doc: arch: Convert README.xtensa to reST

Convert plain text documentation to reStructuredText format and add
it to Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

2 files changed, 100 insertions, 0 deletions

diff --git a/doc/arch/index.rst b/doc/arch/index.rst
index 2980b77dec1..369d8eeb6d1 100644
--- a/doc/arch/index.rst
+++ b/doc/arch/index.rst
@@ -15,3 +15,4 @@ Architecture-specific doc
    sandbox
    sh
    x86
+   xtensa
diff --git a/doc/arch/xtensa.rst b/doc/arch/xtensa.rst
new file mode 100644
index 00000000000..176410d96b9
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+++ b/doc/arch/xtensa.rst
@@ -0,0 +1,99 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Xtensa
+======
+
+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