Bug in app_uart_fifo.c

Question

Hi, 
 I'm attempting to use the UART in a high speed configuration without flow control but I get regularely get two kinds of errors in the output: 
 
 Lost characters 
 Reversed characters 
 
 Expected UART test output: 
 ...
0123456789
0123456789
0123456789
0123456789
0123456789
0123456789
...
 
 Sample of actual UART output: 
 ...
123456798
0123465789
0132456789
123456798
0123465789
0132456789
123456798
0123546789
0213456789
0123457689
...
 
 The output is visible on different hardware: 
 
 Our own nrf51822 boards 
 pca10028 when not running the UART through the Interface chip/USB 
 
 Pseudo code for my setup: 
 
 No softdevice used 
 Only RX/TX, HWFC disabled and RTS/CTS lines disconnected 
 UART_BAUDRATE_BAUDRATE_Baud921600 
 
 
 #define LENGTH 24 
int main(void) {
 char buf[LENGTH] = "0123456789
0123456789
";
 
 while (true)
 {
 for( int i = 0; i < LENGTH; i++) {
 while(app_uart_put(buf[i]) != NRF_SUCCESS);
 }
 }
}
 
 
 I have found potential problem sources in app_uart_fifo.c. To me, the underlying problem seems to be that calling app_uart_put() in rapid succession results in a race condition between direct calls to nrf_drv_uart_tx() in app_uart_put() and the emptying of the fifo also done with calls to nrf_drv_uart_tx() in uart_event_handler(). The former calls are made from thread mode and the later from interrupt (LOW) mode. 
 Example: 
 Say that the app uart tx fifo contains data that is being processed due to a previous call to app_uart_put(). A new call to app_uart_put() calls nrf_drv_uart_tx() precisely after a byte has been sent by the UART driver, but before the TXDRDY signal is serviced. Then: 
 
 Two bytes could be sent in the reversed order, i.e. a byte not in the fifo is sent to the uart driver before a byte is taken from the fifo, or 
 A byte is silently dropped as uart_event_handler() calls nrf_drv_uart_tx() without checking the results. What if nrf_drv_uart_tx() returns NRF_ERROR_BUSY due to the race condition above? 
 
 I modified app_uart_fifo.c successfully by doing the following: 
 
 In app_uart_fifo.c initialize a SWI with the same priority as the UART driver (LOW) 
 In app_uart_put() call app_fifo_put() and then nrf_drv_swi_trigger(). Never call nrf_drv_uart_tx() directly. 
 In both uart_event_handler() and swi_event_handler() call a common function: app_uart_tx() 
 
 
 static void app_uart_tx(void)
{
 app_uart_evt_t app_uart_event;

 // Get next byte from FIFO.
 if (app_fifo_get(&m_tx_fifo, tx_buffer) == NRF_SUCCESS)
 {
 while(nrf_drv_uart_tx(tx_buffer,1) != NRF_ERROR_BUSY);
 }
 if (FIFO_LENGTH(m_tx_fifo) == 0)
 {
 // Last byte from FIFO transmitted, notify the application.
 app_uart_event.evt_type = APP_UART_TX_EMPTY;
 m_event_handler(&app_uart_event);
 }
}
 
 
 This is the same logic used in uart_event_handler() today with the addition of looping the nrf_drv_uart_tx() until success. 
 The changes above result in stable UART output as expected, i.e. no missing or reversed characters. 
 Is there some way of achieving the same results without using a SWI? I tried NVIC_DisableIRQ(UART0_IRQn) together with _DSB and _ISB but never got it to work properly. 
 /Pablo