AP_HAL 分析, 以pixhawk-fmuv2为硬件平台，ChibiOS为底层操作系统:

1. class AP_HAL::AP_HAL，该接口类聚合了所有提供给应用层的硬件接口

class AP_HAL::HAL {
public:HAL(AP_HAL::UARTDriver* _uartA, // consoleAP_HAL::UARTDriver* _uartB, // 1st GPSAP_HAL::UARTDriver* _uartC, // telem1AP_HAL::UARTDriver* _uartD, // telem2AP_HAL::UARTDriver* _uartE, // 2nd GPSAP_HAL::UARTDriver* _uartF, // extra1AP_HAL::UARTDriver* _uartG, // extra2AP_HAL::I2CDeviceManager* _i2c_mgr,AP_HAL::SPIDeviceManager* _spi,AP_HAL::AnalogIn*   _analogin,AP_HAL::Storage*    _storage,AP_HAL::UARTDriver* _console,AP_HAL::GPIO*       _gpio,AP_HAL::RCInput*    _rcin,AP_HAL::RCOutput*   _rcout,AP_HAL::Scheduler*  _scheduler,AP_HAL::Util*       _util,AP_HAL::OpticalFlow *_opticalflow,AP_HAL::Flash *_flash,
#if HAL_WITH_UAVCANAP_HAL::CANManager* _can_mgr[MAX_NUMBER_OF_CAN_DRIVERS])
#elseAP_HAL::CANManager** _can_mgr)
#endif:uartA(_uartA),uartB(_uartB),uartC(_uartC),uartD(_uartD),uartE(_uartE),uartF(_uartF),uartG(_uartG),i2c_mgr(_i2c_mgr),spi(_spi),analogin(_analogin),storage(_storage),console(_console),gpio(_gpio),rcin(_rcin),rcout(_rcout),scheduler(_scheduler),util(_util),opticalflow(_opticalflow),flash(_flash){
#if HAL_WITH_UAVCANif (_can_mgr == nullptr) {for (uint8_t i = 0; i < MAX_NUMBER_OF_CAN_DRIVERS; i++)can_mgr[i] = nullptr;} else {for (uint8_t i = 0; i < MAX_NUMBER_OF_CAN_DRIVERS; i++)can_mgr[i] = _can_mgr[i];}
#endifAP_HAL::init();}struct Callbacks {virtual void setup() = 0;virtual void loop() = 0;};struct FunCallbacks : public Callbacks {FunCallbacks(void (*setup_fun)(void), void (*loop_fun)(void));void setup() override { _setup(); }void loop() override { _loop(); }private:void (*_setup)(void);void (*_loop)(void);};virtual void run(int argc, char * const argv[], Callbacks* callbacks) const = 0;AP_HAL::UARTDriver* uartA;AP_HAL::UARTDriver* uartB;AP_HAL::UARTDriver* uartC;AP_HAL::UARTDriver* uartD;AP_HAL::UARTDriver* uartE;AP_HAL::UARTDriver* uartF;AP_HAL::UARTDriver* uartG;AP_HAL::I2CDeviceManager* i2c_mgr;AP_HAL::SPIDeviceManager* spi;AP_HAL::AnalogIn*   analogin;AP_HAL::Storage*    storage;AP_HAL::UARTDriver* console;AP_HAL::GPIO*       gpio;AP_HAL::RCInput*    rcin;AP_HAL::RCOutput*   rcout;AP_HAL::Scheduler*  scheduler;AP_HAL::Util        *util;AP_HAL::OpticalFlow *opticalflow;AP_HAL::Flash       *flash;
#if HAL_WITH_UAVCANAP_HAL::CANManager* can_mgr[MAX_NUMBER_OF_CAN_DRIVERS];
#elseAP_HAL::CANManager** can_mgr;
#endif
};

继承关系

我们看到ardupilot中有四大系统平台(这里的系统平台指的是基于硬件平台的操作系统和底层驱动的集合，系统平台也可以是虚拟的)实现了该接口类，我们这里分析HAL_ChibiOS

