add esp32 build document and ci Refs #5536

.github/workflows/esp32.yml

@@ -0,0 +1,69 @@
+name: ESP32
+on:
+  push:
+    branches: [master]
+    paths:
+      - '.github/workflows/esp32.yml'
+      - 'CMakeLists.txt'
+      - 'cmake/**'
+      - 'src/*'
+      - 'src/layer/*'
+  pull_request:
+    branches: [master]
+    paths:
+      - '.github/workflows/esp32.yml'
+      - 'CMakeLists.txt'
+      - 'cmake/**'
+      - 'src/*'
+      - 'src/layer/*'
+
+concurrency:
+  group: esp32-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    name: ESP32
+    runs-on: ubuntu-20.04
+
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: true
+
+      - name: Setup Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.8'
+
+      - name: Install dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y cmake ninja-build ccache
+
+      - name: Checkout ESP-IDF
+        uses: actions/checkout@v4
+        with:
+          repository: espressif/esp-idf
+          path: esp-idf-install
+          ref: release/v5.3
+
+      - name: Install ESP-IDF
+        run: |
+          cd esp-idf-install
+          git submodule update --init --recursive
+          ./install.sh
+
+      - name: Set environment and build NCNN for ESP32
+        run: |
+          source esp-idf-install/export.sh
+          echo "IDF_PATH=$IDF_PATH" >> $GITHUB_ENV
+          echo "${IDF_PATH}/tools" >> $GITHUB_PATH
+          echo "${IDF_PATH}/components" >> $GITHUB_PATH
+          mkdir -p build-esp32 && cd build-esp32
+          cmake -DCMAKE_TOOLCHAIN_FILE="../toolchains/esp32.toolchain.cmake" -DCMAKE_BUILD_TYPE=Release -DNCNN_BUILD_EXAMPLES=OFF ..
+          make -j 4
+          make install

docs/how-to-build/how-to-build.md

@@ -27,6 +27,7 @@ git submodule update --init
 - [Build for QNX](#build-for-qnx)
 - [Build for Nintendo 3DS Homebrew Launcher](#build-for-nintendo-3ds-homebrew-launcher)
 - [Build for HarmonyOS with cross-compiling](#build-for-harmonyos-with-cross-compiling)
+- [Build for ESP32 with cross-compiling](#build-for-esp32-with-cross-compiling)
 
 ***
 
@@ -866,3 +867,31 @@ ${HM_SDK}/native/build-tools/cmake/bin/cmake -DOHOS_STL=c++_static -DOHOS_ARCH=a
 make -j$(nproc)
 make install
 ```
+
+***
+
+### Build for ESP32 with cross-compiling
+Download esp-idf sdk
+```shell
+git clone https://github.com/espressif/esp-idf
+cd esp-idf
+git submodule update --init --recursive
+```
+Install esp-idf sdk and configure the environment
+```shell
+sudo sh install.sh
+source export.sh
+```
+Note: python>=3.8, cmake>=3.24.0
+
+Build ncnn library:
+```shell
+mkdir build-esp32
+cd build-esp32
+cmake -DCMAKE_TOOLCHAIN_FILE=../toolchains/esp32.toolchain.cmake -DCMAKE_BUILD_TYPE=Release ..
+make -j 4
+make install
+```
+Note: Make sure to compile in esp-idf environment.
+
+The compiled ncnn library and headers can be put to the esp32 project to test.

toolchains/esp32.toolchain.cmake

@@ -0,0 +1,16 @@
+set(CMAKE_SYSTEM_NAME freertos)
+set(CMAKE_SYSTEM_PROCESSOR xtensa-esp32)
+
+include($ENV{IDF_PATH}/tools/cmake/toolchain-esp32.cmake)
+
+set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
+
+set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
+set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
+set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
+set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)
+
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS}" CACHE STRING "c flags")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}" CACHE STRING "c++ flags")
+
+option(NCNN_BUILD_BENCHMARK "" OFF)