tty: vgacon+sisusb, move scrolldelta to a common helper

The code is mirrorred in scrolldelta implementations of both vgacon
and sisusb. Let's move the code to a separate helper where we will
perform a common cleanup and further changes.

While we are moving the code, make it linear and save one indentation
level. This is done by returning from the "!lines" then-branch
immediatelly. This allows flushing the else-branch 1 level to the
left, obviously.

Few more new lines and comments were added too.

And do not forget to export the helper function given sisusb can be
built as module.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: <linux-fbdev@vger.kernel.org>
Cc: <linux-usb@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vt.c

@@ -4279,6 +4279,44 @@ void vcs_scr_updated(struct vc_data *vc)
 	notify_update(vc);
 }
 
+void vc_scrolldelta_helper(struct vc_data *c, int lines,
+		unsigned int rolled_over, void *base, unsigned int size)
+{
+	unsigned long ubase = (unsigned long)base;
+	int margin = c->vc_size_row * 4;
+	int ul, we, p, st;
+
+	/* Turn scrollback off */
+	if (!lines) {
+		c->vc_visible_origin = c->vc_origin;
+		return;
+	}
+
+	/* Do we have already enough to allow jumping from 0 to the end? */
+	if (rolled_over > (c->vc_scr_end - ubase) + margin) {
+		ul = c->vc_scr_end - ubase;
+		we = rolled_over + c->vc_size_row;
+	} else {
+		ul = 0;
+		we = size;
+	}
+
+	p = (c->vc_visible_origin - ubase - ul + we) % we +
+		lines * c->vc_size_row;
+	st = (c->vc_origin - ubase - ul + we) % we;
+
+	/* Only a little piece would be left? Show all incl. the piece! */
+	if (st < 2 * margin)
+		margin = 0;
+	if (p < margin)
+		p = 0;
+	if (p > st - margin)
+		p = st;
+
+	c->vc_visible_origin = ubase + (p + ul) % we;
+}
+EXPORT_SYMBOL_GPL(vc_scrolldelta_helper);
+
 /*
  *	Visible symbols for modules
  */

drivers/usb/misc/sisusbvga/sisusb_con.c

@@ -686,8 +686,6 @@ static void
 sisusbcon_scrolldelta(struct vc_data *c, int lines)
 {
 	struct sisusb_usb_data *sisusb;
-	int margin = c->vc_size_row * 4;
-	int ul, we, p, st;
 
 	sisusb = sisusb_get_sisusb_lock_and_check(c->vc_num);
 	if (!sisusb)
@@ -700,39 +698,8 @@ sisusbcon_scrolldelta(struct vc_data *c, int lines)
 		return;
 	}
 
-	if (!lines)		/* Turn scrollback off */
-		c->vc_visible_origin = c->vc_origin;
-	else {
-
-		if (sisusb->con_rolled_over >
-				(c->vc_scr_end - sisusb->scrbuf) + margin) {
-
-			ul = c->vc_scr_end - sisusb->scrbuf;
-			we = sisusb->con_rolled_over + c->vc_size_row;
-
-		} else {
-
-			ul = 0;
-			we = sisusb->scrbuf_size;
-
-		}
-
-		p = (c->vc_visible_origin - sisusb->scrbuf - ul + we) % we +
-				lines * c->vc_size_row;
-
-		st = (c->vc_origin - sisusb->scrbuf - ul + we) % we;
-
-		if (st < 2 * margin)
-			margin = 0;
-
-		if (p < margin)
-			p = 0;
-
-		if (p > st - margin)
-			p = st;
-
-		c->vc_visible_origin = sisusb->scrbuf + (p + ul) % we;
-	}
+	vc_scrolldelta_helper(c, lines, sisusb->con_rolled_over,
+			(void *)sisusb->scrbuf, sisusb->scrbuf_size);
 
 	sisusbcon_set_start_address(sisusb, c);
 

drivers/video/console/vgacon.c

@@ -332,31 +332,8 @@ static void vgacon_restore_screen(struct vc_data *c)
 
 static void vgacon_scrolldelta(struct vc_data *c, int lines)
 {
-	if (!lines)		/* Turn scrollback off */
-		c->vc_visible_origin = c->vc_origin;
-	else {
-		int margin = c->vc_size_row * 4;
-		int ul, we, p, st;
-
-		if (vga_rolled_over >
-		    (c->vc_scr_end - vga_vram_base) + margin) {
-			ul = c->vc_scr_end - vga_vram_base;
-			we = vga_rolled_over + c->vc_size_row;
-		} else {
-			ul = 0;
-			we = vga_vram_size;
-		}
-		p = (c->vc_visible_origin - vga_vram_base - ul + we) % we +
-		    lines * c->vc_size_row;
-		st = (c->vc_origin - vga_vram_base - ul + we) % we;
-		if (st < 2 * margin)
-			margin = 0;
-		if (p < margin)
-			p = 0;
-		if (p > st - margin)
-			p = st;
-		c->vc_visible_origin = vga_vram_base + (p + ul) % we;
-	}
+	vc_scrolldelta_helper(c, lines, vga_rolled_over, (void *)vga_vram_base,
+			vga_vram_size);
 	vga_set_mem_top(c);
 }
 #endif /* CONFIG_VGACON_SOFT_SCROLLBACK */

include/linux/vt_kern.h

@@ -191,5 +191,7 @@ extern void vt_set_led_state(int console, int leds);
 extern void vt_kbd_con_start(int console);
 extern void vt_kbd_con_stop(int console);
 
+void vc_scrolldelta_helper(struct vc_data *c, int lines,
+		unsigned int rolled_over, void *_base, unsigned int size);
 
 #endif /* _VT_KERN_H */