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