ardupilot/libraries/AP_HAL_PX4/UARTDriver.h

#pragma once

#include <AP_HAL/utility/RingBuffer.h>

#include "AP_HAL_PX4.h"
#include <systemlib/perf_counter.h>
#include "Semaphores.h"

class PX4::PX4UARTDriver : public AP_HAL::UARTDriver {
public:
    PX4UARTDriver(const char *devpath, const char *perf_name);
    /* PX4 implementations of UARTDriver virtual methods */
    void begin(uint32_t b);
    void begin(uint32_t b, uint16_t rxS, uint16_t txS);
    void end();
    void flush();
    bool is_initialized();
    void set_blocking_writes(bool blocking);
    bool tx_pending();

    /* PX4 implementations of Stream virtual methods */
    uint32_t available() override;
    uint32_t txspace() override;
    int16_t read() override;

    /* PX4 implementations of Print virtual methods */
    size_t write(uint8_t c);
    size_t write(const uint8_t *buffer, size_t size);

    void set_device_path(const char *path) {
	    _devpath = path;
    }

    void _timer_tick(void) override;

    int _get_fd(void) {
	    return _fd;
    }

    void set_flow_control(enum flow_control flow_control);
    enum flow_control get_flow_control(void) { return _flow_control; }

    void configure_parity(uint8_t v);
    void set_stop_bits(int n);
    bool set_unbuffered_writes(bool on);

    /*
      return timestamp estimate in microseconds for when the start of
      a nbytes packet arrived on the uart. This should be treated as a
      time constraint, not an exact time. It is guaranteed that the
      packet did not start being received after this time, but it
      could have been in a system buffer before the returned time.

      This takes account of the baudrate of the link. For transports
      that have no baudrate (such as USB) the time estimate may be
      less accurate.

      A return value of zero means the HAL does not support this API
     */
    uint64_t receive_time_constraint_us(uint16_t nbytes) override;
    
private:
    const char *_devpath;
    int _fd;
    uint32_t _baudrate;
    volatile bool _initialised;
    volatile bool _in_timer;

    bool _nonblocking_writes;
    bool _unbuffered_writes;

    // we use in-task ring buffers to reduce the system call cost
    // of ::read() and ::write() in the main loop
    ByteBuffer _readbuf{0};
    ByteBuffer _writebuf{0};
    perf_counter_t  _perf_uart;

    int _write_fd(const uint8_t *buf, uint16_t n);
    int _read_fd(uint8_t *buf, uint16_t n);
    uint64_t _first_write_time;
    uint64_t _last_write_time;

    void try_initialise(void);
    uint32_t _last_initialise_attempt_ms;

    uint32_t _os_start_auto_space;
    uint32_t _total_read;
    uint32_t _total_written;
    enum flow_control _flow_control;

    // timestamp for receiving data on the UART, avoiding a lock
    uint64_t _receive_timestamp[2];
    uint8_t _receive_timestamp_idx;
    
    Semaphore _semaphore;

    bool _is_usb;
};