[PATCH v=2] serial: implement riscv SBI console support

[Marcelo Politzer]

Implement a console over legacy SBI (version 0.1.0). There is a tiny ringbuffer
to simplify checking for presence and reading characters as separate steps.
---
 arch/riscv/cpu/core.c       | 12 ++++++++
 arch/riscv/lib/sbi.c        | 10 +++++++
 drivers/serial/Kconfig      |  8 +++++
 drivers/serial/Makefile     |  1 +
 drivers/serial/serial_sbi.c | 58 +++++++++++++++++++++++++++++++++++++
 5 files changed, 89 insertions(+)
 create mode 100644 drivers/serial/serial_sbi.c

diff --git a/arch/riscv/cpu/core.c b/arch/riscv/cpu/core.c
index 80730c05b..1d5902a51 100644
--- a/arch/riscv/cpu/core.c
+++ b/arch/riscv/cpu/core.c
@@ -33,6 +33,7 @@ static int riscv_request_stack(void)
 coredevice_initcall(riscv_request_stack);
 
 static struct device_d timer_dev;
+static struct device_d serial_sbi_dev;
 
 static s64 hartid;
 
@@ -75,6 +76,17 @@ static int riscv_probe(struct device_d *parent)
 			return ret;
 	}
 
+	if (IS_ENABLED(CONFIG_SERIAL_SBI) && !serial_sbi_dev.parent) {
+		serial_sbi_dev.id = DEVICE_ID_SINGLE;
+		serial_sbi_dev.device_node = 0;
+		serial_sbi_dev.parent = parent;
+		dev_set_name(&serial_sbi_dev, "riscv-serial-sbi");
+
+		ret = platform_device_register(&serial_sbi_dev);
+		if (ret)
+			return ret;
+	}
+
 	hartid = riscv_hartid();
 	if (hartid >= 0)
 		globalvar_add_simple_uint64("hartid", &hartid, "%llu");
diff --git a/arch/riscv/lib/sbi.c b/arch/riscv/lib/sbi.c
index 45a04fb82..a57c834b4 100644
--- a/arch/riscv/lib/sbi.c
+++ b/arch/riscv/lib/sbi.c
@@ -64,3 +64,13 @@ static int sbi_init(void)
 
 }
 core_initcall(sbi_init);
+
+void sbi_console_putchar(int ch)
+{
+	sbi_ecall(SBI_EXT_0_1_CONSOLE_PUTCHAR, 0, ch, 0, 0, 0, 0, 0);
+}
+
+int sbi_console_getchar(void)
+{
+	return sbi_ecall(SBI_EXT_0_1_CONSOLE_GETCHAR, 0, 0, 0, 0, 0, 0, 0).error;
+}
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index b9750d177..5e30ea388 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -173,4 +173,12 @@ config SERIAL_SIFIVE
 	  contains a SiFive UART IP block.  This type of UART is present on
 	  SiFive FU540 SoCs, among others.
 
+config SERIAL_SBI
+	tristate "RISCV Serial support over SBI's HTIF"
+	depends on OFDEVICE
+	depends on RISCV_SBI
+	help
+	  Select this option if you are building barebox for a RISCV platform
+	  that implements a serial over SBI.
+
 endmenu
diff --git a/drivers/serial/Makefile b/drivers/serial/Makefile
index 5120b1737..b1de436ed 100644
--- a/drivers/serial/Makefile
+++ b/drivers/serial/Makefile
@@ -24,3 +24,4 @@ obj-$(CONFIG_DRIVER_SERIAL_DIGIC)		+= serial_digic.o
 obj-$(CONFIG_DRIVER_SERIAL_LPUART)		+= serial_lpuart.o
 obj-$(CONFIG_VIRTIO_CONSOLE)			+= virtio_console.o
 obj-$(CONFIG_SERIAL_SIFIVE)			+= serial_sifive.o
+obj-$(CONFIG_SERIAL_SBI)			+= serial_sbi.o
diff --git a/drivers/serial/serial_sbi.c b/drivers/serial/serial_sbi.c
new file mode 100644
index 000000000..2ea28fea5
--- /dev/null
+++ b/drivers/serial/serial_sbi.c
@@ -0,0 +1,58 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2021 Marcelo Politzer <marcelo.politzer at cartesi.io>
+ */
+
+#include <common.h>
+#include <driver.h>
+#include <malloc.h>
+#include <asm/sbi.h>
+
+struct sbi_serial_priv {
+	struct console_device cdev;
+	uint8_t b[2], head, tail;
+};
+
+static int sbi_serial_getc(struct console_device *cdev)
+{
+	struct sbi_serial_priv *priv = cdev->dev->priv;
+	if (priv->head == priv->tail)
+		return -1;
+	return priv->b[priv->head++ & 0x1];
+}
+
+static void sbi_serial_putc(struct console_device *cdev, const char ch)
+{
+	sbi_console_putchar(ch);
+}
+
+static int sbi_serial_tstc(struct console_device *cdev)
+{
+	struct sbi_serial_priv *priv = cdev->dev->priv;
+	int c = sbi_console_getchar();
+
+	if (c != -1)
+		priv->b[priv->tail++ & 0x1] = c;
+	return priv->head != priv->tail;
+}
+
+static int sbi_serial_probe(struct device_d *dev)
+{
+	struct sbi_serial_priv *priv;
+
+	priv = dev->priv = xzalloc(sizeof(*priv));
+	priv->cdev.dev    = dev;
+	priv->cdev.putc   = sbi_serial_putc;
+	priv->cdev.getc   = sbi_serial_getc;
+	priv->cdev.tstc   = sbi_serial_tstc;
+	priv->cdev.flush  = 0;
+	priv->cdev.setbrg = 0;
+
+	return console_register(&priv->cdev);
+}
+
+static struct driver_d serial_sbi_driver = {
+	.name   = "riscv-serial-sbi",
+	.probe  = sbi_serial_probe,
+};
+postcore_platform_driver(serial_sbi_driver);
-- 
2.32.0