doc/usage/semihosting.rst

.. SPDX-License-Identifier: GPL-2.0+
.. Copyright 2014 Broadcom Corporation.

Semihosting
===========

Semihosting is ARM's way of having a real or virtual target communicate
with a host or host debugger for basic operations such as file I/O,
console I/O, etc. Please see `Arm's semihosting documentation
<https://developer.arm.com/documentation/100863/latest/>`_ for more
information.

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.

There are two main ARM virtual Fixed Virtual Platform (FVP) models,
`Versatile Express (VE) FVP and BASE FVP
<http://www.arm.com/products/tools/models/fast-models/foundation-model.php>`_.
The initial vexpress64 u-boot board created here runs on the VE virtual
platform using the license-free Foundation_v8 simulator. Fortunately,
the Foundation_v8 simulator also supports the BASE_FVP model which
companies can purchase licenses for and contain much more functionality.
So we can, in U-Boot, run either model by either using the VE FVP (default),
or turning on ``CONFIG_BASE_FVP`` for the more full featured model.

Rather than create a new armv8 board similar to ``armltd/vexpress64``, add
semihosting calls to the existing one, enabled with ``CONFIG_SEMIHOSTING``
and ``CONFIG_BASE_FVP`` both set. Also reuse the existing board config file
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.

Loading files
-------------

The semihosting code adds a "hostfs"::

  smhload <image> <address> [env var]

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.