[PATCH 15/17] serial: Add semihosting driver
Sean Anderson
sean.anderson at seco.com
Thu Mar 3 21:43:58 CET 2022
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 17
smh_puts 160
smh_write with :tt 2000
================== ====
These speeds were mesured 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 imlementations (OpenOCD) which don't implement it (but
*do* implement :tt).
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 at seco.com>
---
drivers/serial/Kconfig | 22 ++++++
drivers/serial/Makefile | 1 +
drivers/serial/serial.c | 2 +
drivers/serial/serial_semihosting.c | 108 ++++++++++++++++++++++++++++
4 files changed, 133 insertions(+)
create mode 100644 drivers/serial/serial_semihosting.c
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index 345d1881f5..cc20759505 100644
--- a/drivers/serial/Kconfig
+++ b/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
@@ -778,6 +787,19 @@ 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
diff --git a/drivers/serial/Makefile b/drivers/serial/Makefile
index 52e70aa191..b68b5e7b2b 100644
--- a/drivers/serial/Makefile
+++ b/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
diff --git a/drivers/serial/serial.c b/drivers/serial/serial.c
index ebbd21916d..6cdbb89841 100644
--- a/drivers/serial/serial.c
+++ b/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();
diff --git a/drivers/serial/serial_semihosting.c b/drivers/serial/serial_semihosting.c
new file mode 100644
index 0000000000..44a67c5cb6
--- /dev/null
+++ b/drivers/serial/serial_semihosting.c
@@ -0,0 +1,108 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2022 Sean Anderson <sean.anderson at seco.com>
+ */
+
+#include <common.h>
+#include <dm.h>
+#include <serial.h>
+#include <semihosting.h>
+
+#if CONFIG_IS_ENABLED(DM_SERIAL)
+static int smh_serial_getc(struct udevice *dev)
+{
+ return smh_getc();
+}
+
+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,
+};
+
+U_BOOT_DRIVER(smh_serial) = {
+ .name = "serial_semihosting",
+ .id = UCLASS_SERIAL,
+ .ops = &smh_serial_ops,
+ .flags = DM_FLAG_PRE_RELOC,
+};
+
+U_BOOT_DRVINFO(smh_serial) = {
+ .name = "serial_semihosting",
+};
+#else /* DM_SERIAL */
+static int outfd = -ENODEV;
+
+static int smh_serial_start(void)
+{
+ 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_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);
+}
+
+static struct serial_device smh_serial_drv = {
+ .name = "serial_smh",
+ .start = smh_serial_start,
+ .stop = smh_serial_stop,
+ .setbrg = smh_serial_setbrg,
+ .getc = smh_getc,
+ .tstc = smh_serial_tstc,
+ .putc = smh_putc,
+ .puts = smh_serial_puts,
+};
+
+void smh_serial_initialize(void)
+{
+ serial_register(&smh_serial_drv);
+}
+
+struct serial_device *default_serial_console(void)
+{
+ return &smh_serial_drv;
+}
+#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
--
2.25.1
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