# Arm(R) Ethos(TM)-U core driver This repository contains a device driver for the Arm(R) Ethos(TM)-U NPU. ## Building The source code comes with a CMake based build system. The driver is expected to be cross compiled for any of the supported Arm Cortex(R)-M CPUs, which requires the user to configure the build to match their system configuration. One such requirement is to define the target CPU, normally by setting `CMAKE_SYSTEM_PROCESSOR`. **Note** that when using the toolchain files provided in [core_platform](https://git.mlplatform.org/ml/ethos-u/ethos-u-core-platform.git), the variable `TARGET_CPU` must be used instead of `CMAKE_SYSTEM_PROCESSOR`. Target CPU is specified on the form "cortex-m", for example: "cortex-m55+nodsp+nofp". Similarly the target NPU configuration is controlled by setting `ETHOSU_TARGET_NPU_CONFIG`, for example "ethos-u55-128". The build configuration can be defined either in the toolchain file or by passing options on the command line. ```[bash] $ cmake -B build \ -DCMAKE_TOOLCHAIN_FILE= \ -DCMAKE_SYSTEM_PROCESSOR=cortex-m \ -DETHOSU_TARGET_NPU_CONFIG=ethos-u- $ cmake --build build ``` or when using toolchain files from [core_platform](https://git.mlplatform.org/ml/ethos-u/ethos-u-core-platform.git) ```[bash] $ cmake -B build \ -DCMAKE_TOOLCHAIN_FILE= \ -DTARGET_CPU=cortex-m \ -DETHOSU_TARGET_NPU_CONFIG=ethos-u- $ cmake --build build ``` ## Driver APIs The driver APIs are defined in `include/ethosu_driver.h` and the related types in `include/ethosu_types.h`. Inferences can be invoked in two manners: synchronously or asynchronously. The two types of invocation can be freely mixed in a single application. ### Synchronous invocation A typical usage of the driver can be the following: ```[C] // reserve a driver to be used (this call could block until a driver is available) struct ethosu_driver *drv = ethosu_reserve_driver(); ... // run one or more inferences int result = ethosu_invoke(drv, custom_data_ptr, custom_data_size, base_addr, base_addr_size, num_base_addr); ... // release the driver for others to use ethosu_release_driver(drv); ``` ### Asynchronous invocation A typical usage of the driver can be the following: ```[C] // reserve a driver to be used (this call could block until a driver is available) struct ethosu_driver *drv = ethosu_reserve_driver(); ... // run one or more inferences int result = ethosu_invoke_async(drv, custom_data_ptr, custom_data_size, base_addr, base_addr_size, num_base_addr, user_arg); ... // do some other work ... int ret; do { // true = blocking, false = non-blocking // ret > 0 means inference not completed (only for non-blocking mode) ret = ethosu_wait(drv, ); } while(ret > 0); ... // release the driver for others to use ethosu_release_driver(drv); ``` Note that if `ethosu_wait` is invoked from a different thread and concurrently with `ethosu_invoke_async`, the user is responsible to guarantee that `ethosu_wait` is called after a successful completion of `ethosu_invoke_async`. Otherwise `ethosu_wait` might fail and not actually wait for the inference completion. ### Driver initialization In order to use a driver it first needs to be initialized by calling the `init` function, which will also register the handle in the list of available drivers. A driver can be torn down by using the `deinit` function, which also removes the driver from the list. The correct mapping is one driver per NPU device. Note that the NPUs must have the same configuration, indeed the NPU configuration can be only one, which is defined at compile time. ## Implementation design The driver is structured in two main parts: the driver, which is responsible to provide an unified API to the user; and the device part, which deals with the details at the hardware level. In order to do its task the driver needs a device implementation. There could be multiple device implementation for different hardware model and/or configurations. Note that the driver can be compiled to target only one NPU configuration by specializing the device part at compile time. ## Data caching For running the driver on Arm CPUs which are configured with data cache, the cache maintenance functions in the driver are exported with weakly linked symbols that should be overridden. An example implementation using the CMSIS primitives found in cachel1_armv7.h could be as below: ```[C++] extern "C" { void ethosu_flush_dcache(uint32_t *p, size_t