lib: utils/serial: Add support for Altera JTAG UART

Altera provides a JTAG UART core that provides virtual UART over JTAG
and can coexist with their virtual JTAG. [1] This core has already been
supported by Linux and the programming interface has always been stable.

Add support for it to OpenSBI to ease JTAG prototype bringing up.

The driver follows the device tree binding in mainline Linux. [2]

[1] https://docs.altera.com/r/docs/683130/25.3/embedded-peripherals-ip-user-guide/jtag-uart-core
[2] https://github.com/torvalds/linux/blob/v6.19-rc1/Documentation/devicetree/bindings/serial/altr%2Cjuart-1.0.yaml

Signed-off-by: Icenowy Zheng <zhengxingda@iscas.ac.cn>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260104065506.70182-1-zhengxingda@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>

include/sbi_utils/serial/altr-juart.h

@@ -0,0 +1,17 @@
+/*
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2025 ISCAS
+ *
+ * Authors:
+ *   Icenowy Zheng <zhengxingda@iscas.ac.cn>
+ */
+
+#ifndef __SERIAL_ALTR_JUART_H__
+#define __SERIAL_ALTR_JUART_H__
+
+#include <sbi/sbi_types.h>
+
+int altr_juart_init(unsigned long base);
+
+#endif

lib/utils/serial/Kconfig

@@ -9,6 +9,11 @@ config FDT_SERIAL
 
 if FDT_SERIAL
 
+config FDT_SERIAL_ALTR_JUART
+	bool "Altera JTAG UART FDT driver"
+	select SERIAL_ALTR_JUART
+	default n
+
 config FDT_SERIAL_CADENCE
 	bool "Cadence UART FDT driver"
 	select SERIAL_CADENCE
@@ -56,6 +61,10 @@ config FDT_SERIAL_XILINX_UARTLITE
 
 endif
 
+config SERIAL_ALTR_JUART
+	bool "Altera JTAG UART support"
+	default n
+
 config SERIAL_CADENCE
 	bool "Cadence UART support"
 	default n

lib/utils/serial/altr-juart.c

@@ -0,0 +1,74 @@
+/*
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2025 ISCAS
+ *
+ * Authors:
+ *   Icenowy Zheng <zhengxingda@iscas.ac.cn>
+ */
+
+#include <sbi/riscv_io.h>
+#include <sbi/sbi_console.h>
+#include <sbi_utils/serial/altr-juart.h>
+
+/* clang-format off */
+
+#define JUART_DATA_OFFSET		0x00
+# define JUART_DATA_DATA_MASK		0x000000ff
+# define JUART_DATA_RVALID		(1 << 15)
+# define JUART_DATA_RAVAIL_MASK		0xffff0000
+# define JUART_DATA_RAVAIL_SHIFT	16
+#define JUART_CTRL_OFFSET		0x04
+# define JUART_CTRL_RE			(1 << 0)
+# define JUART_CTRL_WE			(1 << 1)
+# define JUART_CTRL_RI			(1 << 8)
+# define JUART_CTRL_WI			(1 << 9)
+# define JUART_CTRL_AC			(1 << 10)
+# define JUART_CTRL_WSPACE_MASK		0xffff0000
+# define JUART_CTRL_WSPACE_SHIFT	16
+
+/* clang-format on */
+
+static volatile char *altr_juart_base;
+
+static u32 get_reg(u32 offset)
+{
+	return readl(altr_juart_base + offset);
+}
+
+static void set_reg(u32 offset, u32 val)
+{
+	writel(val, altr_juart_base + offset);
+}
+
+static void altr_juart_putc(char ch)
+{
+	while(!(get_reg(JUART_CTRL_OFFSET) & JUART_CTRL_WSPACE_MASK))
+		;
+
+	set_reg(JUART_DATA_OFFSET, (unsigned char)ch);
+}
+
+static int altr_juart_getc(void)
+{
+	u32 reg = get_reg(JUART_DATA_OFFSET);
+	if (reg & JUART_DATA_RVALID)
+		return reg & JUART_DATA_DATA_MASK;
+
+	return -1;
+}
+
+static struct sbi_console_device altr_juart_console = {
+	.name = "altr-juart",
+	.console_putc = altr_juart_putc,
+	.console_getc = altr_juart_getc
+};
+
+int altr_juart_init(unsigned long base)
+{
+	altr_juart_base = (volatile char *)base;
+
+	sbi_console_set_device(&altr_juart_console);
+
+	return 0;
+}

