imx_serial: set wake bit when we receive a data byte

The Linux kernel added a flood check for RX data recently in commit
496a4471b7c3 ("serial: imx: work-around for hardware RX flood"). This
check uses the wake bit in the UART status register 2. The wake bit
indicates that the receiver detected a start bit on the RX line. If the
kernel sees a number of RX interrupts without the wake bit being set, it
treats this as spurious data and resets the UART port. imx_serial does
never set the wake bit and triggers the kernel's flood check.

This patch adds support for the wake bit. wake is set when we receive a
new character (it's not set for break events). It seems that wake is
cleared by the kernel driver, the hardware does not have to clear it
automatically after data was read.

The wake bit can be configured as an interrupt source. Support this
mechanism as well.

Co-developed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/char/imx_serial.c

@@ -80,7 +80,7 @@ static void imx_update(IMXSerialState *s)
      * TCEN and TXDC are both bit 3
      * RDR and DREN are both bit 0
      */
-    mask |= s->ucr4 & (UCR4_TCEN | UCR4_DREN);
+    mask |= s->ucr4 & (UCR4_WKEN | UCR4_TCEN | UCR4_DREN);
 
     usr2 = s->usr2 & mask;
 
@@ -321,6 +321,9 @@ static void imx_put_data(void *opaque, uint32_t value)
 
 static void imx_receive(void *opaque, const uint8_t *buf, int size)
 {
+    IMXSerialState *s = (IMXSerialState *)opaque;
+
+    s->usr2 |= USR2_WAKE;
     imx_put_data(opaque, *buf);
 }
 

include/hw/char/imx_serial.h

@@ -71,6 +71,7 @@ OBJECT_DECLARE_SIMPLE_TYPE(IMXSerialState, IMX_SERIAL)
 
 #define UCR4_DREN       BIT(0)    /* Receive Data Ready interrupt enable */
 #define UCR4_TCEN       BIT(3)    /* TX complete interrupt enable */
+#define UCR4_WKEN       BIT(7)    /* WAKE interrupt enable */
 
 #define UTS1_TXEMPTY    (1<<6)
 #define UTS1_RXEMPTY    (1<<5)