bytes) { if (p) SCB_CleanDCache_by_Addr(p, bytes); else SCB_CleanDCache(); } void ethosu_invalidate_dcache(uint32_t *p, size_t bytes) { if (p) SCB_InvalidateDCache_by_Addr(p, bytes); else SCB_InvalidateDCache(); } } ``` ## Mutex and semaphores To ensure the correct functionality of the driver mutexes and semaphores are used internally. The default implementations of mutexes and semaphores are designed for a single-threaded baremetal environment. Hence for integration in environemnts where multi-threading is possible, e.g., RTOS, the user is responsible to provide implementation for mutexes and semaphores to be used by the driver. The mutex and semaphores are used as synchronisation mechanisms and unless specified, the timeout is required to be 'forever'. The driver allows for an RTOS to set a timeout for the NPU interrupt semaphore. The timeout can be set with the CMake variable `ETHOSU_INFERENCE_TIMEOUT`, which is then used as `timeout` argument for the interrupt semaphore take call. Note that the unit is implementation defined, the value is shipped as is to the `ethosu_semaphore_take()` function and an override implementation should cast it to the appropriate type and/or convert it to the unit desired. A macro `ETHOSU_SEMAPHORE_WAIT_FOREVER` is defined in the driver header file, and should be made sure to map to the RTOS' equivalent of 'no timeout/wait forever'. Inference timeout value defaults to this if left unset. The macro is used internally in the driver for the available NPU's, thus the driver does NOT support setting a timeout other than forever when waiting for an NPU to become available (global ethosu_semaphore). The mutex and semaphore APIs are defined as weak linked functions that can be overridden by the user. The APIs are the usual ones and described below: ```[C] // create a mutex by returning back a handle void *ethosu_mutex_create(void); // lock the given mutex int ethosu_mutex_lock(void *mutex); // unlock the given mutex int ethosu_mutex_unlock(void *mutex); // create a (binary) semaphore by returning back a handle void *ethosu_semaphore_create(void); // take from the given semaphore, accepting a timeout (unit impl. defined) int ethosu_semaphore_take(void *sem, uint64_t timeout); // give from the given semaphore int ethosu_semaphore_give(void *sem); ``` ## Begin/End inference callbacks The driver provide weak linked functions as hooks to receive callbacks whenever an inference begins and ends. The user can override such functions when needed. To avoid memory leaks, any allocations done in the ethosu_inference_begin() must be balanced by a corresponding free of the memory in the ethosu_inference_end() callback. The end callback will always be called if the begin callback has been called, including in the event of an interrupt semaphore take timeout. ```[C] void ethosu_inference_begin(struct ethosu_driver *drv, void *user_arg); void ethosu_inference_end(struct ethosu_driver *drv, void *user_arg); ``` Note that the `void *user_arg` pointer passed to invoke() function is the same pointer passed to the begin() and end() callbacks. For example: ```[C] void my_function() { ... struct my_data data = {...}; int result = int ethosu_invoke_v3(drv, custom_data_ptr, custom_data_size, base_addr, base_addr_size, num_base_addr, (void *)&data); .... } void ethosu_inference_begin(struct ethosu_driver *drv, void *user_arg) { struct my_data *data = (struct my_data*) user_arg; // use drv and data here } void ethosu_inference_end(struct ethosu_driver *drv, void *user_arg) { struct my_data *data = (struct my_data*) user_arg; // use drv and data here } ``` ## License The Arm Ethos-U core driver is provided under an Apache-2.0 license. Please see [LICENSE.txt](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 \ 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. In order to submit a contribution push your patch to `ssh://@review.mlplatform.org:29418/ml/ethos-u/ethos-u-core-driver`. To do this you will need to sign-in to [review.mlplatform.org](https://review.mlplatform.org) using a GitHub account and add your SSH key under your settings. If there is a problem adding the SSH key make sure there is a valid email address in the Email Addresses field. ## Security Please see [Security](SECURITY.md). ## Trademark notice Arm, Cortex and Ethos are registered trademarks of Arm Limited (or its subsidiaries) in the US and/or elsewhere.