serial: Add semihosting driver

This adds a serial driver which uses semihosting calls to read and write
to the host's console. For convenience, if CONFIG_DM_SERIAL is enabled,
we will instantiate a serial driver. This allows users to enable this
driver (which has no physical device) without modifying their device
trees or board files. We also implement a non-DM driver for SPL, or for
much faster output in U-Boot proper.

There are three ways to print to the console:

Method              Baud
================== =====
smh_putc in a loop   170
smh_puts            1600
smh_write with :tt 20000
================== =====

These speeds were measured using a 175 character message with a J-Link
adapter. For reference, U-Boot typically prints around 2700 characters
during boot on this board. There are two major factors affecting the
speed of these functions. First, each breakpoint incurs a delay. Second,
each debugger memory transaction incurs a delay. smh_putc has a
breakpoint and memory transaction for every character. smh_puts has one
breakpoint, but still has to use a transaction for every character. This
is because we don't know the length up front, so OpenOCD has to check if
each character is nul. smh_write has only one breakpoint and one memory
transfer.

DM serial drivers can only implement a putc interface, so we are stuck
with the slowest API. Non-DM drivers can implement puts, which is vastly
more efficient. When the driver starts up, we try to open :tt. Since
this is an extension, this may fail. If it does, we fall back to
smh_puts. We don't check :semihosting-features, since there are
nonconforming implementations (OpenOCD) which don't implement it (but
*do* implement :tt).

Some semihosting implementations (QEMU) don't handle READC properly. To
work around this, we try to use open/read (much like for stdin) if
possible.

There is no non-blocking I/O available, so we don't implement pending.
This will cause __serial_tstc to always return true. If
CONFIG_SERIAL_RX_BUFFER is enabled, _serial_tstc will try and read
characters forever. To avoid this, we depend on this config being
disabled.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/serial/Kconfig

@@ -399,6 +399,15 @@ config DEBUG_UART_SANDBOX
 	  start up driver model. The driver will be available until the real
 	  driver model serial is running.
 
+config DEBUG_UART_SEMIHOSTING
+	bool "semihosting"
+	depends on SEMIHOSTING_SERIAL
+	help
+	  Select this to enable the debug UART using the semihosting driver.
+	  This provides basic serial output from the console without needing to
+	  start up driver model. The driver will be available until the real
+	  driver model serial is running.
+
 config DEBUG_UART_SIFIVE
 	bool "SiFive UART"
 	depends on SIFIVE_SERIAL
@@ -782,6 +791,19 @@ config SCIF_CONSOLE
 	  on systems with RCar or SH SoCs, say Y to this option. If unsure,
 	  say N.
 
+config SEMIHOSTING_SERIAL
+	bool "Semihosting UART support"
+	depends on SEMIHOSTING && !SERIAL_RX_BUFFER
+	help
+	  Select this to enable a serial UART using semihosting. Special halt
+	  instructions will be issued which an external debugger (such as a
+	  JTAG emulator) may interpret. The debugger will display U-Boot's
+	  console output on the host system.
+
+	  Enable this option only if you are using a debugger which supports
+	  semihosting. If you are not using a debugger, this driver will halt
+	  the boot.
+
 config UNIPHIER_SERIAL
 	bool "Support for UniPhier on-chip UART"
 	depends on ARCH_UNIPHIER

drivers/serial/Makefile

@@ -52,6 +52,7 @@ endif
 obj-$(CONFIG_XILINX_UARTLITE) += serial_xuartlite.o
 obj-$(CONFIG_SANDBOX_SERIAL) += sandbox.o
 obj-$(CONFIG_SCIF_CONSOLE) += serial_sh.o
+obj-$(CONFIG_SEMIHOSTING_SERIAL) += serial_semihosting.o
 obj-$(CONFIG_ZYNQ_SERIAL) += serial_zynq.o
 obj-$(CONFIG_FSL_LPUART) += serial_lpuart.o
 obj-$(CONFIG_FSL_LINFLEXUART) += serial_linflexuart.o

drivers/serial/serial.c

@@ -126,6 +126,7 @@ serial_initfunc(mxc_serial_initialize);
 serial_initfunc(ns16550_serial_initialize);
 serial_initfunc(pl01x_serial_initialize);
 serial_initfunc(pxa_serial_initialize);
+serial_initfunc(smh_serial_initialize);
 serial_initfunc(sh_serial_initialize);
 serial_initfunc(mtk_serial_initialize);
 
