ardupilot/libraries/AP_HAL_ChibiOS/UARTDriver.cpp

/*
 * This file is free software: you can redistribute it and/or modify it
 * under the terms of the GNU General Public License as published by the
 * Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This file is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 * See the GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program.  If not, see <http://www.gnu.org/licenses/>.
 * 
 * Code by Andrew Tridgell and Siddharth Bharat Purohit
 */
#include <AP_HAL/AP_HAL.h>

#if CONFIG_HAL_BOARD == HAL_BOARD_CHIBIOS && !defined(HAL_NO_UARTDRIVER)
#include "UARTDriver.h"
#include "GPIO.h"
#include <usbcfg.h>
#include "shared_dma.h"
#include <AP_Math/AP_Math.h>
#include "Scheduler.h"
#include "hwdef/common/stm32_util.h"

extern const AP_HAL::HAL& hal;

using namespace ChibiOS;

#ifdef HAL_USB_VENDOR_ID
// USB has been configured in hwdef.dat
#define HAVE_USB_SERIAL
#endif

#if HAL_WITH_IO_MCU
extern ChibiOS::UARTDriver uart_io;
#endif

const UARTDriver::SerialDef UARTDriver::_serial_tab[] = { HAL_UART_DEVICE_LIST };

// handle for UART handling thread
thread_t *UARTDriver::uart_thread_ctx;

// table to find UARTDrivers from serial number, used for event handling
UARTDriver *UARTDriver::uart_drivers[UART_MAX_DRIVERS];

// event used to wake up waiting thread. This event number is for
// caller threads
#define EVT_DATA EVENT_MASK(0)

UARTDriver::UARTDriver(uint8_t _serial_num) :
serial_num(_serial_num),
sdef(_serial_tab[_serial_num]),
_baudrate(57600)
{
    osalDbgAssert(serial_num < UART_MAX_DRIVERS, "too many UART drivers");
    uart_drivers[serial_num] = this;
}

/*
  thread for handling UART send/receive

  We use events indexed by serial_num to trigger a more rapid send for
  unbuffered_write uarts, and run at 1kHz for general UART handling
 */
void UARTDriver::uart_thread(void* arg)
{
    uint32_t last_thread_run_us = 0; // last time we did a 1kHz run of uarts

    uart_thread_ctx = chThdGetSelfX();
    while (true) {
        eventmask_t mask = chEvtWaitAnyTimeout(~0, chTimeMS2I(1));
        uint32_t now = AP_HAL::micros();
        if (now - last_thread_run_us >= 1000) {
            // run them all if it's been more than 1ms since we ran
            // them all
            mask = ~0;
            last_thread_run_us = now;
        }
        for (uint8_t i=0; i<UART_MAX_DRIVERS; i++) {
            if (uart_drivers[i] == nullptr) {
                continue;
            }
            if ((mask & EVENT_MASK(i)) &&
                uart_drivers[i]->_initialised) {
                uart_drivers[i]->_timer_tick();
            }
        }
    }
}

/*
  initialise UART thread
 */
void UARTDriver::thread_init(void)
{
    if (uart_thread_ctx) {
        // already initialised
        return;
    }
#if CH_CFG_USE_HEAP == TRUE
    uart_thread_ctx = chThdCreateFromHeap(NULL,
                                          THD_WORKING_AREA_SIZE(2048),
                                          "apm_uart",
                                          APM_UART_PRIORITY,
                                          uart_thread,
                                           this);
#endif
}

#ifndef HAL_STDOUT_SERIAL
/*
  hook to allow printf() to work on hal.console when we don't have a
  dedicated debug console
 */
static int hal_console_vprintf(const char *fmt, va_list arg)
{
    hal.console->vprintf(fmt, arg);
    return 1; // wrong length, but doesn't matter for what this is used for
}
#endif

void UARTDriver::begin(uint32_t b, uint16_t rxS, uint16_t txS)
{
    thread_init();
    
    if (sdef.serial == nullptr) {
        return;
    }
    uint16_t min_tx_buffer = 1024;
    uint16_t min_rx_buffer = 512;

    if (sdef.is_usb) {
        // give more buffer space for log download on USB
        min_tx_buffer *= 4;
    }
    
    // on PX4 we have enough memory to have a larger transmit and
    // receive buffer for all ports. This means we don't get delays
    // while waiting to write GPS config packets
    if (txS < min_tx_buffer) {
        txS = min_tx_buffer;
    }
    if (rxS < min_rx_buffer) {
        rxS = min_rx_buffer;
    }

