[PATCH] serial: sc16is7xx: address RX timeout interrupt errata

[Daniel Mack]

This devices has a silicon bug that makes it report a timeout interrupt
but no data in FIFO.

The datasheet states the following in the errata section 18.1.4:

  "If the host reads the receive FIFO at the at the same time as a
  time-out interrupt condition happens, the host might read 0xCC
  (time-out) in the Interrupt Indication Register (IIR), but bit 0
  of the Line Status Register (LSR) is not set (means there is not
  data in the receive FIFO)."

When this happens, the loop in sc16is7xx_irq() will run forever,
which effectively blocks the i2c bus and breaks the functionality
of the UART.

>From the information above, it is assumed that when the bug is
triggered, the FIFO does in fact have payload in its buffer, but the
fill level reporting is off-by-one. Hence this patch fixes the issue
by reading one byte from the FIFO when that condition is detected.

This clears the interrupt and hence breaks the polling loop.

Signed-off-by: Daniel Mack <daniel@xxxxxxxxxx>
Co-Developed-by: Maxim Popov <maxim.snafu@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 drivers/tty/serial/sc16is7xx.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c
index 289ca7d4e566..76f76e510ed1 100644
--- a/drivers/tty/serial/sc16is7xx.c
+++ b/drivers/tty/serial/sc16is7xx.c
@@ -765,6 +765,18 @@ static bool sc16is7xx_port_irq(struct sc16is7xx_port *s, int portno)
 		case SC16IS7XX_IIR_RTOI_SRC:
 		case SC16IS7XX_IIR_XOFFI_SRC:
 			rxlen = sc16is7xx_port_read(port, SC16IS7XX_RXLVL_REG);
+
+			/*
+			 * There is a silicon bug that makes the chip report a
+			 * time-out interrupt but no data in the FIFO. This is
+			 * described in errata section 18.1.4.
+			 *
+			 * When this happens, read one byte from the FIFO to
+			 * clear the interrupt.
+			 */
+			if (iir == SC16IS7XX_IIR_RTOI_SRC && !rxlen)
+				rxlen = 1;
+
 			if (rxlen)
 				sc16is7xx_handle_rx(port, rxlen, iir);
 			break;
-- 
2.41.0