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.. SPDX-License-Identifier: GPL-2.0+
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Xtensa
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======
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Xtensa Architecture and Diamond Cores
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-------------------------------------
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Xtensa is a configurable processor architecture from Tensilica, Inc.
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Diamond Cores are pre-configured instances available for license and
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SoC cores in the same manner as ARM, MIPS, etc.
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Xtensa licensees create their own Xtensa cores with selected features
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and custom instructions, registers and co-processors. The custom core
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is configured with Tensilica tools and built with Tensilica's Xtensa
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Processor Generator.
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There are an effectively infinite number of CPUs in the Xtensa
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architecture family. It is, however, not feasible to support individual
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Xtensa CPUs in U-Boot. Therefore, there is only a single 'xtensa' CPU
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in the cpu tree of U-Boot.
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In the same manner as the Linux port to Xtensa, U-Boot adapts to an
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individual Xtensa core configuration using a set of macros provided with
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the particular core. This is part of what is known as the hardware
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abstraction layer (HAL). For the purpose of U-Boot, the HAL consists only
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of a few header files. These provide CPP macros that customize sources,
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Makefiles, and the linker script.
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Adding support for an additional processor configuration
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--------------------------------------------------------
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The header files for one particular processor configuration are inside
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a variant-specific directory located in the arch/xtensa/include/asm
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directory. The name of that directory starts with 'arch-' followed by
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the name for the processor configuration, for example, arch-dc233c for
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the Diamond DC233 processor.
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core.h:
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Definitions for the core itself.
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The following files are part of the overlay but not used by U-Boot.
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tie.h:
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Co-processors and custom extensions defined in the Tensilica Instruction
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Extension (TIE) language.
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tie-asm.h:
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Assembly macros to access custom-defined registers and states.
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Global Data Pointer, Exported Function Stubs, and the ABI
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---------------------------------------------------------
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To support standalone applications launched with the "go" command,
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U-Boot provides a jump table of entrypoints to exported functions
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(grep for EXPORT_FUNC). The implementation for Xtensa depends on
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which ABI (or function calling convention) is used.
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Windowed ABI presents unique difficulties with the approach based on
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keeping global data pointer in dedicated register. Because the register
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window rotates during a call, there is no register that is constantly
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available for the gd pointer. Therefore, on xtensa gd is a simple
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global variable. Another difficulty arises from the requirement to have
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an 'entry' at the beginning of a function, which rotates the register
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file and reserves a stack frame. This is an integral part of the
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windowed ABI implemented in hardware. It makes using a jump table to an
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arbitrary (separately compiled) function a bit tricky. Use of a simple
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wrapper is also very tedious due to the need to move all possible
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register arguments and adjust the stack to handle arguments that cannot
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be passed in registers. The most efficient approach is to have the jump
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table perform the 'entry' so as to pretend it's the start of the real
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function. This requires decoding the target function's 'entry'
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instruction to determine the stack frame size, and adjusting the stack
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pointer accordingly, then jumping into the target function just after
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the 'entry'. Decoding depends on the processor's endianness so uses the
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HAL. The implementation (12 instructions) is in examples/stubs.c.
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Access to Invalid Memory Addresses
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----------------------------------
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U-Boot does not check if memory addresses given as arguments to commands
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such as "md" are valid. There are two possible types of invalid
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addresses: an area of physical address space may not be mapped to RAM
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or peripherals, or in the presence of MMU an area of virtual address
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space may not be mapped to physical addresses.
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Accessing first type of invalid addresses may result in hardware lockup,
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reading of meaningless data, written data being ignored or an exception,
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depending on the CPU wiring to the system. Accessing second type of
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invalid addresses always ends with an exception.
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U-Boot for Xtensa provides a special memory exception handler that
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reports such access attempts and resets the board.
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.. Chris Zankel
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.. Ross Morley
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