    /*
      allocate the read buffer
      we allocate buffers before we successfully open the device as we
      want to allocate in the early stages of boot, and cause minimum
      thrashing of the heap once we are up. The ttyACM0 driver may not
      connect for some time after boot
     */
    while (_in_timer) {
        hal.scheduler->delay(1);
    }
    if (rxS != _readbuf.get_size()) {
        _initialised = false;
        _readbuf.set_size(rxS);
    }

    bool clear_buffers = false;
    if (b != 0) {
        // clear buffers on baudrate change, but not on the console (which is usually USB)
        if (_baudrate != b && hal.console != this) {
            clear_buffers = true;
        }
        _baudrate = b;
    }
    
    if (clear_buffers) {
        _readbuf.clear();
    }

    if (rx_bounce_buf == nullptr) {
        rx_bounce_buf = (uint8_t *)hal.util->malloc_type(RX_BOUNCE_BUFSIZE, AP_HAL::Util::MEM_DMA_SAFE);
    }
    if (tx_bounce_buf == nullptr) {
        tx_bounce_buf = (uint8_t *)hal.util->malloc_type(TX_BOUNCE_BUFSIZE, AP_HAL::Util::MEM_DMA_SAFE);
        chVTObjectInit(&tx_timeout);
        tx_bounce_buf_ready = true;
    }
    
    /*
      allocate the write buffer
     */
    while (_in_timer) {
        hal.scheduler->delay(1);
    }
    if (txS != _writebuf.get_size()) {
        _initialised = false;
        _writebuf.set_size(txS);
    }

    if (clear_buffers) {
        _writebuf.clear();
    }

    if (sdef.is_usb) {
#ifdef HAVE_USB_SERIAL
        /*
         * Initializes a serial-over-USB CDC driver.
         */
        if (!_device_initialised) {
            sduObjectInit((SerialUSBDriver*)sdef.serial);
            sduStart((SerialUSBDriver*)sdef.serial, &serusbcfg);
            /*
             * Activates the USB driver and then the USB bus pull-up on D+.
             * Note, a delay is inserted in order to not have to disconnect the cable
             * after a reset.
             */
            usbDisconnectBus(serusbcfg.usbp);
            hal.scheduler->delay_microseconds(1500);
            usbStart(serusbcfg.usbp, &usbcfg);
            usbConnectBus(serusbcfg.usbp);
            _device_initialised = true;
        }
#endif
    } else {
#if HAL_USE_SERIAL == TRUE
        if (_baudrate != 0) {
            bool was_initialised = _device_initialised;            
            //setup Rx DMA
            if(!_device_initialised) {
                if(sdef.dma_rx) {
                    rxdma = STM32_DMA_STREAM(sdef.dma_rx_stream_id);
                    chSysLock();
                    bool dma_allocated = dmaStreamAllocate(rxdma,
                                               12,  //IRQ Priority
                                               (stm32_dmaisr_t)rxbuff_full_irq,
                                               (void *)this);
                    osalDbgAssert(!dma_allocated, "stream already allocated");
                    chSysUnlock();
#if defined(STM32F7)
                    dmaStreamSetPeripheral(rxdma, &((SerialDriver*)sdef.serial)->usart->RDR);
#else
                    dmaStreamSetPeripheral(rxdma, &((SerialDriver*)sdef.serial)->usart->DR);
#endif // STM32F7
                }
                if (sdef.dma_tx) {
                    // we only allow for sharing of the TX DMA channel, not the RX
                    // DMA channel, as the RX side is active all the time, so
                    // cannot be shared
                    dma_handle = new Shared_DMA(sdef.dma_tx_stream_id,
                                                SHARED_DMA_NONE,
                                                FUNCTOR_BIND_MEMBER(&UARTDriver::dma_tx_allocate, void, Shared_DMA *),
                                                FUNCTOR_BIND_MEMBER(&UARTDriver::dma_tx_deallocate, void, Shared_DMA *));
                }
                _device_initialised = true;
            }
            sercfg.speed = _baudrate;

            // start with options from set_options()
            sercfg.cr1 = 0;
            sercfg.cr2 = _cr2_options;
            sercfg.cr3 = _cr3_options;

            if (!sdef.dma_tx && !sdef.dma_rx) {
            } else {
                if (sdef.dma_rx) {
                    sercfg.cr1 |= USART_CR1_IDLEIE;
                    sercfg.cr3 |= USART_CR3_DMAR;
                }
                if (sdef.dma_tx) {
                    sercfg.cr3 |= USART_CR3_DMAT;
                }
            }
            sercfg.cr2 |= USART_CR2_STOP1_BITS;
            sercfg.irq_cb = rx_irq_cb;
            sercfg.ctx = (void*)this;

            sdStart((SerialDriver*)sdef.serial, &sercfg);

