tty: serial: Add earlycon driver for Tegra Combined UART

Add an earlycon driver for platforms with TCU, namely Tegra194.
The driver is compatible with boot parameters passed by NVIDIA
boot chains.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>

drivers/tty/serial/Kconfig

@@ -331,6 +331,18 @@ config SERIAL_TEGRA_TCU_CONSOLE
 
 	  If unsure, say Y.
 
+config SERIAL_TEGRA_TCU_EARLYCON
+	bool "Earlycon on NVIDIA Tegra Combined UART"
+	depends on ARCH_TEGRA || COMPILE_TEST
+	select SERIAL_EARLYCON
+	select SERIAL_CORE_CONSOLE
+	default y if SERIAL_TEGRA_TCU_CONSOLE
+	help
+	  If you say Y here, TCU output will be supported during the earlycon
+	  phase of the boot.
+
+	  If unsure, say Y.
+
 config SERIAL_MAX3100
 	tristate "MAX3100 support"
 	depends on SPI

drivers/tty/serial/Makefile

@@ -72,6 +72,7 @@ obj-$(CONFIG_SERIAL_XILINX_PS_UART) += xilinx_uartps.o
 obj-$(CONFIG_SERIAL_SIRFSOC) += sirfsoc_uart.o
 obj-$(CONFIG_SERIAL_TEGRA) += serial-tegra.o
 obj-$(CONFIG_SERIAL_TEGRA_TCU) += tegra-tcu.o
+obj-$(CONFIG_SERIAL_TEGRA_TCU_EARLYCON) += tegra-tcu-earlycon.o
 obj-$(CONFIG_SERIAL_AR933X)   += ar933x_uart.o
 obj-$(CONFIG_SERIAL_EFM32_UART) += efm32-uart.o
 obj-$(CONFIG_SERIAL_ARC)	+= arc_uart.o

drivers/tty/serial/tegra-tcu-earlycon.c

@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2017-2021, NVIDIA CORPORATION.  All rights reserved.
+ */
+
+#include <linux/console.h>
+#include <linux/io.h>
+#include <linux/serial_core.h>
+
+#define NUM_BYTES_FIELD_BIT	24
+#define FLUSH_BIT		26
+#define INTR_TRIGGER_BIT	31
+
+static u32 update_and_send_mbox(u8 __iomem *addr, u32 mbox_val, char c)
+{
+	int bytes = bytes = (mbox_val >> NUM_BYTES_FIELD_BIT) & 0x3;
+
+	mbox_val |= BIT(INTR_TRIGGER_BIT);
+	mbox_val |= c << (bytes * 8);
+	bytes++;
+	mbox_val = (mbox_val & ~(3 << NUM_BYTES_FIELD_BIT)) |
+		(bytes << NUM_BYTES_FIELD_BIT);
+
+	if (bytes == 3) {
+		/* Send current packet to SPE */
+		while (readl(addr) & BIT(INTR_TRIGGER_BIT))
+			cpu_relax();
+		writel(mbox_val, addr);
+		mbox_val = BIT(INTR_TRIGGER_BIT);
+	}
+
+	return mbox_val;
+}
+
+/*
+ * This function splits the string to be printed (const char *s) into multiple
+ * packets. Each packet contains a max of 3 characters. Packets are sent to the
+ * SPE-based combined UART server for printing. Communication with SPE is done
+ * through mailbox registers which can generate interrupts for SPE.
+ */
+static void early_tcu_write(struct console *console, const char *s, unsigned int count)
+{
+	struct earlycon_device *device = console->data;
+	u8 __iomem *addr = device->port.membase;
+	u32 mbox_val = BIT(INTR_TRIGGER_BIT);
+	unsigned int i;
+
+	/* Loop for processing each 3 char packet */
+	for (i = 0; i < count; i++) {
+		if (s[i] == '\n')
+			mbox_val = update_and_send_mbox(addr, mbox_val, '\r');
+		mbox_val = update_and_send_mbox(addr, mbox_val, s[i]);
+	}
+
+	if ((mbox_val >> NUM_BYTES_FIELD_BIT) & 0x3) {
+		while (readl(addr) & BIT(INTR_TRIGGER_BIT))
+			cpu_relax();
+		writel(mbox_val, addr);
+	}
+}
+
+int __init early_tegra_combined_uart_setup(struct earlycon_device *device, const char *options)
+{
+	if (!(device->port.membase))
+		return -ENODEV;
+
+	device->con->write = early_tcu_write;
+
+	return 0;
+}
+
+EARLYCON_DECLARE(tegra_comb_uart, early_tegra_combined_uart_setup);