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Arm(R) Ethos(TM)-U core platform

Arm(R) Ethos(TM)-U core platform is provided as an example of how to produce a firmware binary for a given target platform. This software is primarily intended for guidance, to demonstrate how to boot up a firmware binary and how to run an inference on an Arm Ethos-U compatible platform.

This repository contains target specific files, like linker scripts. Target agnostic software components are provided in the core_software repository.

Targets

Arm(R) Corstone(TM)-300

The Arm(R) Corstone(TM)-300 is a reference design of how to to build a secure System on Chip (SoC). A fixed virtual platform (FVP) of the Arm Corstone-300 including the Arm Ethos-U can be downloaded from the Ecosystem page at developer.arm.com.

Building

Building with default settings requires CMake for the configuration, make for building and Python 3 for helper scripts.

Before the first time building, install the required Python modules.

$ pip install -r requirements.txt

The following commands will produce an elf file which can be run on the FVP.

$ cmake -B build/corstone-300 targets/corstone-300
$ cd build/corstone-300
$ make

It is also possible to build with a different toolchain.

$ cmake -B build/corstone-300 targets/corstone-300 -DCMAKE_TOOLCHAIN_FILE=$PWD/cmake/toolchain/arm-none-eabi-gcc.cmake
$ cd build/corstone-300
$ make

Please see README_WINDOWS.md for additional information regarding building on a Windows system.

Testing

Fixed Virtual Platform - Corstone-300

Assuming that the Corstone-300 FVP has been downloaded, installed and placed in the PATH variable. Then the software binaries can be tested like this.

$ ctest

Individual applications can also be run directly with the FVP, for example like this.

$ FVP_Corstone_SSE-300_Ethos-U55 applications/freertos/freertos.elf

MPS3 FPGA

The files needed to get started for Corstone-300 can be found on developer.arm.com.

Follow the documentation in the downloaded archive to setup the board with the Corstone-300 FPGA bit files.

The built files can then be ran by adapting the steps in chapter '10 Software', using the extracted binary files from the build process. This is needed for the bootloader on the FPGA to be able to load the memories.

  1. Copy the binary files for the application in the fw/ folder to the board /Software folder, making sure the filename is max 8.3 characters (i.e. 00000000.bin and 70000000.bin for the freertos application)
  2. Navigate to MB/HBI0309C/AN547 and open the images.txt file in a text editor
  3. Remove the lines under the '[IMAGES]' section and replace it with:
TOTALIMAGES: 2

IMAGE0ADDRESS: 0x02000000
IMAGE0UPDATE:  AUTO
IMAGE0FILE:    \SOFTWARE\10000000.bin; sram binary

IMAGE1ADDRESS: 0x0c000000
IMAGE1UPDATE:  AUTO
IMAGE1FILE:    \SOFTWARE\70000000.bin ; ddr binary

The mapping between the Cortex-M55 memory space and the addresse the FPGA MMC bootloader needs is found in section '9.6 MCC Memory mapping' of the documentation in the Corstone-300 FPGA archive. A part of the table is shown below:

Cortex-M55 MMC Bootloader
0x00000000 0x00000000
0x10000000 0x01000000
0x60000000 0x08000000
0x70000000 0x0C000000

The binary that the Cortex-M55 CPU expects at address 0x10000000 must therefor be written to 0x02000000.

Power up the board with the PBON and the application output will be seen on the serial console.

Memory configurations

Embedded systems come in very different configurations, but typically they have a limited amount of high bandwidth low latency memory like SRAM, and some more low bandwidth high latency memory like flash or DRAM.

The Tensorflow Lite for Microcontrollers (TFLu) framework needs two buffers to run an inference, the model and the arena. The model contains static data like weights and biases. The arena contains read write data like activations, IFM, OFM, temporary data etc. Please note that the IFM and OFM are located inside of the arena.

The placement of the model and arena has a big impact on the performance. There are three configurations that make sense for most systems.

Model Arena Spilling Note
SRAM SRAM No
Flash/DRAM SRAM No
Flash/DRAM Flash/DRAM Yes Ethos-U65 only

Model and arena in SRAM

For optimal performance both model and arena should be placed in SRAM.

Model flash/DRAM, Arena SRAM

If both model and arena do not fit in SRAM, then it makes most sense to move the model to flash/DRAM. The performance penalty depends on the network and will need to be measured. For example weight bound networks will experience a larger performance drop than MAC bound networks.

Model and arena in flash/DRAM (Ethos-U65 only)

Moving both model and arena to flash/DRAM comes with quite a hefty performance penalty. To mitigate some of this spilling can be used.

