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