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.
Definitions for the core itself.
The following files are part of the overlay but not used by U-Boot.
Co-processors and custom extensions defined in the Tensilica Instruction Extension (TIE) language.
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.