关于串口回调函数触发的条件，调用HalUARTWrite()触发了串口回调函数，什么原因呢？

static void HalUARTPollDMA(void)
{
uint16 cnt = 0;
uint8 evt = 0;

if (HAL_UART_DMA_NEW_RX_BYTE(dmaCfg.rxHead))
{
rxIdx_t tail = findTail();

// If the DMA has transferred in more Rx bytes, reset the Rx idle timer.
if (dmaCfg.rxTail != tail)
{
dmaCfg.rxTail = tail;

// Re-sync the shadow on any 1st byte(s) received.
if (dmaCfg.rxTick == 0)
{
dmaCfg.rxShdw = ST0;
}
dmaCfg.rxTick = HAL_UART_DMA_IDLE;
}
else if (dmaCfg.rxTick)
{
// Use the LSB of the sleep timer (ST0 must be read first anyway).
uint8 decr = ST0 - dmaCfg.rxShdw;

if (dmaCfg.rxTick > decr)
{
dmaCfg.rxTick -= decr;
dmaCfg.rxShdw = ST0;
}
else
{
dmaCfg.rxTick = 0;
}
}
cnt = HalUARTRxAvailDMA();
}
else
{
dmaCfg.rxTick = 0;
}

if (cnt >= HAL_UART_DMA_FULL)
{
evt = HAL_UART_RX_FULL;
}
else if (cnt >= HAL_UART_DMA_HIGH)
{
evt = HAL_UART_RX_ABOUT_FULL;
PxOUT |= HAL_UART_Px_RTS; // Disable Rx flow.
}
else if (cnt && !dmaCfg.rxTick)
{
evt = HAL_UART_RX_TIMEOUT;
}

if (dmaCfg.txMT)
{
dmaCfg.txMT = FALSE;
evt |= HAL_UART_TX_EMPTY;
}

if (dmaCfg.txShdwValid)
{
uint8 decr = ST0;
decr -= dmaCfg.txShdw;
if (decr > dmaCfg.txTick)
{
// No protection for txShdwValid is required
// because while the shadow was valid, DMA ISR cannot be triggered
// to cause concurrent access to this variable.
dmaCfg.txShdwValid = FALSE;
}
}

if (dmaCfg.txDMAPending && !dmaCfg.txShdwValid)
{
// UART TX DMA is expected to be fired and enough time has lapsed since last DMA ISR
// to know that DBUF can be overwritten
halDMADesc_t *ch = HAL_DMA_GET_DESC1234(HAL_DMA_CH_TX);
halIntState_t intState;

// Clear the DMA pending flag
dmaCfg.txDMAPending = FALSE;

HAL_DMA_SET_SOURCE(ch, dmaCfg.txBuf[dmaCfg.txSel]);
HAL_DMA_SET_LEN(ch, dmaCfg.txIdx[dmaCfg.txSel]);
dmaCfg.txSel ^= 1;
HAL_ENTER_CRITICAL_SECTION(intState);
HAL_DMA_ARM_CH(HAL_DMA_CH_TX);
do
{
asm("NOP");
} while (!HAL_DMA_CH_ARMED(HAL_DMA_CH_TX));
HAL_DMA_CLEAR_IRQ(HAL_DMA_CH_TX);
HAL_DMA_MAN_TRIGGER(HAL_DMA_CH_TX);
HAL_EXIT_CRITICAL_SECTION(intState);
}
else
{
halIntState_t his;

HAL_ENTER_CRITICAL_SECTION(his);
if ((dmaCfg.txIdx[dmaCfg.txSel] != 0) && !HAL_DMA_CH_ARMED(HAL_DMA_CH_TX)
&& !HAL_DMA_CHECK_IRQ(HAL_DMA_CH_TX))
{
HAL_EXIT_CRITICAL_SECTION(his);
HalUARTIsrDMA();
}
else
{
HAL_EXIT_CRITICAL_SECTION(his);
}
}

if (evt && (dmaCfg.uartCB != NULL))
{
dmaCfg.uartCB(HAL_UART_DMA-1, evt);
}
}