[PATCH v2 1/5] vt: Add cursor-size helpers

[Thomas Zimmermann <tzimmermann@suse.de>]

Cursors in the VT subsystem are blocks within a character cell that are
filled with the foreground color. The new helpers vc_cursor_start() and
vc_cursor_end() return the scanlines in which the cursor block starts rsp.
ends. This is compatible with VGA hardware

In terms of cursor design, the new cursor-size helpers follow established
styles in fbcon. The only exception is in underline cursors for fonts with
a size larger than 10. The underlining dash is now one pixel closer to
the font-glyph data, so that the cursor looks less detached. This follows
the style used by vgacon.

Similar code in vgacon and fbcon ignores the cursor's default size stored
in vt.cur_default. The consoles default to full-block cursors, while vt
defaults to underline cursors. The new helper fall back to cur_default and
then underline cursors; in this order.

v2:
- export non-font interface for vgacon
- avoid interference from concurrent user space (Sashiko)
- fix function docs (Sashiko)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Helge Deller <deller@gmx.de>
---
 drivers/tty/vt/vt.c            | 93 ++++++++++++++++++++++++++++++++++
 include/linux/console_struct.h |  7 +++
 2 files changed, 100 insertions(+)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 57edf37495a8..5d2583cd6e79 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -71,6 +71,7 @@
  * by Adam Tla/lka <atlka@pg.gda.pl>, Aug 2006
  */
 
+#include <linux/compiler.h>
 #include <linux/module.h>
 #include <linux/types.h>
 #include <linux/sched/signal.h>
@@ -264,6 +265,98 @@ unsigned int vc_font_size(const struct vc_font *font)
 }
 EXPORT_SYMBOL_GPL(vc_font_size);
 
+/*
+ * Cursors
+ */
+
+/**
+ * vc_cursor_start - Calculates the cursor's first scanline within a character cell
+ * @cell_height: The overall height of the character cell
+ * @cursor_size: The size constant of cursor pattern
+ *
+ * The parameter @cell_height is the height of the character cell as
+ * displayed by the console. The argument given in @cursor_size is one
+ * of the CUR_ constants, as stored in struct @vc_data.vc_cursor_type.
+ * For unknown values, the helper draws the default cursor or an underline
+ * dash.
+ *
+ * Returns:
+ * The index of the cursor's first scanline within the character cell
+ */
+unsigned int vc_cursor_start(unsigned int cell_height, unsigned int cursor_size)
+{
+	const unsigned int cursor_default_size = CUR_SIZE(READ_ONCE(cur_default));
+
+retry:
+	switch (cursor_size) {
+	case CUR_NONE:
+		return cell_height;
+	case CUR_UNDERLINE:
+		if (cell_height < 10)
+			return cell_height - 1;
+		else
+			return cell_height - 3;
+	case CUR_LOWER_THIRD:
+		return cell_height - cell_height / 3;
+	case CUR_LOWER_HALF:
+		return cell_height - cell_height / 2;
+	case CUR_TWO_THIRDS:
+		return cell_height - (cell_height * 2) / 3;
+	case CUR_BLOCK:
+		return 0;
+	default:
+		pr_warn_once("Unknown cursor %u\n", cursor_size);
+		if (cursor_size != cursor_default_size)
+			cursor_size = cursor_default_size;
+		else
+			cursor_size = CUR_UNDERLINE;
+		goto retry;
+	}
+}
+EXPORT_SYMBOL_GPL(vc_cursor_start);
+
+/**
+ * vc_cursor_end - Calculates the first scanline after the cursor within a character cell
+ * @cell_height: The overall height of the character cell
+ * @cursor_size: The size constant of cursor pattern
+ *
+ * The parameter @cell_height is the height of the character cell as
+ * displayed by the console. The argument given in @cursor_size is one
+ * of the CUR_ constants, as stored in struct @vc_data.vc_cursor_type.
+ * For unknown values, the helper draws the default cursor or an underline
+ * dash.
+ *
+ * Returns:
+ * The index of the first scanline after the cursor within the character cell
+ */
+unsigned int vc_cursor_end(unsigned int cell_height, unsigned int cursor_size)
+{
+	const unsigned int cursor_default_size = CUR_SIZE(READ_ONCE(cur_default));
+
+retry:
+	switch (cursor_size) {
+	case CUR_UNDERLINE:
+		if (cell_height < 10)
+			return cell_height;
+		else
+			return cell_height - 1;
+	case CUR_NONE:
+	case CUR_LOWER_THIRD:
+	case CUR_LOWER_HALF:
+	case CUR_TWO_THIRDS:
+	case CUR_BLOCK:
+		return cell_height;
+	default:
+		pr_warn_once("Unknown cursor %u\n", cursor_size);
+		if (cursor_size != cursor_default_size)
+			cursor_size = cursor_default_size;
+		else
+			cursor_size = CUR_UNDERLINE;
+		goto retry;
+	}
+}
+EXPORT_SYMBOL_GPL(vc_cursor_end);
+
 /*
  * /sys/class/tty/tty0/
  *
diff --git a/include/linux/console_struct.h b/include/linux/console_struct.h
index fe915afdece5..1d37c7898ea0 100644
--- a/include/linux/console_struct.h
+++ b/include/linux/console_struct.h
@@ -204,6 +204,10 @@ struct vc {
 extern struct vc vc_cons [MAX_NR_CONSOLES];
 extern void vc_SAK(struct work_struct *work);
 
+/*
+ * Cursors
+ */
+
 #define CUR_MAKE(size, change, set)	((size) | ((change) << 8) |	\
 		((set) << 16))
 #define CUR_SIZE(c)		 ((c) & 0x00000f)
@@ -222,6 +226,9 @@ extern void vc_SAK(struct work_struct *work);
 #define CUR_CHANGE(c)		 ((c) & 0x00ff00)
 #define CUR_SET(c)		(((c) & 0xff0000) >> 8)
 
+unsigned int vc_cursor_start(unsigned int cell_height, unsigned int cursor_size);
+unsigned int vc_cursor_end(unsigned int cell_height, unsigned int cursor_size);
+
 bool con_is_visible(const struct vc_data *vc);
 
 #endif /* _LINUX_CONSOLE_STRUCT_H */
-- 
2.55.0