serial: 8250_port: Don't service RX FIFO if throttled

When port's throttle callback is called, it should stop pushing any more
data into TTY buffer to avoid buffer overflow. This means driver has to
stop HW from receiving more data and assert the HW flow control. For
UARTs with auto HW flow control (such as 8250_omap) manual assertion of
flow control line is not possible and only way is to allow RX FIFO to
fill up, thus trigger auto HW flow control logic.

Therefore make sure that 8250 generic IRQ handler does not drain data
when port is stopped (i.e UART_LSR_DR is unset in read_status_mask). Not
servicing, RX FIFO would trigger auto HW flow control when FIFO
occupancy reaches preset threshold, thus halting RX.
Since, error conditions in UART_LSR register are cleared just by reading
the register, data has to be drained in case there are FIFO errors, else
error information will lost.
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lore.kernel.org/r/20200319103230.16867-2-vigneshr@ti.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_port.c

@@ -1883,6 +1883,7 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
 	unsigned char status;
 	unsigned long flags;
 	struct uart_8250_port *up = up_to_u8250p(port);
+	bool skip_rx = false;
 
 	if (iir & UART_IIR_NO_INT)
 		return 0;
@@ -1891,7 +1892,20 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
 
 	status = serial_port_in(port, UART_LSR);
 
-	if (status & (UART_LSR_DR | UART_LSR_BI)) {
+	/*
+	 * If port is stopped and there are no error conditions in the
+	 * FIFO, then don't drain the FIFO, as this may lead to TTY buffer
+	 * overflow. Not servicing, RX FIFO would trigger auto HW flow
+	 * control when FIFO occupancy reaches preset threshold, thus
+	 * halting RX. This only works when auto HW flow control is
+	 * available.
+	 */
+	if (!(status & (UART_LSR_FIFOE | UART_LSR_BRK_ERROR_BITS)) &&
+	    (port->status & (UPSTAT_AUTOCTS | UPSTAT_AUTORTS)) &&
+	    !(port->read_status_mask & UART_LSR_DR))
+		skip_rx = true;
+
+	if (status & (UART_LSR_DR | UART_LSR_BI) && !skip_rx) {
 		if (!up->dma || handle_rx_dma(up, iir))
 			status = serial8250_rx_chars(up, status);
 	}