u-boot/doc/README.semihosting

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

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
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/Bgbjjgij.html 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 (See
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_aemv8a.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.

The semihosting code adds a command:

  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.