serial: Add serial driver for Intel MID

Add a specific serial driver for Intel MID platforms.

It has special fractional divider which can be programmed via UART_PS,
UART_MUL, and UART_DIV registers.

The UART clock is calculated as

	UART clock = XTAL * UART_MUL / UART_DIV

The baudrate is calculated as

	baud rate = UART clock / UART_PS / DLAB

Initialize fractional divider correctly for Intel Edison platform.

For backward compatibility we have to set initial DLAB value to 16
and speed to 115200 baud, where initial frequency is 29491200Hz, and
XTAL frequency is 38.4MHz.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>

drivers/serial/Kconfig

@@ -376,6 +376,15 @@ config SYS_NS16550
 	  be used. It can be a constant or a function to get clock, eg,
 	  get_serial_clock().
 
+config INTEL_MID_SERIAL
+	bool "Intel MID platform UART support"
+	depends on DM_SERIAL && OF_CONTROL
+	depends on INTEL_MID
+	select SYS_NS16550
+	help
+	  Select this to enable a UART for Intel MID platforms.
+	  This uses the ns16550 driver as a library.
+
 config ROCKCHIP_SERIAL
 	bool "Rockchip on-chip UART support"
 	depends on DM_SERIAL && SPL_OF_PLATDATA

drivers/serial/Makefile

@@ -28,6 +28,7 @@ obj-$(CONFIG_S5P) += serial_s5p.o
 obj-$(CONFIG_MXC_UART) += serial_mxc.o
 obj-$(CONFIG_PXA_SERIAL) += serial_pxa.o
 obj-$(CONFIG_MESON_SERIAL) += serial_meson.o
+obj-$(CONFIG_INTEL_MID_SERIAL) += serial_intel_mid.o
 ifdef CONFIG_SPL_BUILD
 obj-$(CONFIG_ROCKCHIP_SERIAL) += serial_rockchip.o
 endif

drivers/serial/serial_intel_mid.c

@@ -0,0 +1,69 @@
+/*
+ * Copyright (c) 2017 Intel Corporation
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#include <common.h>
+#include <dm.h>
+#include <ns16550.h>
+#include <serial.h>
+
+/*
+ * The UART clock is calculated as
+ *
+ *	UART clock = XTAL * UART_MUL / UART_DIV
+ *
+ * The baudrate is calculated as
+ *
+ *	baud rate = UART clock / UART_PS / DLAB
+ */
+#define UART_PS		0x30
+#define UART_MUL	0x34
+#define UART_DIV	0x38
+
+static void mid_writel(struct ns16550_platdata *plat, int offset, int value)
+{
+	unsigned char *addr;
+
+	offset *= 1 << plat->reg_shift;
+	addr = (unsigned char *)plat->base + offset;
+
+	writel(value, addr + plat->reg_offset);
+}
+
+static int mid_serial_probe(struct udevice *dev)
+{
+	struct ns16550_platdata *plat = dev_get_platdata(dev);
+
+	/*
+	 * Initialize fractional divider correctly for Intel Edison
+	 * platform.
+	 *
+	 * For backward compatibility we have to set initial DLAB value
+	 * to 16 and speed to 115200 baud, where initial frequency is
+	 * 29491200Hz, and XTAL frequency is 38.4MHz.
+	 */
+	mid_writel(plat, UART_MUL, 96);
+	mid_writel(plat, UART_DIV, 125);
+	mid_writel(plat, UART_PS, 16);
+
+	return ns16550_serial_probe(dev);
+}
+
+static const struct udevice_id mid_serial_ids[] = {
+	{ .compatible = "intel,mid-uart" },
+	{}
+};
+
+U_BOOT_DRIVER(serial_intel_mid) = {
+	.name	= "serial_intel_mid",
+	.id	= UCLASS_SERIAL,
+	.of_match = mid_serial_ids,
+	.ofdata_to_platdata = ns16550_serial_ofdata_to_platdata,
+	.platdata_auto_alloc_size = sizeof(struct ns16550_platdata),
+	.priv_auto_alloc_size = sizeof(struct NS16550),
+	.probe	= mid_serial_probe,
+	.ops	= &ns16550_serial_ops,
+	.flags	= DM_FLAG_PRE_RELOC,
+};