ARM: OMAP5/DRA7: 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 * 375 / 1220.
This is unfortunately not integer math so the actual arch_timer_freq
needs to be precalculated.

Also the SYSCLK1 frequencies that have never been used by an omap
evaluation board were all missing a 0.

diff --git a/arch/arm/mach-omap2/timer.c b/arch/arm/mach-omap2/timer.c
index 4f61148..2e81511 100644
--- a/arch/arm/mach-omap2/timer.c
+++ b/arch/arm/mach-omap2/timer.c
@@ -513,11 +513,11 @@ static void __init realtime_counter_init(void)
 	rate = clk_get_rate(sys_clk);
 	/* Numerator/denumerator values refer TRM Realtime Counter section */
 	switch (rate) {
-	case 1200000:
+	case 12000000:
 		num = 64;
 		den = 125;
 		break;
-	case 1300000:
+	case 13000000:
 		num = 768;
 		den = 1625;
 		break;
@@ -529,11 +529,11 @@ static void __init realtime_counter_init(void)
 		num = 192;
 		den = 625;
 		break;
-	case 2600000:
+	case 26000000:
 		num = 384;
 		den = 1625;
 		break;
-	case 2700000:
+	case 27000000:
 		num = 256;
 		den = 1125;
 		break;
@@ -557,6 +557,73 @@ static void __init realtime_counter_init(void)
 	writel_relaxed(reg, base + INCREMENTER_DENUMERATOR_RELOAD_OFFSET);
 
 	arch_timer_freq = (rate / den) * num;
+
+	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 what it
+			 * was expected to be.	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.
+			 *
+			 * Precalculate the arch_timer_freq, since
+			 * rate/610 isn't integer math and accuracy is
+			 * desirable here.
+			 */
+			if (reg) {
+				switch (rate) {
+				case 19200000:
+					num = 375;
+					den = 1220;
+					arch_timer_freq = 5901639;
+					break;
+				case 27000000:
+					num = 375;
+					den = 1220;
+					arch_timer_freq = 8299180;
+					break;
+				case 20000000:
+				default:
+					num = 375;
+					den = 1220;
+					arch_timer_freq = 6147541;
+					break;
+				}
+				reg = readl_relaxed(base + INCREMENTER_NUMERATOR_OFFSET) &
+						NUMERATOR_DENUMERATOR_MASK;
+				reg |= num;
+				writel_relaxed(reg, base + INCREMENTER_NUMERATOR_OFFSET);
+
+				reg = readl_relaxed(base + INCREMENTER_DENUMERATOR_RELOAD_OFFSET) &
+						NUMERATOR_DENUMERATOR_MASK;
+				reg |= den;
+				writel_relaxed(reg, base + INCREMENTER_DENUMERATOR_RELOAD_OFFSET);
+
+				printk(KERN_WARNING "DRA7xx CP15 compensated for sloppy internal 32KHz clock.\n");
+			}
+		}
+	}
+
 	set_cntfreq();
 
 	iounmap(base);

-- 
Len Sorensen
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