lib/utils/serial/fdt_serial_altr_juart.c

@@ -0,0 +1,40 @@
+/*
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2025 ISCAS
+ *
+ * Authors:
+ *   Icenowy Zheng <zhengxingda@iscas.ac.cn>
+ */
+
+#include <sbi/sbi_error.h>
+#include <sbi_utils/fdt/fdt_helper.h>
+#include <sbi_utils/serial/fdt_serial.h>
+#include <sbi_utils/serial/altr-juart.h>
+
+static int serial_altr_juart_init(const void *fdt, int nodeoff,
+				  const struct fdt_match *match)
+{
+	uint64_t reg_addr, reg_size;
+	int rc;
+
+	if (nodeoff < 0 || !fdt)
+		return SBI_ENODEV;
+
+	rc = fdt_get_node_addr_size(fdt, nodeoff, 0, &reg_addr, &reg_size);
+	/* Two 32-bit registers */
+	if (rc < 0 || !reg_addr || !reg_size || reg_size < 0x8)
+		return SBI_ENODEV;
+
+	return altr_juart_init(reg_addr);
+}
+
+static const struct fdt_match serial_altr_juart_match[] = {
+	{ .compatible = "altr,juart-1.0" },
+	{ },
+};
+
+const struct fdt_driver fdt_serial_altr_juart = {
+	.match_table = serial_altr_juart_match,
+	.init = serial_altr_juart_init,
+};

lib/utils/serial/objects.mk

@@ -10,6 +10,9 @@
 libsbiutils-objs-$(CONFIG_FDT_SERIAL) += serial/fdt_serial.o
 libsbiutils-objs-$(CONFIG_FDT_SERIAL) += serial/fdt_serial_drivers.carray.o
 
+carray-fdt_serial_drivers-$(CONFIG_FDT_SERIAL_ALTR_JUART) += fdt_serial_altr_juart
+libsbiutils-objs-$(CONFIG_FDT_SERIAL_ALTR_JUART) += serial/fdt_serial_altr_juart.o
+
 carray-fdt_serial_drivers-$(CONFIG_FDT_SERIAL_CADENCE) += fdt_serial_cadence
 libsbiutils-objs-$(CONFIG_FDT_SERIAL_CADENCE) += serial/fdt_serial_cadence.o
 
@@ -37,6 +40,7 @@ libsbiutils-objs-$(CONFIG_FDT_SERIAL_UART8250) += serial/fdt_serial_uart8250.o
 carray-fdt_serial_drivers-$(CONFIG_FDT_SERIAL_XILINX_UARTLITE) += fdt_serial_xlnx_uartlite
 libsbiutils-objs-$(CONFIG_FDT_SERIAL_XILINX_UARTLITE) += serial/fdt_serial_xlnx_uartlite.o
 
+libsbiutils-objs-$(CONFIG_SERIAL_ALTR_JUART) += serial/altr-juart.o
 libsbiutils-objs-$(CONFIG_SERIAL_CADENCE) += serial/cadence-uart.o
 libsbiutils-objs-$(CONFIG_SERIAL_GAISLER) += serial/gaisler-uart.o
 libsbiutils-objs-$(CONFIG_SERIAL_RENESAS_SCIF) += serial/renesas_scif.o

platform/generic/configs/defconfig

@@ -51,6 +51,7 @@ CONFIG_FDT_RESET_SG2042_HWMON_MCU=y
 CONFIG_FDT_RESET_SUNXI_WDT=y
 CONFIG_FDT_RESET_SYSCON=y
 CONFIG_FDT_SERIAL=y
+CONFIG_FDT_SERIAL_ALTR_JUART=y
 CONFIG_FDT_SERIAL_CADENCE=y
 CONFIG_FDT_SERIAL_GAISLER=y
 CONFIG_FDT_SERIAL_HTIF=y