mpfs_serial.c: Add RX flowcontrol

[jpaali]

This is very basic functionality what we have here as we don't have RTS / CTS Flow Control in Saluki.
Disable RX interrupts and clear the FIFO in case of full RX buffer. Enable RX interrupts in case of empty buffer.

[jlaitine]

I didn't understand this... so if the rx buffer is full ( 1 queued ? ), we disable rx interrupt and start loosing data?

What is the issue this PR fixes?

[jpaali]

I didn't understand this... so if the rx buffer is full ( 1 queued ? ), we disable rx interrupt and start loosing data?

What is the issue this PR fixes?

DP-10022.
UART4 (gps) goes to situation where it is unable to get out of while loop in interrupt handler (DR bit does not get cleared -> rxbuf gets full).

nuttx/drivers/serial/serial_io.c

Line 149 in 85fe8fc

while (uart_rxavailable(dev))

Looping for a while also RX FIFO of UART goes overrun condition.

[jlaitine]

Looping for a while also RX FIFO of UART goes overrun condition.

Got it, thanks!

[jlaitine]

Is there some bug in upper layers, or what is the reason for buffer empty check (nbuffered == 0)?

Just out of curiosity, it is fine to have it there

[jlaitine]

Please upstream the same patch to apache/nuttx before merging to our master, if possible!