libpayload/i8042: Add API to peek on keyboard input queue

Change-Id: I60699e044b5bacd3f5292fed7edbf529ae133284
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47592
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/libpayload/drivers/i8042/i8042.c

@@ -112,6 +112,19 @@ static u8 fifo_pop(struct fifo *fifo)
 	return ret;
 }
 
+/** Peek on the head of fifo queue.
+ * Returns the oldest object on the queue if any.
+ * In case the queue is empty 0 is returned.
+ * @fifo: Fifo to use
+ */
+static u8 fifo_peek(struct fifo *fifo)
+{
+	if (fifo_is_empty(fifo))
+		return 0;
+
+	return fifo->buf[fifo->rx];
+}
+
 /** Destroys a fifo queue.
  * @fifo: Fifo to use
  */
@@ -390,6 +403,14 @@ u8 i8042_read_data_ps2(void)
 	return fifo_pop(ps2_fifo);
 }
 
+/**
+ * Returns available keyboard data without advancing the queue.
+ */
+u8 i8042_peek_data_ps2(void)
+{
+	return fifo_peek(ps2_fifo);
+}
+
 /**
  * Returns available mouse data, if any.
  */

payloads/libpayload/include/libpayload.h

@@ -233,6 +233,7 @@ u8 i8042_data_ready_ps2(void);
 u8 i8042_data_ready_aux(void);
 
 u8 i8042_read_data_ps2(void);
+u8 i8042_peek_data_ps2(void);
 u8 i8042_read_data_aux(void);
 
 int i8042_wait_read_ps2(void);