vt: make vt_console_print() compatible with the unicode screen buffer

When kernel messages are printed to the console, they appear blank on
the unicode screen. This is because vt_console_print() is lacking a call
to vc_uniscr_putc(). However the later function assumes vc->vc_x is
always up to date when called, which is not the case here as
vt_console_print() uses it to mark the beginning of the display update.

This patch reworks (and simplifies) vt_console_print() so that vc->vc_x
is always valid and keeps the start of display update in a local variable
instead, which finally allows for adding the missing vc_uniscr_putc()
call.
Signed-off-by: Nicolas Pitre <nico@linaro.org>
Cc: stable@vger.kernel.org # v4.19+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vt.c

@@ -2884,8 +2884,7 @@ static void vt_console_print(struct console *co, const char *b, unsigned count)
 	unsigned char c;
 	static DEFINE_SPINLOCK(printing_lock);
 	const ushort *start;
-	ushort cnt = 0;
-	ushort myx;
+	ushort start_x, cnt;
 	int kmsg_console;
 
 	/* console busy or not yet initialized */
@@ -2898,10 +2897,6 @@ static void vt_console_print(struct console *co, const char *b, unsigned count)
 	if (kmsg_console && vc_cons_allocated(kmsg_console - 1))
 		vc = vc_cons[kmsg_console - 1].d;
 
-	/* read `x' only after setting currcons properly (otherwise
-	   the `x' macro will read the x of the foreground console). */
-	myx = vc->vc_x;
-
 	if (!vc_cons_allocated(fg_console)) {
 		/* impossible */
 		/* printk("vt_console_print: tty %d not allocated ??\n", currcons+1); */
@@ -2916,53 +2911,41 @@ static void vt_console_print(struct console *co, const char *b, unsigned count)
 		hide_cursor(vc);
 
 	start = (ushort *)vc->vc_pos;
-
-	/* Contrived structure to try to emulate original need_wrap behaviour
-	 * Problems caused when we have need_wrap set on '\n' character */
+	start_x = vc->vc_x;
+	cnt = 0;
 	while (count--) {
 		c = *b++;
 		if (c == 10 || c == 13 || c == 8 || vc->vc_need_wrap) {
-			if (cnt > 0) {
-				if (con_is_visible(vc))
-					vc->vc_sw->con_putcs(vc, start, cnt, vc->vc_y, vc->vc_x);
-				vc->vc_x += cnt;
-				if (vc->vc_need_wrap)
-					vc->vc_x--;
+			if (cnt && con_is_visible(vc))
+				vc->vc_sw->con_putcs(vc, start, cnt, vc->vc_y, start_x);
 			cnt = 0;
-			}
 			if (c == 8) {		/* backspace */
 				bs(vc);
 				start = (ushort *)vc->vc_pos;
-				myx = vc->vc_x;
+				start_x = vc->vc_x;
 				continue;
 			}
 			if (c != 13)
 				lf(vc);
 			cr(vc);
 			start = (ushort *)vc->vc_pos;
-			myx = vc->vc_x;
+			start_x = vc->vc_x;
 			if (c == 10 || c == 13)
 				continue;
 		}
+		vc_uniscr_putc(vc, c);
 		scr_writew((vc->vc_attr << 8) + c, (unsigned short *)vc->vc_pos);
 		notify_write(vc, c);
 		cnt++;
-		if (myx == vc->vc_cols - 1) {
+		if (vc->vc_x == vc->vc_cols - 1) {
 			vc->vc_need_wrap = 1;
-			continue;
-		}
+		} else {
 			vc->vc_pos += 2;
-		myx++;
-	}
-	if (cnt > 0) {
-		if (con_is_visible(vc))
-			vc->vc_sw->con_putcs(vc, start, cnt, vc->vc_y, vc->vc_x);
-		vc->vc_x += cnt;
-		if (vc->vc_x == vc->vc_cols) {
-			vc->vc_x--;
-			vc->vc_need_wrap = 1;
+			vc->vc_x++;
 		}
 	}
+	if (cnt && con_is_visible(vc))
+		vc->vc_sw->con_putcs(vc, start, cnt, vc->vc_y, start_x);
 	set_cursor(vc);
 	notify_update(vc);