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    Subject[PATCH] vt: change SGR 21 to follow the standards
    Date
    From: Mike Frysinger <vapier@chromium.org>

    ECMA-48 [1] (aka ISO 6429) has defined SGR 21 as "doubly underlined"
    since at least March 1984. The Linux kernel has treated it as SGR 22
    "normal intensity" since it was added in Linux-0.96b in June 1992.
    Before that, it was simply ignored. Other terminal emulators have
    either ignored it, or treat it as double underline now. xterm for
    example added support in its 304 release (May 2014) [2] where it was
    previously ignoring it.

    Changing this behavior shouldn't be an issue:
    - It isn't a named capability in ncurses's terminfo database, so no
    script is using libtinfo/libcurses to look this up, or using tput
    to query & output the right sequence.
    - Any script assuming SGR 21 will reset intensity in all terminals
    already do not work correctly on non-Linux VTs (including running
    under screen/tmux/etc...).
    - If someone has written a script that only runs in the Linux VT, and
    they're using SGR 21 (instead of SGR 22), the output should still
    be readable.

    imo it's important to change this as the Linux VT's non-conformance
    is sometimes used as an argument for other terminal emulators to not
    implement SGR 21 at all, or do so incorrectly.

    [1]: https://www.ecma-international.org/publications/standards/Ecma-048.htm
    [2]: https://github.com/ThomasDickey/xterm-snapshots/commit/2fd29cb98d214cb536bcafbee00bc73b3f1eeb9d

    Signed-off-by: Mike Frysinger <vapier@chromium.org>
    ---
    drivers/tty/vt/vt.c | 6 +++++-
    1 file changed, 5 insertions(+), 1 deletion(-)

    diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
    index 88b902c525d7..16c0fcd6904a 100644
    --- a/drivers/tty/vt/vt.c
    +++ b/drivers/tty/vt/vt.c
    @@ -1354,6 +1354,11 @@ static void csi_m(struct vc_data *vc)
    case 3:
    vc->vc_italic = 1;
    break;
    + case 21:
    + /*
    + * No console drivers support double underline, so
    + * convert it to a single underline.
    + */
    case 4:
    vc->vc_underline = 1;
    break;
    @@ -1389,7 +1394,6 @@ static void csi_m(struct vc_data *vc)
    vc->vc_disp_ctrl = 1;
    vc->vc_toggle_meta = 1;
    break;
    - case 21:
    case 22:
    vc->vc_intensity = 1;
    break;
    --
    2.16.1
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