tty: vt: separate ESesc state handling into handle_esc()

Similar to the ASCII handling, the ESC handling can be easily moved away
from do_con_trol(). So create a new handle_esc() for that.

And add a comment with an example.
Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20240202065608.14019-13-jirislaby@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vt.c

@@ -2267,84 +2267,94 @@ static bool handle_ascii(struct tty_struct *tty, struct vc_data *vc, u8 c)
 	return false;
 }
 
-/* console_lock is held */
-static void do_con_trol(struct tty_struct *tty, struct vc_data *vc, u8 c)
-{
-	/*
-	 *  Control characters can be used in the _middle_
-	 *  of an escape sequence, aside from ANSI control strings.
+/*
+ * Handle a character (@c) following an ESC (when @vc is in the ESesc state).
+ * E.g. previous ESC with @c == '[' here yields the ESsquare state (that is:
+ * CSI).
  */
-	if (ansi_control_string(vc->vc_state) && c >= ASCII_IGNORE_FIRST &&
-	    c <= ASCII_IGNORE_LAST)
-		return;
-
-	if (handle_ascii(tty, vc, c))
-		return;
-
-	switch(vc->vc_state) {
-	case ESesc:
+static void handle_esc(struct tty_struct *tty, struct vc_data *vc, u8 c)
+{
 	vc->vc_state = ESnormal;
 	switch (c) {
 	case '[':
 		vc->vc_state = ESsquare;
-			return;
+		break;
 	case ']':
 		vc->vc_state = ESnonstd;
-			return;
+		break;
 	case '_':
 		vc->vc_state = ESapc;
-			return;
+		break;
 	case '^':
 		vc->vc_state = ESpm;
-			return;
+		break;
 	case '%':
 		vc->vc_state = ESpercent;
-			return;
+		break;
 	case 'E':
 		cr(vc);
 		lf(vc);
-			return;
+		break;
 	case 'M':
 		ri(vc);
-			return;
+		break;
 	case 'D':
 		lf(vc);
-			return;
+		break;
 	case 'H':
 		if (vc->state.x < VC_TABSTOPS_COUNT)
 			set_bit(vc->state.x, vc->vc_tab_stop);
-			return;
+		break;
 	case 'P':
 		vc->vc_state = ESdcs;
-			return;
+		break;
 	case 'Z':
 		respond_ID(tty);
-			return;
+		break;
 	case '7':
 		save_cur(vc);
-			return;
+		break;
 	case '8':
 		restore_cur(vc);
-			return;
+		break;
 	case '(':
 		vc->vc_state = ESsetG0;
-			return;
+		break;
 	case ')':
 		vc->vc_state = ESsetG1;
-			return;
+		break;
 	case '#':
 		vc->vc_state = EShash;
-			return;
+		break;
 	case 'c':
 		reset_terminal(vc, 1);
-			return;
+		break;
 	case '>':  /* Numeric keypad */
 		clr_kbd(vc, kbdapplic);
-			return;
+		break;
 	case '=':  /* Appl. keypad */
 		set_kbd(vc, kbdapplic);
-			return;
+		break;
 	}
+}
+
+/* console_lock is held */
+static void do_con_trol(struct tty_struct *tty, struct vc_data *vc, u8 c)
+{
+	/*
+	 *  Control characters can be used in the _middle_
+	 *  of an escape sequence, aside from ANSI control strings.
+	 */
+	if (ansi_control_string(vc->vc_state) && c >= ASCII_IGNORE_FIRST &&
+	    c <= ASCII_IGNORE_LAST)
+		return;
+
+	if (handle_ascii(tty, vc, c))
+		return;
+
+	switch(vc->vc_state) {
+	case ESesc:
+		handle_esc(tty, vc, c);
 		return;
 	case ESnonstd:
 		if (c=='P') {   /* palette escape sequence */