Speed up vt(4) by keeping a record of the most recently drawn character and

the foreground and background colours.  In bitblt_text functions, compare
values to this cache and don't re-draw the characters if they haven't changed.
When invalidating the display, clear this cache in order to force characters
to be redrawn; also force full redraws between suspend/resume pairs since odd
artifacts can otherwise result.

When scrolling the display (which is where most time is spent within the vt
driver) this yields a significant performance improvement if most lines are
less than the width of the terminal, since this avoids re-drawing blanks on
top of blanks.

(Note that "re-drawing" here includes writing to the VGA text mode buffer; on
virtualized systems this can be extremely slow since it triggers a glyph
being rendered onto a 640x480 screen).

On a c5.4xlarge EC2 instance (with emulated text mode VGA) this cuts the time
spent in vt(4) during the kernel boot from 1200 ms to 700ms; on my laptop
(with a 3200x1800 display) the corresponding time is reduced from 970 ms down
to 155 ms.

Reviewed by:	imp, cem
Approved by:	re (gjb)
Relnotes:	Significant speedup in vt(4) and the system boot generally.
Differential Revision:	https://reviews.freebsd.org/D16723

1 files changed, 7 insertions, 0 deletions

diff --git a/sys/dev/vt/vt.h b/sys/dev/vt/vt.h
index 745f9de836cb..712da20b7634 100644
--- a/sys/dev/vt/vt.h
+++ b/sys/dev/vt/vt.h
@@ -156,11 +156,15 @@ struct vt_device {
 #define	VDF_INITIALIZED	0x20	/* vtterm_cnprobe already done. */
 #define	VDF_MOUSECURSOR	0x40	/* Mouse cursor visible. */
 #define	VDF_QUIET_BELL	0x80	/* Disable bell. */
+#define	VDF_SUSPENDED	0x100	/* Device has been suspended. */
 #define	VDF_DOWNGRADE	0x8000	/* The driver is being downgraded. */
 	int			 vd_keyboard;	/* (G) Keyboard index. */
 	unsigned int		 vd_kbstate;	/* (?) Device unit. */
 	unsigned int		 vd_unit;	/* (c) Device unit. */
 	int			 vd_altbrk;	/* (?) Alt break seq. state */
+	term_char_t		*vd_drawn;	/* (?) Most recent char drawn. */
+	term_color_t		*vd_drawnfg;	/* (?) Most recent fg color drawn. */
+	term_color_t		*vd_drawnbg;	/* (?) Most recent bg color drawn. */
 };
 
 #define	VD_PASTEBUF(vd)	((vd)->vd_pastebuf.vpb_buf)
@@ -320,6 +324,8 @@ typedef void vd_postswitch_t(struct vt_device *vd);
 typedef void vd_blank_t(struct vt_device *vd, term_color_t color);
 typedef void vd_bitblt_text_t(struct vt_device *vd, const struct vt_window *vw,
     const term_rect_t *area);
+typedef void vd_invalidate_text_t(struct vt_device *vd,
+    const term_rect_t *area);
 typedef void vd_bitblt_bmp_t(struct vt_device *vd, const struct vt_window *vw,
     const uint8_t *pattern, const uint8_t *mask,
     unsigned int width, unsigned int height,
@@ -345,6 +351,7 @@ struct vt_driver {
 	vd_drawrect_t	*vd_drawrect;
 	vd_setpixel_t	*vd_setpixel;
 	vd_bitblt_text_t *vd_bitblt_text;
+	vd_invalidate_text_t *vd_invalidate_text;
 	vd_bitblt_bmp_t	*vd_bitblt_bmp;
 
 	/* Framebuffer ioctls, if present. */