vt: make use of ucs_get_fallback() when glyph is unavailable

Attempt to display a fallback character when given character doesn't
have an available glyph. The substitution may not be as good as the
original character but still way more helpful than a squared question
mark.

Example substitutions: À -> A, ç -> c, ø -> o, ─ -> -, © -> C, etc.

See gen_ucs_fallback_table.py for a comprehensive list.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Link: https://lore.kernel.org/r/20250507141535.40655-8-nico@fluxnic.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 556af82a9231a40f5554ac56db8489d871751e16..efb7614541666cdeeb2b226af0e14b737f6f24a1 100644 (file)
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -3007,6 +3007,19 @@ static int vc_get_glyph(struct vc_data *vc, int tc)
                return tc;
        }
 
+       /*
+        * The Unicode screen memory is allocated only when required.
+        * This is one such case: we're about to "cheat" with the displayed
+        * character meaning the simple screen buffer won't hold the original
+        * information, whereas the Unicode screen buffer always does.
+        */
+       vc_uniscr_check(vc);
+
+       /* Try getting a simpler fallback character. */
+       tc = ucs_get_fallback(tc);
+       if (tc)
+               return vc_get_glyph(vc, tc);
+
        /* Display U+FFFD (Unicode Replacement Character). */
        return conv_uni_to_pc(vc, UCS_REPLACEMENT);
 }