            if(sdef.dma_rx) {
                //Configure serial driver to skip handling RX packets
                //because we will handle them via DMA
                ((SerialDriver*)sdef.serial)->usart->CR1 &= ~USART_CR1_RXNEIE;
                //Start DMA
                if(!was_initialised) {
                    uint32_t dmamode = STM32_DMA_CR_DMEIE | STM32_DMA_CR_TEIE;
                    dmamode |= STM32_DMA_CR_CHSEL(STM32_DMA_GETCHANNEL(sdef.dma_rx_stream_id,
                                                                       sdef.dma_rx_channel_id));
                    dmamode |= STM32_DMA_CR_PL(0);
                    dmaStreamSetMemory0(rxdma, rx_bounce_buf);
                    dmaStreamSetTransactionSize(rxdma, RX_BOUNCE_BUFSIZE);
                    dmaStreamSetMode(rxdma, dmamode    | STM32_DMA_CR_DIR_P2M |
                                         STM32_DMA_CR_MINC | STM32_DMA_CR_TCIE);
                    dmaStreamEnable(rxdma);
                }
            }
        }
#endif // HAL_USE_SERIAL
    }

    if (_writebuf.get_size() && _readbuf.get_size()) {
        _initialised = true;
    }
    _uart_owner_thd = chThdGetSelfX();

    // setup flow control
    set_flow_control(_flow_control);

    if (serial_num == 0 && _initialised) {
#ifndef HAL_STDOUT_SERIAL
        // setup hal.console to take printf() output
        vprintf_console_hook = hal_console_vprintf;
#endif
    }
}

void UARTDriver::dma_tx_allocate(Shared_DMA *ctx)
{
#if HAL_USE_SERIAL == TRUE
    osalDbgAssert(txdma == nullptr, "double DMA allocation");
    txdma = STM32_DMA_STREAM(sdef.dma_tx_stream_id);
    chSysLock();
    bool dma_allocated = dmaStreamAllocate(txdma,
                                           12,  //IRQ Priority
                                           (stm32_dmaisr_t)tx_complete,
                                           (void *)this);
    osalDbgAssert(!dma_allocated, "stream already allocated");
    chSysUnlock();
#if defined(STM32F7)
    dmaStreamSetPeripheral(txdma, &((SerialDriver*)sdef.serial)->usart->TDR);
#else
    dmaStreamSetPeripheral(txdma, &((SerialDriver*)sdef.serial)->usart->DR);
#endif // STM32F7
#endif // HAL_USE_SERIAL
}

void UARTDriver::dma_tx_deallocate(Shared_DMA *ctx)
{
    chSysLock();
    dmaStreamRelease(txdma);
    txdma = nullptr;
    chSysUnlock();
}

/*
  DMA transmit complettion interrupt handler
 */
void UARTDriver::tx_complete(void* self, uint32_t flags)
{
    UARTDriver* uart_drv = (UARTDriver*)self;
    chSysLockFromISR();
    if (!uart_drv->tx_bounce_buf_ready) {
        // reset timeout 
        chVTResetI(&uart_drv->tx_timeout);
        
        uart_drv->_last_write_completed_us = AP_HAL::micros();
        uart_drv->tx_bounce_buf_ready = true;
        if (uart_drv->unbuffered_writes && uart_drv->_writebuf.available()) {
            // trigger a rapid send of next bytes
            chEvtSignalI(uart_thread_ctx, EVENT_MASK(uart_drv->serial_num));
        }
        uart_drv->dma_handle->unlock_from_IRQ();
    }
    chSysUnlockFromISR();
}


void UARTDriver::rx_irq_cb(void* self)
{
#if HAL_USE_SERIAL == TRUE
    UARTDriver* uart_drv = (UARTDriver*)self;
    if (!uart_drv->sdef.dma_rx) {
        return;
    }
#if defined(STM32F7)
    //disable dma, triggering DMA transfer complete interrupt
    uart_drv->rxdma->stream->CR &= ~STM32_DMA_CR_EN;
#else
    volatile uint16_t sr = ((SerialDriver*)(uart_drv->sdef.serial))->usart->SR;
    if(sr & USART_SR_IDLE) {
        volatile uint16_t dr = ((SerialDriver*)(uart_drv->sdef.serial))->usart->DR;
        (void)dr;
        //disable dma, triggering DMA transfer complete interrupt
        uart_drv->rxdma->stream->CR &= ~STM32_DMA_CR_EN;
    }
#endif // STM32F7
#endif // HAL_USE_SERIAL
}

void UARTDriver::rxbuff_full_irq(void* self, uint32_t flags)
{
#if HAL_USE_SERIAL == TRUE
    UARTDriver* uart_drv = (UARTDriver*)self;
    if (uart_drv->_lock_rx_in_timer_tick) {
        return;
    }
    if (!uart_drv->sdef.dma_rx) {
        return;
    }
    uint8_t len = RX_BOUNCE_BUFSIZE - dmaStreamGetTransactionSize(uart_drv->rxdma);
    if (len == 0) {
        return;
    }

    uart_drv->_readbuf.write(uart_drv->rx_bounce_buf, len);

    uart_drv->receive_timestamp_update();
    
