doc: smh: Update semihosting documentation

This documents how to use semihosting, the new semihosting features, and
how to migrate from smhload.
Signed-off-by: NSean Anderson <sean.anderson@seco.com>

doc/usage/semihosting.rst

@@ -10,6 +10,12 @@ console I/O, etc. Please see `Arm's semihosting documentation
 <https://developer.arm.com/documentation/100863/latest/>`_ for more
 information.
 
+Platform Support
+----------------
+
+Versatile Express
+^^^^^^^^^^^^^^^^^
+
 For developing on armv8 virtual fastmodel platforms, semihosting is a
 valuable tool since it allows access to image/configuration files before
 eMMC or other NV media are available.
@@ -31,13 +37,70 @@ vexpress_aemv8.h but differentiate the two models by the presence or
 absence of ``CONFIG_BASE_FVP``. This change is tested and works on both the
 Foundation and Base fastmodel simulators.
 
+QEMU
+^^^^
+
+Another ARM emulator which supports semihosting is `QEMU
+<https://www.qemu.org/>`_. To enable semihosting, enable
+``CONFIG_SERIAL_PROBE_ALL`` when configuring U-Boot, and use
+``-semihosting`` when invoking QEMU. Adding ``-nographic`` can also be
+helpful. When using a semihosted serial console, QEMU will block waiting
+for input. This will cause the GUI to become unresponsive. To mitigate
+this, try adding ``-nographic``. For more information about building and
+running QEMU, refer to the :doc:`board documentation
+<../board/emulation/qemu-arm>`.
+
+OpenOCD
+^^^^^^^
+
+Any ARM platform can use semihosting with an attached debugger. One such
+debugger with good support for a variety of boards and JTAG adapters is
+`OpenOCD <https://openocd.org/>`_. Semihosting is not enabled by default,
+so you will need to enable it::
+
+    $ openocd -f <your board config> -c init -c halt -c \
+          '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.
+
 Loading files
 -------------
 
-The semihosting code adds a "hostfs"::
+The semihosting code adds a "semihosting filesystem"::
 
-  smhload <image> <address> [env var]
+  load hostfs - <address> <image>
 
 That will load an image from the host filesystem into RAM at the specified
-address and optionally store the load end address in the specified
-environment variable.
+address. If you are using U-Boot SPL, you can also use ``BOOT_DEVICE_SMH``
+which will load ``CONFIG_SPL_FS_LOAD_PAYLOAD_NAME``.
+
+Host console
+------------
+
+U-Boot can use the host's console instead of a physical serial device by
+enabling ``CONFIG_SERIAL_SEMIHOSTING``. If you don't have
+``CONFIG_DM_SERIAL`` enabled, make sure you disable any other serial
+drivers.
+
+Migrating from ``smhload``
+--------------------------
+
+If you were using the ``smhload`` command, you can migrate commands like::
+
+    smhload <file> <address> [<end var>]
+
+to a generic load command like::
+
+    load hostfs - <address> <file>
+
+The ``load`` command will set the ``filesize`` variable with the size of
+the file. The ``fdt chosen`` command has been updated to take a size
+instead of an end address. If you were adding the initramfs to your device
+tree like::
+
+    fdt chosen <address> <end var>
+
+you can now run::
+
+    fdt chosen <address> $filesize