ls1046ardb: Add support for JTAG boot

This adds support for booting entirely from JTAG while using a
hard-coded RCW. With these steps, it is not necessary to program a
"good" RCW using CodeWarrior. The method here can be performed with any
JTAG adapter supported by OpenOCD, including the on-board CMSIS-DAP
(albeit very slowly).

These steps require LS1046A support in OpenOCD, which was added in [1].

[1] 5b70c1f679/

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
[trini: Add reference to doc/board/nxp/ls1046ardb.rst]

arch/arm/cpu/armv8/fsl-layerscape/spl.c

@@ -27,6 +27,8 @@ DECLARE_GLOBAL_DATA_PTR;
 
 u32 spl_boot_device(void)
 {
+	if (IS_ENABLED(CONFIG_SPL_SEMIHOSTING))
+		return BOOT_DEVICE_SMH;
 #ifdef CONFIG_SPL_MMC
 	return BOOT_DEVICE_MMC1;
 #endif

board/freescale/ls1046ardb/ls1046ardb.c

@@ -7,6 +7,8 @@
 #include <i2c.h>
 #include <fdt_support.h>
 #include <init.h>
+#include <semihosting.h>
+#include <serial.h>
 #include <asm/global_data.h>
 #include <asm/io.h>
 #include <asm/arch/clock.h>
@@ -27,6 +29,14 @@
 
 DECLARE_GLOBAL_DATA_PTR;
 
+struct serial_device *default_serial_console(void)
+{
+#if IS_ENABLED(CONFIG_SEMIHOSTING_SERIAL)
+	return &serial_smh_device;
+#endif
+	return &eserial1_device;
+}
+
 int board_early_init_f(void)
 {
 	fsl_lsch2_early_init_f();

doc/board/nxp/ls1046ardb.rst

@@ -110,6 +110,79 @@ SD boot and eMMC boot
 ``{ SW5[0:8], SW4[0] }`` should be ``0010_0000_0``. eMMC is selected only if
 there is no SD card in the slot.
 
+.. _ls1046ardb_jtag:
+
+JTAG boot
+^^^^^^^^^
+
+To recover a bricked board, or to perform initial programming, the ls1046
+supports using two hard-coded Reset Configuration Words (RCWs). Unfortunately,
+this configuration disables most functionality, including the uarts and ethernet.
+However, the SD/MMC and flash controllers are still functional. To get around
+the lack of a serial console, we will use ARM semihosting instead. When
+enabled, OpenOCD will interpret certain instructions as calls to the host
+operating system. This allows U-Boot to use the console, read/write files, or
+run arbitrary commands (!).
+
+When configuring U-Boot, ensure that ``CONFIG_SEMIHOSTING``,
+``CONFIG_SPL_SEMIHOSTING``, and ``CONFIG_SEMIHOSTING_SERIAL`` are enabled.
+``{ SW5[0:8], SW4[0] }`` should be ``0100_1111_0``. Additionally, ``SW4[7]``
+should be set to ``0``. Connect to the "console" USB connector on the front of
+the enclosure.
+
+Create a new file called ``u-boot.tcl`` (or whatever you choose) with the
+following contents::
+
+    # Load the configuration for the LS1046ARDB
+    source [find board/nxp_rdb-ls1046a.cfg]
+    # Initialize the scan chain
+    init
+    # Stop the processor
+    halt
+    # Enable semihosting
+    arm semihosting enable
+    # Load U-Boot SPL
+    load_image spl/u-boot-spl 0 elf
+    # Start executing SPL at the beginning of OCRAM
+    resume 0x10000000
+
+Then, launch openocd like::
+
+    openocd -f u-boot.tcl
+
+You should see the U-boot SPL banner followed by the banner for U-Boot proper
+in the output of openocd. The CMSIS-DAP adapter is slow, so this can take a
+long time. If you don't see it, something has gone wrong. After a while, you
+should see the prompt. You can load an image using semihosting by running::
+
+    => load hostfs - $loadaddr <name of file>
+
+Note that openocd's terminal is "cooked," so commands will only be sent to
+U-Boot when you press enter, and all commands will be echoed twice.
+Additionally, openocd will block when waiting for input, ignoring gdb, JTAG
+events, and Ctrl-Cs. To make openocd process these events, just hit enter.
+
+Using an external JTAG adapter
+""""""""""""""""""""""""""""""
+
+The CMSIS-DAP adapter can be rather slow. To speed up booting, use an external
+JTAG adapter. The following examples assume you are using a J-Link, though any
+adapter supported by OpenOCD will do. Ensure that ``SW4[7]`` is ``1``. Attach
+your jtag adapter to J22. Modify ``u-boot.tcl`` and replace the first two lines
+with the following::
+
+    # Load the J-Link configuration (or whatever your adapter is)
+    source [find interface/jlink.cfg]
+    # Use JTAG, since the J-Link also supports SWD
+    transport select jtag
+    # The reset pin resets the whole CPU
+    reset_config srst_only
+    # Load the LS1046A config
+    source [find target/ls1046a.cfg]
+
+You can proceed as normal through the rest of the steps above. I got a speedup
+of around 100x by using a J-Link.
+
 Debug UART
 ----------
 

doc/usage/semihosting.rst

@@ -62,7 +62,8 @@ so you will need to enable it::
           'arm semihosting enable' -c resume
 
 Note that enabling semihosting can only be done after attaching to the
-board with ``init``, and must be done while the CPU is halted.
+board with ``init``, and must be done while the CPU is halted. For a more
+extended example, refer to the :ref:`LS1046ARDB docs <ls1046ardb_jtag>`.
 
 Loading files
 -------------

include/configs/ls1046ardb.h

@@ -140,6 +140,8 @@
 #endif
 #endif
 
+#define CONFIG_SPL_FS_LOAD_PAYLOAD_NAME "u-boot.img"
+
 #include <asm/fsl_secure_boot.h>
 
 #endif /* __LS1046ARDB_H__ */