mirror of
https://github.com/AsahiLinux/u-boot
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55 lines
2.5 KiB
Text
55 lines
2.5 KiB
Text
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/*
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* Copyright 2014 Broadcom Corporation.
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*
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* SPDX-License-Identifier: GPL-2.0+
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*/
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Semihosting is ARM's way of having a real or virtual target communicate
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with a host or host debugger for basic operations such as file I/O,
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console I/O, etc. Please see
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http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/Bgbjjgij.html for more information.
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For developing on armv8 virtual fastmodel platforms, semihosting is a
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valuable tool since it allows access to image/configuration files before
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eMMC or other NV media are available.
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There are two main ARM virtual Fixed Virtual Platform (FVP) models,
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Versatile Express (VE) FVP and BASE FVP (See
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http://www.arm.com/products/tools/models/fast-models/foundation-model.php)
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The initial vexpress64 u-boot board created here runs on the VE virtual
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platform using the license-free Foundation_v8 simulator. Fortunately,
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the Foundation_v8 simulator also supports the BASE_FVP model which
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companies can purchase licenses for and contain much more functionality.
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So we can, in u-boot, run either model by either using the VE FVP (default),
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or turning on CONFIG_BASE_FVP for the more full featured model.
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Rather than create a new armv8 board similar to armltd/vexpress64, add
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semihosting calls to the existing one, enabled with CONFIG_SEMIHOSTING
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and CONFIG_BASE_FVP both set. Also reuse the existing board config file
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vexpress_aemv8a.h but differentiate the two models by the presence or
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absence of CONFIG_BASE_FVP. This change is tested and works on both the
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Foundation and Base fastmodel simulators.
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The level of semihosting support is minimal, restricted to just what it
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takes to load images to memory. If more semihosting functionality is
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required, such as file seek, outputting strings, reading characters, etc,
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then it can be easily added later.
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We require that the board include file define these env variables:
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- kernel_name e.g. "uImage"
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- kernel_addr_r e.g. "0x80000000"
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- initrd_name e.g. "ramdisk.img"
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- initrd_addr_r e.g. "0x88000000"
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- fdt_name e.g. "devtree.dtb"
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- fdt_addr_r e.g. "0x83000000"
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Optionally, "fdt_high" and "initrd_high" can be specified as per
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their rules for allowing or preventing copying of these images.
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For the "fdt chosen" startup macro, this code will then define:
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- initrd_end (based on retrieving initrd_addr_r plus actual initrd_size)
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We will then load the kernel, initrd, and fdt into the specified
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locations in memory in a similar way that the ATF fastmodel code
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uses semihosting calls to load other boot stages and u-boot itself.
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