    //restart the DMA transfers
    dmaStreamSetMemory0(uart_drv->rxdma, uart_drv->rx_bounce_buf);
    dmaStreamSetTransactionSize(uart_drv->rxdma, RX_BOUNCE_BUFSIZE);
    dmaStreamEnable(uart_drv->rxdma);
    if (uart_drv->_wait.thread_ctx && uart_drv->_readbuf.available() >= uart_drv->_wait.n) {
        chSysLockFromISR();
        chEvtSignalI(uart_drv->_wait.thread_ctx, EVT_DATA);                    
        chSysUnlockFromISR();
    }
    if (uart_drv->_rts_is_active) {
        uart_drv->update_rts_line();
    }
#endif // HAL_USE_SERIAL
}

void UARTDriver::begin(uint32_t b)
{
    begin(b, 0, 0);
}

void UARTDriver::end()
{
    _initialised = false;
    while (_in_timer) hal.scheduler->delay(1);

    if (sdef.is_usb) {
#ifdef HAVE_USB_SERIAL

        sduStop((SerialUSBDriver*)sdef.serial);
#endif
    } else {
#if HAL_USE_SERIAL == TRUE
        sdStop((SerialDriver*)sdef.serial);
#endif
    }
    _readbuf.set_size(0);
    _writebuf.set_size(0);
}

void UARTDriver::flush()
{
    if (sdef.is_usb) {
#ifdef HAVE_USB_SERIAL

        sduSOFHookI((SerialUSBDriver*)sdef.serial);
#endif
    } else {
        //TODO: Handle this for other serial ports
    }
}

bool UARTDriver::is_initialized()
{
    return _initialised;
}

void UARTDriver::set_blocking_writes(bool blocking)
{
    _blocking_writes = blocking;
}

bool UARTDriver::tx_pending() { return false; }

/* Empty implementations of Stream virtual methods */
uint32_t UARTDriver::available() {
    if (!_initialised) {
        return 0;
    }
    if (sdef.is_usb) {
#ifdef HAVE_USB_SERIAL

        if (((SerialUSBDriver*)sdef.serial)->config->usbp->state != USB_ACTIVE) {
            return 0;
        }
#endif
    }
    return _readbuf.available();
}

uint32_t UARTDriver::txspace()
{
    if (!_initialised) {
        return 0;
    }
    return _writebuf.space();
}

int16_t UARTDriver::read()
{
    if (_uart_owner_thd != chThdGetSelfX()){
        return -1;
    }
    if (!_initialised) {
        return -1;
    }

    uint8_t byte;
    if (!_readbuf.read_byte(&byte)) {
        return -1;
    }
    if (!_rts_is_active) {
        update_rts_line();
    }
    
    return byte;
}

/* Empty implementations of Print virtual methods */
size_t UARTDriver::write(uint8_t c)
{
    if (lock_key != 0 || !_write_mutex.take_nonblocking()) {
        return 0;
    }
    
    if (!_initialised) {
        _write_mutex.give();
        return 0;
    }

    while (_writebuf.space() == 0) {
        if (!_blocking_writes) {
            _write_mutex.give();
            return 0;
        }
        hal.scheduler->delay(1);
    }
    size_t ret = _writebuf.write(&c, 1);
    if (unbuffered_writes) {
        write_pending_bytes();
    }
    _write_mutex.give();
    return ret;
}

size_t UARTDriver::write(const uint8_t *buffer, size_t size)
{
    if (!_initialised || lock_key != 0) {
		return 0;
	}

    if (!_write_mutex.take_nonblocking()) {
        return 0;
    }

    if (_blocking_writes && !unbuffered_writes) {
        /*
          use the per-byte delay loop in write() above for blocking writes
         */
        _write_mutex.give();
        size_t ret = 0;
        while (size--) {
            if (write(*buffer++) != 1) break;
            ret++;
        }
        return ret;
    }

    size_t ret = _writebuf.write(buffer, size);
    if (unbuffered_writes) {
        write_pending_bytes();
    }
    _write_mutex.give();
    return ret;
}

/*
  lock the uart for exclusive use by write_locked() with the right key
 */
bool UARTDriver::lock_port(uint32_t key)
{
    if (lock_key && key != lock_key && key != 0) {
        // someone else is using it
        return false;
    }
    lock_key = key;
    return true;
}

/* 
   write to a locked port. If port is locked and key is not correct then 0 is returned
   and write is discarded. All writes are non-blocking
*/
size_t UARTDriver::write_locked(const uint8_t *buffer, size_t size, uint32_t key)
{
    if (lock_key != 0 && key != lock_key) {
        return 0;
    }
    if (!_write_mutex.take_nonblocking()) {
        return 0;
    }
    size_t ret = _writebuf.write(buffer, size);

