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diff --git a/doc/README.semihosting b/doc/README.semihosting new file mode 100644 index 0000000000..7248560780 --- /dev/null +++ b/doc/README.semihosting @@ -0,0 +1,54 @@ +/* + * Copyright 2014 Broadcom Corporation. + * + * SPDX-License-Identifier: GPL-2.0+ + */ + +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 level of semihosting support is minimal, restricted to just what it +takes to load images to memory. If more semihosting functionality is +required, such as file seek, outputting strings, reading characters, etc, +then it can be easily added later. + +We require that the board include file define these env variables: +- kernel_name e.g. "uImage" +- kernel_addr_r e.g. "0x80000000" +- initrd_name e.g. "ramdisk.img" +- initrd_addr_r e.g. "0x88000000" +- fdt_name e.g. "devtree.dtb" +- fdt_addr_r e.g. "0x83000000" + +Optionally, "fdt_high" and "initrd_high" can be specified as per +their rules for allowing or preventing copying of these images. + +For the "fdt chosen" startup macro, this code will then define: +- initrd_end (based on retrieving initrd_addr_r plus actual initrd_size) + +We will then load the kernel, initrd, and fdt into the specified +locations in memory in a similar way that the ATF fastmodel code +uses semihosting calls to load other boot stages and u-boot itself. |