2. class HAL_ChibiOS 实现了AP_HAL::HAL接口类

class HAL_ChibiOS : public AP_HAL::HAL {
public:HAL_ChibiOS();void run(int argc, char* const* argv, Callbacks* callbacks) const override;
};
在实现文件中我们看到：
static HAL_UARTA_DRIVER; ///< #define HAL_UARTA_DRIVER ChibiOS::UARTDriver uartADriver(0)
static HAL_UARTB_DRIVER; ///< #define HAL_UARTB_DRIVER ChibiOS::UARTDriver uartBDriver(1)
static HAL_UARTC_DRIVER; ///< #define HAL_UARTC_DRIVER ChibiOS::UARTDriver uartCDriver(2)
static HAL_UARTD_DRIVER; ///< #define HAL_UARTD_DRIVER ChibiOS::UARTDriver uartDDriver(3)
static HAL_UARTE_DRIVER; ///< #define HAL_UARTE_DRIVER ChibiOS::UARTDriver uartEDriver(4)
static HAL_UARTF_DRIVER; ///< #define HAL_UARTF_DRIVER ChibiOS::UARTDriver uartFDriver(5)
static HAL_UARTG_DRIVER; ///< #define HAL_UARTG_DRIVER Empty::UARTDriver uartGDriver
///< 前面这些宏定义在build/fmuv2/hwdef.h头文件中, 该头文件在编译时由python脚本从hwdef.dat文件中解析然后生成
///< 我们这里的硬件平台的硬件定义文件位于：ardupilot/libraries/AP_HAL_ChibiOS/hwdef/fmuv2/hwdef.dat
static ChibiOS::I2CDeviceManager i2cDeviceManager;
static ChibiOS::SPIDeviceManager spiDeviceManager;
static ChibiOS::AnalogIn analogIn;
static ChibiOS::Storage storageDriver;
static ChibiOS::GPIO gpioDriver;
static ChibiOS::RCInput rcinDriver;
static ChibiOS::RCOutput rcoutDriver;
static ChibiOS::Scheduler schedulerInstance;
static ChibiOS::Util utilInstance;
static Empty::OpticalFlow opticalFlowDriver;
static ChibiOS::Flash flashDriver;

在构造函数中，这些对象会被传递进去：

HAL_ChibiOS::HAL_ChibiOS() :AP_HAL::HAL(&uartADriver,&uartBDriver,&uartCDriver,&uartDDriver,&uartEDriver,&uartFDriver,&uartGDriver,&i2cDeviceManager,&spiDeviceManager,&analogIn,&storageDriver,&uartADriver,&gpioDriver,&rcinDriver,&rcoutDriver,&schedulerInstance,&utilInstance,&opticalFlowDriver,&flashDriver,nullptr)
{}

