	0x00020000	/* large decremeter */
 #define   LPCR_PECE	0x0001f000	/* powersave exit cause enable */
 #define     LPCR_PECEDP	0x00010000	/* directed priv dbells cause exit */
 #define     LPCR_PECEDH	0x00008000	/* directed hyp dbells cause exit */
diff --git a/arch/powerpc/include/asm/time.h b/arch/powerpc/include/asm/time.h
index 1092fdd7e737..09211640a0e0 100644
--- a/arch/powerpc/include/asm/time.h
+++ b/arch/powerpc/include/asm/time.h
@@ -146,7 +146,7 @@ static inline void set_tb(unsigned int upper, unsigned int lower)
  * in auto-reload mode.  The problem is PIT stops counting when it
  * hits zero.  If it would wrap, we could use it just like a decrementer.
  */
-static inline unsigned int get_dec(void)
+static inline u64 get_dec(void)
 {
 #if defined(CONFIG_40x)
 	return (mfspr(SPRN_PIT));
@@ -160,10 +160,10 @@ static inline unsigned int get_dec(void)
  * in when the decrementer generates its interrupt: on the 1 to 0
  * transition for Book E/4xx, but on the 0 to -1 transition for others.
  */
-static inline void set_dec(int val)
+static inline void set_dec(u64 val)
 {
 #if defined(CONFIG_40x)
-	mtspr(SPRN_PIT, val);
+	mtspr(SPRN_PIT, (u32) val);
 #else
 #ifndef CONFIG_BOOKE
 	--val;
diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 3ed9a5a21d77..fe66f1c8d8b2 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -96,7 +96,8 @@ static struct clocksource clocksource_timebase = {
 	.read         = timebase_read,
 };
 
-#define DECREMENTER_MAX	0x7fffffff
+#define DECREMENTER_DEFAULT_MAX 0x7FFFFFFF
+u64 decrementer_max = DECREMENTER_DEFAULT_MAX;
 
 static int decrementer_set_next_event(unsigned long evt,
 				      struct clock_event_device *dev);
@@ -504,8 +505,8 @@ static void __timer_interrupt(void)
 		__this_cpu_inc(irq_stat.timer_irqs_event);
 	} else {
 		now = *next_tb - now;
-		if (now <= DECREMENTER_MAX)
-			set_dec((int)now);
+		if (now <= decrementer_max)
+			set_dec(now);
 		/* We may have raced with new irq work */
 		if (test_irq_work_pending())
 			set_dec(1);
@@ -535,7 +536,7 @@ void timer_interrupt(struct pt_regs * regs)
 	/* Ensure a positive value is written to the decrementer, or else
 	 * some CPUs will continue to take decrementer exceptions.
 	 */
-	set_dec(DECREMENTER_MAX);
+	set_dec(decrementer_max);
 
 	/* Some implementations of hotplug will get timer interrupts while
 	 * offline, just ignore these and we also need to set
@@ -583,9 +584,9 @@ static void generic_suspend_disable_irqs(void)
 	 * with suspending.
 	 */
 
-	set_dec(DECREMENTER_MAX);
+	set_dec(decrementer_max);
 	local_irq_disable();
-	set_dec(DECREMENTER_MAX);
+	set_dec(decrementer_max);
 }
 
 static void generic_suspend_enable_irqs(void)
@@ -866,7 +867,7 @@ static int decrementer_set_next_event(unsigned long evt,
 
 static int decrementer_shutdown(struct clock_event_device *dev)
 {
-	decrementer_set_next_event(DECREMENTER_MAX, dev);
+	decrementer_set_next_event(decrementer_max, dev);
 	return 0;
 }
 
@@ -892,6 +893,76 @@ static void register_decrementer_clockevent(int cpu)
 	clockevents_register_device(dec);
 }
 
+static inline bool cpu_has_large_dec(void)
+{
+	return cpu_has_feature(CPU_FTR_ARCH_300);
+}
+
+static inline bool large_dec_enabled(void)
+{
+	return (mfspr(SPRN_LPCR) & LPCR_LD) == LPCR_LD;
+}
+
+/* enables the large decrementer for the current CPU */
+static void enable_large_decrementer(void)
+{
+	/* do we have a large decrementer? */
+	if (!cpu_has_large_dec())
+		return;
+
+	/* do we need a large decrementer? */
+	if (decrementer_max <= DECREMENTER_DEFAULT_MAX)
+		return;
+
+	mtspr(SPRN_LPCR, mfspr(SPRN_LPCR) | LPCR_LD);
+
+	if (!large_dec_enabled()) {
+		decrementer_max = DECREMENTER_DEFAULT_MAX;
+
+		pr_warn_once("time_init: Failed to enable large decrementer on CPU %d\n",
+			smp_processor_id());
+	}
+}
+
+static void __init set_decrementer_max(void)
+{
+	struct device_node *cpu;
+	const __be32 *fp;
+	u64 bits = 32;
+
+	cpu = of_find_node_by_type(NULL, "cpu");
+	if (cpu)
+		fp = of_get_property(cpu, "ibm,dec-bits", NULL);
+
+	if (cpu && fp) {
+		u64 dt_bits = of_read_number(fp, 1);
+
+		bits = dt_bits;
+		if (bits > 64)
+			bits = 64;
+		else if (bits < 32)
+			bits = 32;
+
+		WARN(bits != dt_bits, "firmware supplied invalid ibm,dec-bits");
+
+		/*
+		 * Firmware says we support large dec but this cpu doesn't we
+		 * should warn about it. We can still limp along with default
+		 * 32 bit dec, but something is broken.
+		 */
+		if (!cpu_has_large_dec()) {
+			WARN_ON(bits > 32);
+			bits = 32;
+		}
+
+		/* calculate the signed maximum given this many bits */
+		decrementer_max = (1ul << (bits - 1)) - 1;
+	}
+
+	pr_info("time_init: %llu bit decrementer (max: %llx)\n",
+		bits, decrementer_max);
+}
+
 static void __init init_decrementer_clockevent(void)
 {
 	int cpu = smp_processor_id();
@@ -899,7 +970,7 @@ static void __init init_decrementer_clockevent(void)
 	clockevents_calc_mult_shift(&decrementer_clockevent, ppc_tb_freq, 4);
 
 	decrementer_clockevent.max_delta_ns =
-		clockevent_delta2ns(DECREMENTER_MAX, &decrementer_clockevent);
+		clockevent_delta2ns(decrementer_max, &decrementer_clockevent);
 	decrementer_clockevent.min_delta_ns =
 		clockevent_delta2ns(2, &decrementer_clockevent);
 
@@ -908,6 +979,9 @@ static void __init init_decrementer_clockevent(void)
 
 void secondary_cpu_time_init(void)
 {
+	/* Enable the large decrementer (if we need to) */
+	enable_large_decrementer();
+
 	/* Start the decrementer on CPUs that have manual control
 	 * such as BookE
 	 */
@@ -973,6 +1047,10 @@ void __init time_init(void)
 	vdso_data->tb_update_count = 0;
 	vdso_data->tb_ticks_per_sec = tb_ticks_per_sec;
 
+	/* initialise and enable the large decrementer (if we have one) */
+	set_decrementer_max();
+	enable_large_decrementer();
+
 	/* Start the decrementer on CPUs that have manual control
 	 * such as BookE
 	 */
-- 
2.5.5