tty/serial: add arm/arm64 semihosting earlycon

Add earlycon support for the arm/arm64 semihosting debug serial
interface. This allows enabling a debug console when early_params are
processed. This is based on the arm64 earlyprintk smh support and is
intended to replace it.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

4 files changed, 74 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 2609ead7ff55..4946d8e58d53 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -899,6 +899,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                         must already be setup and configured. Options are not
                         yet supported.
 
+                smh     Use ARM semihosting calls for early console.
+
         earlyprintk=    [X86,SH,BLACKFIN,ARM]
                         earlyprintk=vga
                         earlyprintk=efi
diff --git a/drivers/tty/serial/Kconfig b/drivers/tty/serial/Kconfig
index 4290d05875b1..988fa2b6243b 100644
--- a/drivers/tty/serial/Kconfig
+++ b/drivers/tty/serial/Kconfig
@@ -73,6 +73,16 @@ config SERIAL_AMBA_PL011_CONSOLE
           your boot loader (lilo or loadlin) about how to pass options to the
           kernel at boot time.)
 
+config SERIAL_EARLYCON_ARM_SEMIHOST
+        bool "Early console using ARM semihosting"
+        depends on ARM64 || ARM
+        select SERIAL_EARLYCON
+        help
+          Support for early debug console using ARM semihosting. This enables
+          the console before standard serial driver is probed. This is enabled
+          with "earlycon=smh" on the kernel command line. The console is
+          enabled when early_param is processed.
+
 config SERIAL_SB1250_DUART
         tristate "BCM1xxx on-chip DUART serial support"
         depends on SIBYTE_SB1xxx_SOC=y
diff --git a/drivers/tty/serial/Makefile b/drivers/tty/serial/Makefile
index 28048178f308..3a5be4633333 100644
--- a/drivers/tty/serial/Makefile
+++ b/drivers/tty/serial/Makefile
@@ -6,6 +6,7 @@ obj-$(CONFIG_SERIAL_CORE) += serial_core.o
 obj-$(CONFIG_SERIAL_21285) += 21285.o
 
 obj-$(CONFIG_SERIAL_EARLYCON) += earlycon.o
+obj-$(CONFIG_SERIAL_EARLYCON_ARM_SEMIHOST) += earlycon-arm-semihost.o
 
 # These Sparc drivers have to appear before others such as 8250
 # which share ttySx minor node space.  Otherwise console device
diff --git a/drivers/tty/serial/earlycon-arm-semihost.c b/drivers/tty/serial/earlycon-arm-semihost.c
new file mode 100644
index 000000000000..383db10fbb49
--- /dev/null
+++ b/drivers/tty/serial/earlycon-arm-semihost.c
@@ -0,0 +1,61 @@
+/*
+ * Copyright (C) 2012 ARM Ltd.
+ * Author: Marc Zyngier <marc.zyngier@arm.com>
+ *
+ * Adapted for ARM and earlycon:
+ * Copyright (C) 2014 Linaro Ltd.
+ * Author: Rob Herring <robh@kernel.org>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#include <linux/kernel.h>
+#include <linux/console.h>
+#include <linux/init.h>
+#include <linux/serial_core.h>
+
+#ifdef CONFIG_THUMB2_KERNEL
+#define SEMIHOST_SWI    "0xab"
+#else
+#define SEMIHOST_SWI    "0x123456"
+#endif
+
+/*
+ * Semihosting-based debug console
+ */
+static void smh_putc(struct uart_port *port, int c)
+{
+#ifdef CONFIG_ARM64
+        asm volatile("mov  x1, %0\n"
+                     "mov  x0, #3\n"
+                     "hlt  0xf000\n"
+                     : : "r" (&c) : "x0", "x1", "memory");
+#else
+        asm volatile("mov  r1, %0\n"
+                     "mov  r0, #3\n"
+                     "svc  " SEMIHOST_SWI "\n"
+                     : : "r" (&c) : "r0", "r1", "memory");
+#endif
+}
+
+static void smh_write(struct console *con, const char *s, unsigned n)
+{
+        struct earlycon_device *dev = con->data;
+        uart_console_write(&dev->port, s, n, smh_putc);
+}
+
+int __init early_smh_setup(struct earlycon_device *device, const char *opt)
+{
+        device->con->write = smh_write;
+        return 0;
+}
+EARLYCON_DECLARE(smh, early_smh_setup);