@@ -180,6 +181,7 @@ int serial_initialize(void)
 	ns16550_serial_initialize();
 	pl01x_serial_initialize();
 	pxa_serial_initialize();
+	smh_serial_initialize();
 	sh_serial_initialize();
 	mtk_serial_initialize();
 

drivers/serial/serial_semihosting.c

@@ -0,0 +1,147 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2022 Sean Anderson <sean.anderson@seco.com>
+ */
+
+#include <common.h>
+#include <dm.h>
+#include <serial.h>
+#include <semihosting.h>
+
+/**
+ * struct smh_serial_priv - Semihosting serial private data
+ * @infd: stdin file descriptor (or error)
+ */
+struct smh_serial_priv {
+	int infd;
+	int outfd;
+};
+
+#if CONFIG_IS_ENABLED(DM_SERIAL)
+static int smh_serial_getc(struct udevice *dev)
+{
+	char ch = 0;
+	struct smh_serial_priv *priv = dev_get_priv(dev);
+
+	if (priv->infd < 0)
+		return smh_getc();
+
+	smh_read(priv->infd, &ch, sizeof(ch));
+	return ch;
+}
+
+static int smh_serial_putc(struct udevice *dev, const char ch)
+{
+	smh_putc(ch);
+	return 0;
+}
+
+static const struct dm_serial_ops smh_serial_ops = {
+	.putc = smh_serial_putc,
+	.getc = smh_serial_getc,
+};
+
+static int smh_serial_probe(struct udevice *dev)
+{
+	struct smh_serial_priv *priv = dev_get_priv(dev);
+
+	priv->infd = smh_open(":tt", MODE_READ);
+	return 0;
+}
+
+U_BOOT_DRIVER(smh_serial) = {
+	.name	= "serial_semihosting",
+	.id	= UCLASS_SERIAL,
+	.probe	= smh_serial_probe,
+	.priv_auto = sizeof(struct smh_serial_priv),
+	.ops	= &smh_serial_ops,
+	.flags	= DM_FLAG_PRE_RELOC,
+};
+
+U_BOOT_DRVINFO(smh_serial) = {
+	.name = "serial_semihosting",
+};
+#else /* DM_SERIAL */
+static int infd = -ENODEV;
+static int outfd = -ENODEV;
+
+static int smh_serial_start(void)
+{
+	infd = smh_open(":tt", MODE_READ);
+	outfd = smh_open(":tt", MODE_WRITE);
+	return 0;
+}
+
+static int smh_serial_stop(void)
+{
+	if (outfd >= 0)
+		smh_close(outfd);
+	return 0;
+}
+
+static void smh_serial_setbrg(void)
+{
+}
+
+static int smh_serial_getc(void)
+{
+	char ch = 0;
+
+	if (infd < 0)
+		return smh_getc();
+
+	smh_read(infd, &ch, sizeof(ch));
+	return ch;
+}
+
+static int smh_serial_tstc(void)
+{
+	return 1;
+}
+
+static void smh_serial_puts(const char *s)
+{
+	ulong unused;
+
+	if (outfd < 0)
+		smh_puts(s);
+	else
+		smh_write(outfd, s, strlen(s), &unused);
+}
+
+struct serial_device serial_smh_device = {
+	.name	= "serial_smh",
+	.start	= smh_serial_start,
+	.stop	= smh_serial_stop,
+	.setbrg	= smh_serial_setbrg,
+	.getc	= smh_serial_getc,
+	.tstc	= smh_serial_tstc,
+	.putc	= smh_putc,
+	.puts	= smh_serial_puts,
+};
+
+void smh_serial_initialize(void)
+{
+	serial_register(&serial_smh_device);
+}
+
+__weak struct serial_device *default_serial_console(void)
+{
+	return &serial_smh_device;
+}
+#endif
+
+#ifdef CONFIG_DEBUG_UART_SEMIHOSTING
+#include <debug_uart.h>
+
+static inline void _debug_uart_init(void)
+{
+}
+
+static inline void _debug_uart_putc(int c)
+{
+	smh_putc(c);
+}
+
+DEBUG_UART_FUNCS
+#endif

include/serial.h

@@ -23,6 +23,7 @@ struct serial_device {
 void default_serial_puts(const char *s);
 
 extern struct serial_device serial_smc_device;
+extern struct serial_device serial_smh_device;
 extern struct serial_device serial_scc_device;
 extern struct serial_device *default_serial_console(void);