    _write_mutex.give();

    return ret;
}

/*
  wait for data to arrive, or a timeout. Return true if data has
  arrived, false on timeout
 */
bool UARTDriver::wait_timeout(uint16_t n, uint32_t timeout_ms)
{
    chEvtGetAndClearEvents(EVT_DATA);
    if (available() >= n) {
        return true;
    }
    _wait.n = n;
    _wait.thread_ctx = chThdGetSelfX();
    eventmask_t mask = chEvtWaitAnyTimeout(EVT_DATA, chTimeMS2I(timeout_ms));
    return (mask & EVT_DATA) != 0;
}

/*
  check for DMA completed for TX
 */
void UARTDriver::check_dma_tx_completion(void)
{
    chSysLock();
    if (!tx_bounce_buf_ready) {
        if (!(txdma->stream->CR & STM32_DMA_CR_EN)) {
            if (dmaStreamGetTransactionSize(txdma) == 0) {
                tx_bounce_buf_ready = true;
                _last_write_completed_us = AP_HAL::micros();
                chVTResetI(&tx_timeout);
                dma_handle->unlock_from_lockzone();
            }
        }
    }
    chSysUnlock();
}

/*
  handle a TX timeout. This can happen with using hardware flow
  control if CTS pin blocks transmit
 */
void UARTDriver::handle_tx_timeout(void *arg)
{
    UARTDriver* uart_drv = (UARTDriver*)arg;
    chSysLockFromISR();
    if (!uart_drv->tx_bounce_buf_ready) {
        dmaStreamDisable(uart_drv->txdma);
        uart_drv->tx_len -= dmaStreamGetTransactionSize(uart_drv->txdma);
        uart_drv->tx_bounce_buf_ready = true;
        uart_drv->dma_handle->unlock_from_IRQ();
    }
    chSysUnlockFromISR();
}

/*
  write out pending bytes with DMA
 */
void UARTDriver::write_pending_bytes_DMA(uint32_t n)
{
    check_dma_tx_completion();

    if (!tx_bounce_buf_ready) {
        return;
    }

    /* TX DMA channel preparation.*/
    _total_written += tx_len;
    _writebuf.advance(tx_len);
    tx_len = _writebuf.peekbytes(tx_bounce_buf, MIN(n, TX_BOUNCE_BUFSIZE));
    if (tx_len == 0) {
        return;
    }
    if (!dma_handle->lock_nonblock()) {
        tx_len = 0;
        return;
    }
    if (dma_handle->has_contention()) {
        /*
          someone else is using this same DMA channel. To reduce
          latency we will drop the TX size with DMA on this UART to
          keep TX times below 250us. This can still suffer from long
          times due to CTS blockage
         */
        uint32_t max_tx_bytes = 1 + (_baudrate * 250UL / 1000000UL);
        if (tx_len > max_tx_bytes) {
            tx_len = max_tx_bytes;
        }
    }
    tx_bounce_buf_ready = false;
    osalDbgAssert(txdma != nullptr, "UART TX DMA allocation failed");
    dmaStreamSetMemory0(txdma, tx_bounce_buf);
    dmaStreamSetTransactionSize(txdma, tx_len);
    uint32_t dmamode = STM32_DMA_CR_DMEIE | STM32_DMA_CR_TEIE;
    dmamode |= STM32_DMA_CR_CHSEL(STM32_DMA_GETCHANNEL(sdef.dma_tx_stream_id,
                                                       sdef.dma_tx_channel_id));
    dmamode |= STM32_DMA_CR_PL(0);
    dmaStreamSetMode(txdma, dmamode | STM32_DMA_CR_DIR_M2P |
                     STM32_DMA_CR_MINC | STM32_DMA_CR_TCIE);
    dmaStreamEnable(txdma);
    uint32_t timeout_us = ((1000000UL * (tx_len+2) * 10) / _baudrate) + 500;
    chVTSet(&tx_timeout, chTimeUS2I(timeout_us), handle_tx_timeout, this);
}

/*
  write any pending bytes to the device, non-DMA method
 */
void UARTDriver::write_pending_bytes_NODMA(uint32_t n)
{
    ByteBuffer::IoVec vec[2];
    const auto n_vec = _writebuf.peekiovec(vec, n);
    for (int i = 0; i < n_vec; i++) {
        int ret = -1;
        if (sdef.is_usb) {
            ret = 0;
#ifdef HAVE_USB_SERIAL
            ret = chnWriteTimeout((SerialUSBDriver*)sdef.serial, vec[i].data, vec[i].len, TIME_IMMEDIATE);
#endif
        } else {
#if HAL_USE_SERIAL == TRUE
            ret = chnWriteTimeout((SerialDriver*)sdef.serial, vec[i].data, vec[i].len, TIME_IMMEDIATE);
#endif
        }
        if (ret < 0) {
            break;
        }
        if (ret > 0) {
            _last_write_completed_us = AP_HAL::micros();
            _total_written += ret;
        }
        _writebuf.advance(ret);
        
