[PATCH 17/20] fbconsole: implement VT100 deferred wrap (last column flag)

[Ahmad Fatoum]

Quoting Mattias Engdegård's wraptest project[1]:

  DEC VT terminals use a mechanism sometimes called "soft-wrap",
  "deferred wrap" or the "VT100 glitch", mainly to permit full-length
  newline-terminated text lines to be written without being followed
  by unsightly blank lines. This is what complicates matters.

  The auto-wrap mode flag, DECAWM, controls whether text will wrap around
  the edges at all. It can be set and cleared using the DECSET and DECRST
  control sequences; terminal emulators usually get this bit right.
  In the following text, we assume that auto-wrap is enabled (DECAWM set).

  When the terminal receives a printing character and the cursor is in the
  rightmost column, what happens depends on the hidden Last Column Flag:

  - If the LCF is 0, then the character is drawn at the cursor position
    as usual, but instead of advancing the cursor afterwards, the LCF is
    set to 1.

  - If the LCF is 1, then the cursor is moved to the first column of the
    next line, scrolling if necessary. Then LCF is set to 0, the character
    is drawn, and the cursor advanced as usual.

  The Last Column flag starts out as 0 when the terminal is powered on.

Implement this into barebox, so full screen TUIs do not immediately roll
off their first line off the framebuffer console.

[1]: https://github.com/mattiase/wraptest

Signed-off-by: Ahmad Fatoum <a.fatoum at barebox.org>
---
 drivers/video/fbconsole.c | 33 ++++++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/drivers/video/fbconsole.c b/drivers/video/fbconsole.c
index 71e0778368ed..bed55de7571e 100644
--- a/drivers/video/fbconsole.c
+++ b/drivers/video/fbconsole.c
@@ -55,6 +55,8 @@ struct fbc_screen_state {
 #define SGR_ATTRIBUTES		(ANSI_FLAG_INVERT | ANSI_FLAG_BRIGHT)
 #define HIDE_CURSOR		(1 << 2)
 	unsigned flags;
+
+	bool pending_wrap;	/* deferred wrap: cursor at last column */
 };
 
 struct fbc_priv {
@@ -471,6 +473,7 @@ static void printchar(struct fbc_priv *priv, int c)
 	case '\007': /* bell: ignore */
 		break;
 	case '\b':
+		cur->pending_wrap = false;
 		if (cur->x > 0) {
 			cur->x--;
 		} else if (cur->y > 0) {
@@ -480,25 +483,47 @@ static void printchar(struct fbc_priv *priv, int c)
 		break;
 	case '\n':
 	case '\013': /* Vertical tab is the same as Line Feed */
+		cur->pending_wrap = false;
 		cur->y++;
 		break;
 
 	case '\r':
+		cur->pending_wrap = false;
 		cur->x = 0;
 		break;
 
 	case '\t':
+		cur->pending_wrap = false;
 		cur->x = (cur->x + 8) & ~0x3;
 		break;
 
 	default:
-		drawchar(priv, priv->cur.x, priv->cur.y, c);
+		/*
+		 * VT100 deferred wrap: if the previous character landed at
+		 * the last column, perform the wrap NOW before drawing the
+		 * next character.  This prevents a scroll from occurring
+		 * merely by writing to the bottom-right corner cell.
+		 */
+		if (cur->pending_wrap) {
+			cur->pending_wrap = false;
+			cur->x = 0;
+			cur->y++;
+
+			if (cur->y >= priv->rows) {
+				fb_scroll_up(priv);
+				cur->y = priv->rows - 1;
+			}
+		}
+
+		drawchar(priv, cur->x, cur->y, c);
 
 		cur->x++;
 		if (cur->x >= priv->cols) {
-			cur->y++;
-			cur->x = 0;
+			/* Defer the wrap to the next printable character */
+			cur->x = priv->cols - 1;
+			cur->pending_wrap = true;
 		}
+		break;
 	}
 
 	if (cur->y >= priv->rows) {
@@ -660,6 +685,7 @@ static bool fbc_parse_csi(struct fbc_priv *priv)
 		return true;
 	case 'H':
 		toggle_cursor_visibility(priv);
+		priv->cur.pending_wrap = false;
 
 		pos = simple_strtoul(priv->csi, &end, 10);
 		fbc_set_cursor_row(priv, pos - 1);
@@ -672,6 +698,7 @@ static bool fbc_parse_csi(struct fbc_priv *priv)
 	case 'A' ... 'D': {
 		pos = simple_strtoul(priv->csi, &end, 10) ?: 1;
 		toggle_cursor_visibility(priv);
+		priv->cur.pending_wrap = false;
 
 		switch (last) {
 		case 'A': /* cursor up */
-- 
2.47.3