3. 我们进入到具体的uartADriver实例对应的类ChibiOS::UARTDriver中，其属于AP_HAL::UARTDriver接口的实现

可以看到，四大系统平台都分别实现了AP_HAL::UARTDriver接口

class ChibiOS::UARTDriver : public AP_HAL::UARTDriver { ///<
public:UARTDriver(uint8_t serial_num);void begin(uint32_t b) override;void begin(uint32_t b, uint16_t rxS, uint16_t txS) override;void end() override;void flush() override;bool is_initialized() override;void set_blocking_writes(bool blocking) override;bool tx_pending() override;uint32_t available() override;uint32_t txspace() override;int16_t read() override;int16_t read_locked(uint32_t key) override;void _timer_tick(void) override;size_t write(uint8_t c) override;size_t write(const uint8_t *buffer, size_t size) override;// lock a port for exclusive use. Use a key of 0 to unlockbool lock_port(uint32_t write_key, uint32_t read_key) override;// control optional featuresbool set_options(uint8_t options) override;// write to a locked port. If port is locked and key is not correct then 0 is returned// and write is discardedsize_t write_locked(const uint8_t *buffer, size_t size, uint32_t key) override;struct SerialDef {BaseSequentialStream* serial;bool is_usb;bool dma_rx;uint8_t dma_rx_stream_id;uint32_t dma_rx_channel_id;bool dma_tx;uint8_t dma_tx_stream_id;uint32_t dma_tx_channel_id; ioline_t rts_line;int8_t rxinv_gpio;uint8_t rxinv_polarity;int8_t txinv_gpio;uint8_t txinv_polarity;uint8_t get_index(void) const {return uint8_t(this - &_serial_tab[0]);}};bool wait_timeout(uint16_t n, uint32_t timeout_ms) override;void set_flow_control(enum flow_control flow_control) override;enum flow_control get_flow_control(void) override { return _flow_control; }// allow for low latency writesbool set_unbuffered_writes(bool on) override;void configure_parity(uint8_t v) override;void set_stop_bits(int n) override;/*return timestamp estimate in microseconds for when the start ofa nbytes packet arrived on the uart. This should be treated as atime constraint, not an exact time. It is guaranteed that thepacket did not start being received after this time, but itcould have been in a system buffer before the returned time.This takes account of the baudrate of the link. For transportsthat have no baudrate (such as USB) the time estimate may beless accurate.A return value of zero means the HAL does not support this API*/uint64_t receive_time_constraint_us(uint16_t nbytes) override;uint32_t bw_in_kilobytes_per_second() const override {if (sdef.is_usb) {return 200;}return _baudrate/(9*1024);}private:bool tx_bounce_buf_ready;const SerialDef &sdef;// thread used for all UARTsstatic thread_t *uart_thread_ctx;// table to find UARTDrivers from serial number, used for event handlingstatic UARTDriver *uart_drivers[UART_MAX_DRIVERS];// index into uart_drivers tableuint8_t serial_num;// key for a locked portuint32_t lock_write_key;uint32_t lock_read_key;uint32_t _baudrate;uint16_t tx_len;
#if HAL_USE_SERIAL == TRUESerialConfig sercfg;
#endifconst thread_t* _uart_owner_thd;struct {// thread waiting for datathread_t *thread_ctx;// number of bytes neededuint16_t n;} _wait;// we use in-task ring buffers to reduce the system call cost// of ::read() and ::write() in the main loop，使用ringbuffer降低mainloop中read/write系统调用开销uint8_t *rx_bounce_buf;///< 接收弹性bufferuint8_t *tx_bounce_buf;///< 发送弹性bufferByteBuffer _readbuf{0}; ///< 接收ringbufferByteBuffer _writebuf{0};///< 发送ringbufferSemaphore _write_mutex;const stm32_dma_stream_t* rxdma; ///< 接收dmaconst stm32_dma_stream_t* txdma; ///< 发送dmavirtual_timer_t tx_timeout;    bool _in_timer;bool _blocking_writes;bool _initialised;bool _device_initialised;bool _lock_rx_in_timer_tick = false;Shared_DMA *dma_handle;static const SerialDef _serial_tab[];// timestamp for receiving data on the UART, avoiding a lockuint64_t _receive_timestamp[2];uint8_t _receive_timestamp_idx;// handling of flow controlenum flow_control _flow_control = FLOW_CONTROL_DISABLE;bool _rts_is_active;uint32_t _last_write_completed_us;uint32_t _first_write_started_us;uint32_t _total_written;// we remember cr2 and cr2 options from set_options to apply on sdStart()uint32_t _cr3_options;uint32_t _cr2_options;// half duplex control. After writing we throw away bytes for 4 byte widths to// prevent reading our own bytes backbool half_duplex;uint32_t hd_read_delay_us;uint32_t hd_write_us;void half_duplex_setup_delay(uint16_t len);// set to true for unbuffered writes (low latency writes)bool unbuffered_writes;static void rx_irq_cb(void* sd);static void rxbuff_full_irq(void* self, uint32_t flags);static void tx_complete(void* self, uint32_t flags);static void handle_tx_timeout(void *arg);void dma_tx_allocate(Shared_DMA *ctx);void dma_tx_deallocate(Shared_DMA *ctx);void update_rts_line(void);void check_dma_tx_completion(void);void write_pending_bytes_DMA(uint32_t n);void write_pending_bytes_NODMA(uint32_t n);void write_pending_bytes(void);void receive_timestamp_update(void);void thread_init();static void uart_thread(void *);
};

QA1.

首先，HAL_ChibiOS 是 AP_HAL::HAL这个抽象类的以ChibiOS为具体操作系统的派生类，所以按照C++惯例，子类构造函数中显式调用基类构造函数即AP_HAL::HAL();
HAL_ChibiOS 相当于已经落实到了具象的操作系统软件硬件平台，比如uartA，他提供具体的:

static HAL_UARTA_DRIVER;

而宏在具体的硬件定义比如fmuv2/ArduPilot_743头文件hwdef.h中：

#define HAL_UARTA_DRIVER ChibiOS::UARTDriver uartADriver(0)

替换完成就是:

ChibiOS::UARTDriver uartADriver(0)

也就是使用ChibiOS领域(namespace)内的UARTDriver 实例化一个对象 uartADriver 并将其地址传入到AP_HAL::HAL基类构造函数中，利用多态特性完成抽象到具体的特化，因为具体的业务逻辑比如ArduCopter或APMrover等凡是在涉及具体软硬件平台方面的操作都是面向接口编程的

故而可以总结业务逻辑层：               ArduCopter ... APMrover\       |         / -----AP_HAL 层---||----------------------------------/                |                 \软硬件平台层：ChibiOS+fmuv2 ...ChibiOS+fmuv5... linux+树莓派等

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