        /* We wrote less than we asked for, stop */
        if ((unsigned)ret != vec[i].len) {
            break;
        }
    }
}


/*
  write any pending bytes to the device
 */
void UARTDriver::write_pending_bytes(void)
{
    uint32_t n;

    if (sdef.dma_tx) {
        check_dma_tx_completion();
    }

    // write any pending bytes
    n = _writebuf.available();
    if (n <= 0) {
        return;
    }
    
    if (!sdef.dma_tx) {
        write_pending_bytes_NODMA(n);
    } else {
        write_pending_bytes_DMA(n);
    }
    
    // handle AUTO flow control mode
    if (_flow_control == FLOW_CONTROL_AUTO) {
        if (_first_write_started_us == 0) {
            _first_write_started_us = AP_HAL::micros();
        }
        if (sdef.dma_tx) {
            // when we are using DMA we have a reliable indication that a write
            // has completed from the DMA completion interrupt
            if (_last_write_completed_us != 0) {
                _flow_control = FLOW_CONTROL_ENABLE;
                return;
            }
        } else {
            // without DMA we need to look at the number of bytes written into the queue versus the
            // remaining queue space
            uint32_t space = qSpaceI(&((SerialDriver*)sdef.serial)->oqueue);
            uint32_t used = SERIAL_BUFFERS_SIZE - space;
            // threshold is 8 for the GCS_Common code to unstick SiK radios, which
            // sends 6 bytes with flow control disabled
            const uint8_t threshold = 8;
            if (_total_written > used && _total_written - used > threshold) {
                _flow_control = FLOW_CONTROL_ENABLE;
                return;
            }
        }
        if (AP_HAL::micros() - _first_write_started_us > 500*1000UL) {
            // it doesn't look like hw flow control is working
            hal.console->printf("disabling flow control on serial %u\n", sdef.get_index());
            set_flow_control(FLOW_CONTROL_DISABLE);
        }
    }
}

/*
  push any pending bytes to/from the serial port. This is called at
  1kHz in the timer thread. Doing it this way reduces the system call
  overhead in the main task enormously.
 */
void UARTDriver::_timer_tick(void)
{
    if (!_initialised) return;

    if (sdef.dma_rx && rxdma) {
        _lock_rx_in_timer_tick = true;
        //Check if DMA is enabled
        //if not, it might be because the DMA interrupt was silenced
        //let's handle that here so that we can continue receiving
        if (!(rxdma->stream->CR & STM32_DMA_CR_EN)) {
            uint8_t len = RX_BOUNCE_BUFSIZE - dmaStreamGetTransactionSize(rxdma);
            if (len != 0) {
                _readbuf.write(rx_bounce_buf, len);

                receive_timestamp_update();
                
                if (_wait.thread_ctx && _readbuf.available() >= _wait.n) {
                    chEvtSignal(_wait.thread_ctx, EVT_DATA);                    
                }
                if (_rts_is_active) {
                    update_rts_line();
                }
            }
            //DMA disabled by idle interrupt never got a chance to be handled
            //we will enable it here
            dmaStreamSetMemory0(rxdma, rx_bounce_buf);
            dmaStreamSetTransactionSize(rxdma, RX_BOUNCE_BUFSIZE);
            dmaStreamEnable(rxdma);
        }
        _lock_rx_in_timer_tick = false;
    }

    // don't try IO on a disconnected USB port
    if (sdef.is_usb) {
#ifdef HAVE_USB_SERIAL
        if (((SerialUSBDriver*)sdef.serial)->config->usbp->state != USB_ACTIVE) {
            return;
        }
#endif
    }
    if(sdef.is_usb) {
#ifdef HAVE_USB_SERIAL
        ((GPIO *)hal.gpio)->set_usb_connected();
#endif
    }
    _in_timer = true;

    if (!sdef.dma_rx) {
        // try to fill the read buffer
        ByteBuffer::IoVec vec[2];

        const auto n_vec = _readbuf.reserve(vec, _readbuf.space());
        for (int i = 0; i < n_vec; i++) {
            int ret = 0;
            //Do a non-blocking read
            if (sdef.is_usb) {
    #ifdef HAVE_USB_SERIAL
                ret = chnReadTimeout((SerialUSBDriver*)sdef.serial, vec[i].data, vec[i].len, TIME_IMMEDIATE);
    #endif
            } else {
#if HAL_USE_SERIAL == TRUE
                ret = chnReadTimeout((SerialDriver*)sdef.serial, vec[i].data, vec[i].len, TIME_IMMEDIATE);
#endif
            }
            if (ret < 0) {
                break;
            }
            _readbuf.commit((unsigned)ret);

            receive_timestamp_update();
            
