serial: add semihosting console

There is no way to implement a proper tstc() with the semihosting
console, so for now, let's add it as output-only console.

Currently, the console is only usable when not running as EFI payload to
avoid duplicate output when it's exposed over regular EFI protocol.

In the future, we could make it opt-in by providing a way to create the
semihosting device from shell.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://lore.barebox.org/20240611065923.2900168-7-a.fatoum@pengutronix.de
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Author: a3f

drivers/serial/Kconfig

@@ -151,4 +151,12 @@ config SERIAL_SBI
 	  Select this option if you are building barebox for a RISCV platform
 	  that implements a serial over SBI.
 
+config SERIAL_SEMIHOSTING
+	bool "Semihosting console"
+	depends on SEMIHOSTING
+	help
+	  Select this option if you want barebox to be able to output to
+	  the semihosting console implemented by a debugger or emulator.
+	  This console can not be read from.
+
 endmenu

drivers/serial/Makefile

@@ -25,3 +25,4 @@ obj-$(CONFIG_VIRTIO_CONSOLE)			+= virtio_console.o
 obj-$(CONFIG_SERIAL_SIFIVE)			+= serial_sifive.o
 obj-$(CONFIG_SERIAL_SBI)			+= serial_sbi.o
 obj-$(CONFIG_SOC_LITEX)				+= serial_litex.o
+obj-$(CONFIG_SERIAL_SEMIHOSTING)		+= serial_semihosting.o

drivers/serial/serial_semihosting.c

@@ -0,0 +1,44 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+#include <console.h>
+#include <asm/semihosting.h>
+#include <efi/efi-mode.h>
+
+static void smh_serial_putc(struct console_device *cdev, char ch)
+{
+	semihosting_writec(ch);
+}
+
+static int smh_serial_probe(struct device *dev)
+{
+	struct console_device *cdev;
+
+	cdev = xzalloc(sizeof(*cdev));
+
+	cdev->dev = dev;
+	cdev->putc = smh_serial_putc;
+
+	return console_register(cdev);
+}
+
+static struct driver serial_semihosting_driver = {
+	.name   = "serial_semihosting",
+	.probe  = smh_serial_probe,
+};
+
+static int __init serial_semihosting_register(void)
+{
+	if (!efi_is_payload()) {
+		struct device *dev;
+		int ret;
+
+		dev = device_alloc("serial_semihosting", 0);
+
+		ret = platform_device_register(dev);
+		if (ret)
+			return ret;
+	}
+
+	return platform_driver_register(&serial_semihosting_driver);
+}
+console_initcall(serial_semihosting_register);