kdb: Simplify code to fetch characters from console

Currently kdb_read_get_key() contains complex control flow that, on
close inspection, turns out to be unnecessary. In particular:

1. It is impossible to enter the branch conditioned on (escape_delay == 1)
   except when the loop enters with (escape_delay == 2) allowing us to
   combine the branches.

2. Most of the code conditioned on (escape_delay == 2) simply modifies
   local data and then breaks out of the loop causing the function to
   return escape_data[0].

3. Based on #2 there is not actually any need to ever explicitly set
   escape_delay to 2 because we it is much simpler to directly return
   escape_data[0] instead.

4. escape_data[0] is, for all but one exit path, known to be '\e'.

Simplify the code based on these observations.

There is a subtle (and harmless) change of behaviour resulting from this
simplification: instead of letting the escape timeout after ~1998
milliseconds we now timeout after ~2000 milliseconds

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20191025073328.643-3-daniel.thompson@linaro.org

kernel/debug/kdb/kdb_io.c

@@ -124,25 +124,18 @@ static int kdb_read_get_key(char *buffer, size_t bufsize)
 			touch_nmi_watchdog();
 			f = &kdb_poll_funcs[0];
 		}
-		if (escape_delay == 2) {
-			*ped = '\0';
-			ped = escape_data;
-			--escape_delay;
-		}
-		if (escape_delay == 1) {
-			key = *ped++;
-			if (!*ped)
-				--escape_delay;
-			break;
-		}
+
 		key = (*f)();
+
 		if (key == -1) {
 			if (escape_delay) {
 				udelay(ESCAPE_UDELAY);
-				--escape_delay;
+				if (--escape_delay == 0)
+					return '\e';
 			}
 			continue;
 		}
+
 		if (bufsize <= 2) {
 			if (key == '\r')
 				key = '\n';
@@ -150,27 +143,24 @@ static int kdb_read_get_key(char *buffer, size_t bufsize)
 			*buffer = '\0';
 			return -1;
 		}
+
 		if (escape_delay == 0 && key == '\e') {
 			escape_delay = ESCAPE_DELAY;
 			ped = escape_data;
 			f_escape = f;
 		}
 		if (escape_delay) {
-			*ped++ = key;
-			if (f_escape != f) {
-				escape_delay = 2;
-				continue;
-			}
+			if (f_escape != f)
+				return '\e';
 
+			*ped++ = key;
 			key = kdb_handle_escape(escape_data, ped - escape_data);
-			if (key > 0) {
-				escape_data[0] = key;
-				escape_data[1] = '\0';
-			}
-			if (key)
-				escape_delay = 2;
-			continue;
+			if (key < 0)
+				return '\e';
+			if (key == 0)
+				continue;
 		}
+
 		break;	/* A key to process */
 	}
 	return key;