            /* stop reading as we read less than we asked for */
            if ((unsigned)ret < vec[i].len) {
                break;
            }
        }
    }

    if (unbuffered_writes) {
        // now send pending bytes. If we are doing "unbuffered" writes
        // then the send normally happens as soon as the bytes are
        // provided by the write() call, but if the write is larger
        // than the DMA buffer size then there can be extra bytes to
        // send here, and they must be sent with the write lock held
        _write_mutex.take(HAL_SEMAPHORE_BLOCK_FOREVER);
        write_pending_bytes();
        _write_mutex.give();
    } else {
        write_pending_bytes();
    }

    _in_timer = false;
}

/*
  change flow control mode for port
 */
void UARTDriver::set_flow_control(enum flow_control flowcontrol)
{
    if (sdef.rts_line == 0 || sdef.is_usb) {
        // no hw flow control available
        return;
    }    
#if HAL_USE_SERIAL == TRUE
    SerialDriver *sd = (SerialDriver*)(sdef.serial);
    _flow_control = flowcontrol;
    if (!_initialised) {
        // not ready yet, we just set variable for when we call begin
        return;
    }
    switch (_flow_control) {
        
    case FLOW_CONTROL_DISABLE:
        // force RTS active when flow disabled
        palSetLineMode(sdef.rts_line, 1);
        palClearLine(sdef.rts_line);
        _rts_is_active = true;
        // disable hardware CTS support
        chSysLock();
        if ((sd->usart->CR3 & (USART_CR3_CTSE | USART_CR3_RTSE)) != 0) {
            sd->usart->CR1 &= ~USART_CR1_UE;
            sd->usart->CR3 &= ~(USART_CR3_CTSE | USART_CR3_RTSE);
            sd->usart->CR1 |= USART_CR1_UE;
        }
        chSysUnlock();
        break;

    case FLOW_CONTROL_AUTO:
        // reset flow control auto state machine
        _first_write_started_us = 0;
        _last_write_completed_us = 0;
        FALLTHROUGH;

    case FLOW_CONTROL_ENABLE:
        // we do RTS in software as STM32 hardware RTS support toggles
        // the pin for every byte which loses a lot of bandwidth
        palSetLineMode(sdef.rts_line, 1);
        palClearLine(sdef.rts_line);
        _rts_is_active = true;
        // enable hardware CTS support, disable RTS support as we do that in software
        chSysLock();
        if ((sd->usart->CR3 & (USART_CR3_CTSE | USART_CR3_RTSE)) != USART_CR3_CTSE) {
            // CTSE and RTSE can only be written when uart is disabled
            sd->usart->CR1 &= ~USART_CR1_UE;
            sd->usart->CR3 |= USART_CR3_CTSE;
            sd->usart->CR3 &= ~USART_CR3_RTSE;
            sd->usart->CR1 |= USART_CR1_UE;
        }
        chSysUnlock();
        break;
    }
#endif // HAL_USE_SERIAL
}

/*
  software update of rts line. We don't use the HW support for RTS as
  it has no hysteresis, so it ends up toggling RTS on every byte
 */
void UARTDriver::update_rts_line(void)
{
    if (sdef.rts_line == 0 || _flow_control == FLOW_CONTROL_DISABLE) {
        return;
    }
    uint16_t space = _readbuf.space();
    if (_rts_is_active && space < 16) {
        _rts_is_active = false;
        palSetLine(sdef.rts_line);
    } else if (!_rts_is_active && space > 32) {
        _rts_is_active = true;
        palClearLine(sdef.rts_line);
    }
}

/* 
   setup unbuffered writes for lower latency
 */
bool UARTDriver::set_unbuffered_writes(bool on)
{
    if (on && !sdef.dma_tx) {
        // we can't implement low latemcy writes safely without TX DMA
        return false;
    }
    unbuffered_writes = on;
    return true;
}

/*
  setup parity
 */
void UARTDriver::configure_parity(uint8_t v)
{
    if (sdef.is_usb) {
        // not possible
        return;
    }
#if HAL_USE_SERIAL == TRUE
    // stop and start to take effect
    sdStop((SerialDriver*)sdef.serial);
    
    switch (v) {
    case 0:
        // no parity
        sercfg.cr1 &= ~(USART_CR1_PCE | USART_CR1_PS);
        break;
    case 1:
        // odd parity
        // setting USART_CR1_M ensures extra bit is used as parity
        // not last bit of data
        sercfg.cr1 |= USART_CR1_M | USART_CR1_PCE;
        sercfg.cr1 |= USART_CR1_PS;
        break;
    case 2:
        // even parity
        sercfg.cr1 |= USART_CR1_M | USART_CR1_PCE;
        sercfg.cr1 &= ~USART_CR1_PS;
        break;
    }

    sdStart((SerialDriver*)sdef.serial, &sercfg);
    if(sdef.dma_rx) {
        //Configure serial driver to skip handling RX packets
        //because we will handle them via DMA
        ((SerialDriver*)sdef.serial)->usart->CR1 &= ~USART_CR1_RXNEIE;
    }
#endif // HAL_USE_SERIAL
}

