Fixed syslink DMA by guarding a none-atimic read.

[tobbeanton]

The hangup of the syslink RX DMA read was due to that the USART_DMACmd function is not atomic and called from uartslkSendDataDmaBlocking as well as uartslkReceiveDMA which is run from an interrupt.

void USART_DMACmd(USART_TypeDef* USARTx, uint16_t USART_DMAReq, FunctionalState NewState)
{
  /* Check the parameters */
  assert_param(IS_USART_ALL_PERIPH(USARTx));
  assert_param(IS_USART_DMAREQ(USART_DMAReq));  
  assert_param(IS_FUNCTIONAL_STATE(NewState)); 

  if (NewState != DISABLE)
  {
    /* Enable the DMA transfer for selected requests by setting the DMAT and/or
       DMAR bits in the USART CR3 register */
    USARTx->CR3 |= USART_DMAReq;
  }
  else
  {
    /* Disable the DMA transfer for selected requests by clearing the DMAT and/or
       DMAR bits in the USART CR3 register */
    USARTx->CR3 &= (uint16_t)~USART_DMAReq;
  }
}

The USARTx->CR3 |= USART_DMAReq; translates to a read-modify-write operation and if the uart RX interupt happened just in the modify state it would write the wrong value to the register.

Syslink DMA RX will be used with only one byte payload as well as it tuns out to be more efficient.

Syslink DMA RX has been enabled again in kbuild.

[evoggy]

Great!