[PATCH 2/2] ARM: omap5/dra7xx: Fix counter frequency drift for AM572x errata i856.

[Lennart Sorensen]

Errata i856 for the AM572x (DRA7xx) points out that the 32.768KHz external
crystal is not enabled at power up.  Instead the CPU falls back to using
an emulation for the 32KHz clock which is SYSCLK1/610.  SYSCLK1 is usually
20MHz on boards so far (which gives an emulated frequency of 32.786KHz),
but can also be 19.2 or 27MHz which result in much larger drift.

Since this is used to drive the master counter at 32.768KHz * 375 /
2 = 6.144MHz, the emulated speed for 20MHz is of by 570ppm, or about 43
seconds per day, and more than the 500ppm NTP is able to tolerate.

Checking the CTRL_CORE_BOOTSTRAP register can determine if the CPU
is using the real 32.768KHz crystal or the emulated SYSCLK1/610, and
by known that the real counter frequency can be determined and used.
The real speed is then SYSCLK1 / 610 * 375 / 2 or SYSCLK1 * 75 / 244.

Signed-off-by: Len Sorensen <lsorense at csclub.uwaterloo.ca>
---
 arch/arm/mach-omap2/timer.c |  120 +++++++++++++++++++++++++++++++------------
 1 file changed, 87 insertions(+), 33 deletions(-)

diff --git a/arch/arm/mach-omap2/timer.c b/arch/arm/mach-omap2/timer.c
index fb0cb2b..f00e4b4 100644
--- a/arch/arm/mach-omap2/timer.c
+++ b/arch/arm/mach-omap2/timer.c
@@ -497,6 +497,7 @@ static void __init realtime_counter_init(void)
 	static struct clk *sys_clk;
 	unsigned long rate;
 	unsigned int reg, num, den;
+	bool errata_i856_workaround = false;
 
 	base = ioremap(REALTIME_COUNTER_BASE, SZ_32);
 	if (!base) {
@@ -510,39 +511,93 @@ static void __init realtime_counter_init(void)
 		return;
 	}
 
+	if (soc_is_dra7xx()) {
+		#define CTRL_CORE_BOOTSTRAP 0x4A0026C4
+		#define SPEEDSELECT_MASK 0x00000300
+		void __iomem *corebase;
+		corebase = ioremap(CTRL_CORE_BOOTSTRAP, SZ_4);
+		if (!corebase)
+			pr_err("%s: ioremap failed\n", __func__);
+		else {
+			reg = readl_relaxed(corebase) & SPEEDSELECT_MASK;
+			iounmap(corebase);
+			/*
+			 * Errata i856 says the 32.768KHz crystal does
+			 * not start at power on, so the CPU falls back in
+			 * an emulated 32KHz clock instead.  This causes
+			 * the master counter frequency to not be 6.144MHz
+			 * This affects at least the AM572x 1.0 and
+			 * 1.1 revisions.
+			 *
+			 * Of course any board built without a populated
+			 * 32.768KHz crystal would also need this fix
+			 * even if the CPU is fixed later.
+			 *
+			 * If the two speedselect bits are not 0, then the
+			 * 32.768KHz clock driving the course counter that
+			 * corrects the fine counter every time it ticks is
+			 * actually rate/610 rather than 32.768KHz and we
+			 * should compensate to avoid the 570ppm (At 20MHz,
+			 * much worse at other rates) too fast system time.
+			 */
+			if (reg) {
+				errata_i856_workaround = true;
+			}
+		}
+	}
+
 	rate = clk_get_rate(sys_clk);
-	/* Numerator/denumerator values refer TRM Realtime Counter section */
-	switch (rate) {
-	case 12000000:
-		num = 64;
-		den = 125;
-		break;
-	case 13000000:
-		num = 768;
-		den = 1625;
-		break;
-	case 19200000:
-		num = 8;
-		den = 25;
-		break;
-	case 20000000:
-		num = 192;
-		den = 625;
-		break;
-	case 26000000:
-		num = 384;
-		den = 1625;
-		break;
-	case 27000000:
-		num = 256;
-		den = 1125;
-		break;
-	case 38400000:
-	default:
-		/* Program it for 38.4 MHz */
-		num = 4;
-		den = 25;
-		break;
+	if (errata_i856_workaround) {
+		/*
+		 * Realtime Counter frequency is not based on a real
+		 * 32.768KHz time source, so calculate the real resulting
+		 * frequency instead.  It is not 6.144MHz in this case.
+		 *
+		 * The frequency is always rate / 610 + 375 / 2 which
+		 * is rate * 244 / 75 and will fit in 32 bit for all rates.
+		 *
+		 * The multiplication has to be first to keep accuracy
+		 * with integer math as high as possible.
+		 */
+		num = 75;
+		den = 244;
+		arch_timer_freq = (rate * num) / den;
+	} else {
+		/* Numerator/denumerator values refer TRM Realtime Counter section */
+		switch (rate) {
+		case 12000000:
+			num = 64;
+			den = 125;
+			break;
+		case 13000000:
+			num = 768;
+			den = 1625;
+			break;
+		case 19200000:
+			num = 8;
+			den = 25;
+			break;
+		case 20000000:
+			num = 192;
+			den = 625;
+			break;
+		case 26000000:
+			num = 384;
+			den = 1625;
+			break;
+		case 27000000:
+			num = 256;
+			den = 1125;
+			break;
+		case 38400000:
+		default:
+			/* Program it for 38.4 MHz */
+			num = 4;
+			den = 25;
+			break;
+		}
+		/* This divides cleanly and fits in 32 bits */
+		arch_timer_freq = (rate / den) * num;
 	}
 
 	/* Program numerator and denumerator registers */
@@ -556,7 +611,6 @@ static void __init realtime_counter_init(void)
 	reg |= den;
 	writel_relaxed(reg, base + INCREMENTER_DENUMERATOR_RELOAD_OFFSET);
 
-	arch_timer_freq = (rate / den) * num;
 	set_cntfreq();
 
 	iounmap(base);
-- 
1.7.10.4