[PATCH] serial: 8250: Avoid "too much work" from bogus rx timeout interrupt

[Douglas Anderson]

On a Rockchip rk3399-based board during suspend/resume testing, we
found that we could get the console UART into a state where it would
print this to the console a lot:
  serial8250: too much work for irq42

Followed eventually by:
  NMI watchdog: BUG: soft lockup - CPU#0 stuck for 11s!

Upon debugging I found that we're in this state:
  iir = 0x000000cc
  lsr = 0x00000060

It appears that somehow we have a RX Timeout interrupt but there is no
actual data present to receive.  When we're in this state the UART
driver claims that it handled the interrupt but it actually doesn't
really do anything.  This means that we keep getting the interrupt
over and over again.

Normally we don't actually need to do anything special to handle a RX
Timeout interrupt.  We'll notice that there is some data ready and
we'll read it, which will end up clearing the RX Timeout.  In this
case we have a problem specifically because we got the RX TImeout
without any data.  Reading a bogus byte is confirmed to get us out of
this state.

It's unclear how exactly the UART got into this state, but it is known
that the UART lines are essentially undriven and unpowered during
suspend, so possibly during resume some garbage / half transmitted
bits are seen on the line and put the UART into this state.

The UART on the rk3399 is a DesignWare based 8250 UART but I have
placed this fix in the general 8250 code because it shouldn't hurt to
have this detection on all 8250 UARTs and it's plausible some other
UART could get into the same state.  If these two extra lines of code
are too much overhead, we can certainly move it into the DesignWare
driver or even only do it for Rockchip UARTs.

Signed-off-by: Douglas Anderson <dianders at chromium.org>
---
Testing and development done on a kernel-4.4 based tree, then picked
to ToT, where the code applied cleanly.

 drivers/tty/serial/8250/8250_port.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index fe4399b41df6..8582c068c3d1 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1824,6 +1824,12 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
 	if (status & (UART_LSR_DR | UART_LSR_BI)) {
 		if (!up->dma || handle_rx_dma(up, iir))
 			status = serial8250_rx_chars(up, status);
+	} else if ((iir & 0x3f) == UART_IIR_RX_TIMEOUT) {
+		/*
+		 * On some systems we saw the timeout interrupt even when
+		 * there was no data ready.  Do a bogus read to clear it.
+		 */
+		(void) serial_port_in(port, UART_RX);
 	}
 	serial8250_modem_status(up);
 	if ((!up->dma || up->dma->tx_err) && (status & UART_LSR_THRE))
-- 
2.8.0.rc3.226.g39d4020