Spilling means that a small buffer is reserved in SRAM that acts like a cache for frequently accessed data. When spilling is enabled Vela will prepend and append extra instructions to the command stream to DMA copy data between the arena and the spilling buffer.

Some of the data stored in the spilling buffer must be copied back to the arena, which is done as DMA transfer over AXI 1. This is only supported by Ethos-U65, because Ethos-U55 is equipped with a readonly AXI 1 interface.

Multi NPU

The Tensorflow Lite for Microcontrollers (TFLu) framework supports running multiple parallel inferences. Each parallel inference requires a TFLu arena (costs memory) and a stack (requires an RTOS). The examples provided in this repo are implemented in the application layer, which means that any RTOS could be used.

The Ethos-U NPU driver is implemented in plain C. To enable thread safety in a multi-threading environment the driver defines a set of weak functions that the application is expected to override, providing implementations for mutex and semaphore primitives.

The weak function can be found in ethosu_driver.c. An example based on FreeRTOS how to override and implement these functions can be found in applications/freertos/main.cpp.

The sequence diagram below illustrates the call stack for a multi NPU system. Please note how the ethosu_mutex_* and ethosu_semaphore_* functions are implemented in the application layer. Mutexes are used for thread safety and semaphores for sleeping.

Multi NPU

Tradeoffs

A single Cortex-M is capable of driving multiple Ethos-U. What the optimal number of Ethos-U is, that is impossible to tell without knowing which network to run or without detailed knowledge about the limitations of the embedded system.

Each parallel inference requires an arena. The arena should for optimal performance be placed in a high bandwidth low latency memory like SRAM, which is a cost that has to be considered. The size of the arena varies greatly depending on the network.

For networks that map fully to Ethos-U, the memory bandwidth might become a limiting factor. For networks that run partly in software, the Cortex-M might become the limiting factor. The placement of the TFLu model and arena (flash, DRAM, SRAM, etc) will also have a big impact on the performance.

Startup

The applications in this repo use CMSIS Device to startup the Cortex-M. The standard procedure is to copy and modify the CMSIS templates, but in this repo we have chosen to include the unmodified templates directly from CMSIS.

The sequence diagram below describes what happens after the Cortex-M reset is lifted, up until the execution enters the application main().

Startup

CMSIS Device

First thing that happens is that the CPU loads index 0 from the interrupt vector into the SP register and index 1 into the PC register, and then starts executing from the PC location.

Index 1 in the VTOR is referred to as the reset handler and is resposible for initializing the CPU. If the CPU for example has a FPU or MVE extension, then these are enabled.

Compiler runtime

The entry function for the compiler runtime setup varies depending on which compiler that is used. For Arm Clang this function is called __main(), not to be confused with the application main()!

The runtime is responsible for initializing the memory segments and setting up the runtime environment. Please refer to the compiler documentation for detailed information about the runtime setup.

Target

The init() is defined as a constructor, which will be called before the application main(). We use this constructor to run targetSetup() to initialize the platform.

For each target there is a targets/<target> directory, which contains linker scripts and code needed to setup the target. targetSetup() is implemented in this folder and is responsible for initializing drivers, configuring the MPU, enabling caches etc.

Adding a new target would involve creating a new targets/<target> directory, providing linker scripts and implementing targetSetup().

Application

Finally the runtime calls application main(). Ideally the application code should be generic and have no knowledge about which target it is executing on.

License

The Arm Ethos-U core platform is provided under an Apache-2.0 license. Please see LICENSE.txt for more information.

Contributions

The Arm Ethos-U project welcomes contributions under the Apache-2.0 license.

Before we can accept your contribution, you need to certify its origin and give us your permission. For this process we use the Developer Certificate of Origin (DCO) V1.1 (https://developercertificate.org).

To indicate that you agree to the terms of the DCO, you "sign off" your contribution by adding a line with your name and e-mail address to every git commit message. You must use your real name, no pseudonyms or anonymous contributions are accepted. If there are more than one contributor, everyone adds their name and e-mail to the commit message.

Author: John Doe \<john.doe@example.org\>
Date:   Mon Feb 29 12:12:12 2016 +0000

Title of the commit

Short description of the change.

Signed-off-by: John Doe john.doe@example.org
Signed-off-by: Foo Bar foo.bar@example.org

The contributions will be code reviewed by Arm before they can be accepted into the repository.

Security

Please see Security.

Trademark notice

Arm, Cortex, Corstone and Ethos are registered trademarks of Arm Limited (or its subsidiaries) in the US and/or elsewhere.