mips: ingenic: Enable EXT/2 divider on JZ4750/55/60 if EXT is 24 MHz

The JZ4750, JZ4755 and JZ4760 (non-B version) support using a 24 MHz
external crystal oscillator instead of the typical 12 MHz one.

However, most of the SoC's IP blocks only work with a 12 MHz clock.
Thanksfully, there is a /2 divider we can enable when a 24 MHz external
crystal is present.

Force-enable this /2 divider when the oscillator is 24 MHz, so that the
SoC always uses a 12 MHz clock internally.

It is done here, and not in the clocks driver, because we need the EXT
clock to be 12 MHz for the early console to work, and the clocks driver
probes way too late.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/generic/board-ingenic.c

@@ -17,6 +17,7 @@
 #include <linux/types.h>
 
 #include <asm/bootinfo.h>
+#include <asm/io.h>
 #include <asm/machine.h>
 #include <asm/reboot.h>
 
@@ -60,6 +61,50 @@ static __init char *ingenic_get_system_type(unsigned long machtype)
 	}
 }
 
+#define INGENIC_CGU_BASE	0x10000000
+#define JZ4750_CGU_CPCCR_ECS	BIT(30)
+#define JZ4760_CGU_CPCCR_ECS	BIT(31)
+
+static __init void ingenic_force_12M_ext(const void *fdt, unsigned int mask)
+{
+	const __be32 *prop;
+	unsigned int cpccr;
+	void __iomem *cgu;
+	bool use_div;
+	int offset;
+
+	offset = fdt_path_offset(fdt, "/ext");
+	if (offset < 0)
+		return;
+
+	prop = fdt_getprop(fdt, offset, "clock-frequency", NULL);
+	if (!prop)
+		return;
+
+	/*
+	 * If the external oscillator is 24 MHz, enable the /2 divider to
+	 * drive it down to 12 MHz, since this is what the hardware can work
+	 * with.
+	 * The 16 MHz cutoff value is arbitrary; setting it to 12 MHz would not
+	 * work as the crystal frequency (as reported in the Device Tree) might
+	 * be slightly above this value.
+	 */
+	use_div = be32_to_cpup(prop) >= 16000000;
+
+	cgu = ioremap(INGENIC_CGU_BASE, 0x4);
+	if (!cgu)
+		return;
+
+	cpccr = ioread32(cgu);
+	if (use_div)
+		cpccr |= mask;
+	else
+		cpccr &= ~mask;
+	iowrite32(cpccr, cgu);
+
+	iounmap(cgu);
+}
+
 static __init const void *ingenic_fixup_fdt(const void *fdt, const void *match_data)
 {
 	/*
@@ -73,6 +118,18 @@ static __init const void *ingenic_fixup_fdt(const void *fdt, const void *match_d
 	mips_machtype = (unsigned long)match_data;
 	system_type = ingenic_get_system_type(mips_machtype);
 
+	switch (mips_machtype) {
+	case MACH_INGENIC_JZ4750:
+	case MACH_INGENIC_JZ4755:
+		ingenic_force_12M_ext(fdt, JZ4750_CGU_CPCCR_ECS);
+		break;
+	case MACH_INGENIC_JZ4760:
+		ingenic_force_12M_ext(fdt, JZ4760_CGU_CPCCR_ECS);
+		break;
+	default:
+		break;
+	}
+
 	return fdt;
 }