/*
  set stop bits
 */
void UARTDriver::set_stop_bits(int n)
{
    if (sdef.is_usb) {
        // not possible
        return;
    }
#if HAL_USE_SERIAL
    // stop and start to take effect
    sdStop((SerialDriver*)sdef.serial);
    
    switch (n) {
    case 1:
        sercfg.cr2 = _cr2_options | USART_CR2_STOP1_BITS;
        break;
    case 2:
        sercfg.cr2 = _cr2_options | USART_CR2_STOP2_BITS;
        break;
    }

    sdStart((SerialDriver*)sdef.serial, &sercfg);
    if(sdef.dma_rx) {
        //Configure serial driver to skip handling RX packets
        //because we will handle them via DMA
        ((SerialDriver*)sdef.serial)->usart->CR1 &= ~USART_CR1_RXNEIE;
    }
#endif // HAL_USE_SERIAL
}


// record timestamp of new incoming data 
void UARTDriver::receive_timestamp_update(void)
{
    _receive_timestamp[_receive_timestamp_idx^1] = AP_HAL::micros64();
    _receive_timestamp_idx ^= 1;
}

/*
  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 UARTDriver::receive_time_constraint_us(uint16_t nbytes)
{
    uint64_t last_receive_us = _receive_timestamp[_receive_timestamp_idx];
    if (_baudrate > 0 && !sdef.is_usb) {
        // assume 10 bits per byte. For USB we assume zero transport delay
        uint32_t transport_time_us = (1000000UL * 10UL / _baudrate) * (nbytes + available());
        last_receive_us -= transport_time_us;
    }
    return last_receive_us;
}

// set optional features, return true on success
bool UARTDriver::set_options(uint8_t options)
{
    if (sdef.is_usb) {
        // no options allowed on USB
        return (options == 0);
    }
    bool ret = true;

#if HAL_USE_SERIAL == TRUE
    SerialDriver *sd = (SerialDriver*)(sdef.serial);
    uint32_t cr2 = sd->usart->CR2;
    uint32_t cr3 = sd->usart->CR3;
    bool was_enabled = (sd->usart->CR1 & USART_CR1_UE);

#ifdef STM32F7
    // F7 has built-in support for inversion in all uarts
    if (options & OPTION_RXINV) {
        cr2 |= USART_CR2_RXINV;
        _cr2_options |= USART_CR2_RXINV;
    } else {
        cr2 &= ~USART_CR2_RXINV;
    }
    if (options & OPTION_TXINV) {
        cr2 |= USART_CR2_TXINV;
        _cr2_options |= USART_CR2_TXINV;
    } else {
        cr2 &= ~USART_CR2_TXINV;
    }
    // F7 can also support swapping RX and TX pins
    if (options & OPTION_SWAP) {
        cr2 |= USART_CR2_SWAP;
        _cr2_options |= USART_CR2_SWAP;
    } else {
        cr2 &= ~USART_CR2_SWAP;
    }
#else // STM32F4
    // F4 can do inversion by GPIO if enabled in hwdef.dat, using
    // TXINV and RXINV options
    if (options & OPTION_RXINV) {
        if (sdef.rxinv_gpio >= 0) {
            hal.gpio->write(sdef.rxinv_gpio, sdef.rxinv_polarity);
        } else {
            ret = false;
        }
    }
    if (options & OPTION_TXINV) {
        if (sdef.txinv_gpio >= 0) {
            hal.gpio->write(sdef.txinv_gpio, sdef.txinv_polarity);
        } else {
            ret = false;
        }
    }
    if (options & OPTION_SWAP) {
        ret = false;
    }
#endif // STM32xx

    // both F4 and F7 can do half-duplex
    if (options & OPTION_HDPLEX) {
        cr3 |= USART_CR3_HDSEL;
        _cr3_options |= USART_CR3_HDSEL;
    } else {
        cr3 &= ~USART_CR3_HDSEL;
    }

    if (sd->usart->CR2 == cr2 &&
        sd->usart->CR3 == cr3) {
        // no change
        return ret;
    }

    if (was_enabled) {
        sd->usart->CR1 &= ~USART_CR1_UE;
    }

    sd->usart->CR2 = cr2;
    sd->usart->CR3 = cr3;

    if (was_enabled) {
        sd->usart->CR1 |= USART_CR1_UE;
    }
#endif // HAL_USE_SERIAL == TRUE
    return ret;
}

#endif //CONFIG_HAL_BOARD == HAL_BOARD_CHIBIOS