efi/earlycon: Speed up scrolling by disregarding empty space

Currently the scroll copies the full screen which is slow on high
resolution displays. At the same time, most of the screen is an empty
space which has no need to be copied over and over.

Optimize the scrolling algorithm by caching the x coordinates of the
last printed lines and scroll in accordance with the maximum x in that
cache.

On my Microsoft Surface Book (the first version) this produces a
significant speedup of the console 90 seconds vs. 168 seconds with the
kernel command line having

	ignore_loglevel earlycon=efifb keep_bootcon
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/earlycon.c

@@ -16,6 +16,8 @@
 
 static const struct console *earlycon_console __initdata;
 static const struct font_desc *font;
+static u16 cur_line_y, max_line_y;
+static u32 efi_x_array[1024];
 static u32 efi_x, efi_y;
 static u64 fb_base;
 static bool fb_wb;
@@ -86,9 +88,17 @@ static void efi_earlycon_clear_scanline(unsigned int y)
 static void efi_earlycon_scroll_up(void)
 {
 	unsigned long *dst, *src;
+	u16 maxlen = 0;
 	u16 len;
 	u32 i, height;
 
+	/* Find the cached maximum x coordinate */
+	for (i = 0; i < max_line_y; i++) {
+		if (efi_x_array[i] > maxlen)
+			maxlen = efi_x_array[i];
+	}
+	maxlen *= 4;
+
 	len = screen_info.lfb_linelength;
 	height = screen_info.lfb_height;
 
@@ -103,7 +113,7 @@ static void efi_earlycon_scroll_up(void)
 			return;
 		}
 
-		memmove(dst, src, len);
+		memmove(dst, src, maxlen);
 
 		efi_earlycon_unmap(src, len);
 		efi_earlycon_unmap(dst, len);
@@ -136,6 +146,7 @@ static void
 efi_earlycon_write(struct console *con, const char *str, unsigned int num)
 {
 	struct screen_info *si;
+	u32 cur_efi_x = efi_x;
 	unsigned int len;
 	const char *s;
 	void *dst;
@@ -176,6 +187,7 @@ efi_earlycon_write(struct console *con, const char *str, unsigned int num)
 		str += count;
 
 		if (num > 0 && *s == '\n') {
+			cur_efi_x = efi_x;
 			efi_x = 0;
 			efi_y += font->height;
 			str++;
@@ -183,6 +195,7 @@ efi_earlycon_write(struct console *con, const char *str, unsigned int num)
 		}
 
 		if (efi_x + font->width > si->lfb_width) {
+			cur_efi_x = efi_x;
 			efi_x = 0;
 			efi_y += font->height;
 		}
@@ -190,6 +203,9 @@ efi_earlycon_write(struct console *con, const char *str, unsigned int num)
 		if (efi_y + font->height > si->lfb_height) {
 			u32 i;
 
+			efi_x_array[cur_line_y] = cur_efi_x;
+			cur_line_y = (cur_line_y + 1) % max_line_y;
+
 			efi_y -= font->height;
 			efi_earlycon_scroll_up();
 
@@ -230,7 +246,15 @@ static int __init efi_earlycon_setup(struct earlycon_device *device,
 	if (!font)
 		return -ENODEV;
 
-	efi_y = rounddown(yres, font->height) - font->height;
+	/* Fill the cache with maximum possible value of x coordinate */
+	memset32(efi_x_array, rounddown(xres, font->width), ARRAY_SIZE(efi_x_array));
+	efi_y = rounddown(yres, font->height);
+
+	/* Make sure we have cache for the x coordinate for the full screen */
+	max_line_y = efi_y / font->height + 1;
+	cur_line_y = 0;
+
+	efi_y -= font->height;
 	for (i = 0; i < (yres - efi_y) / font->height; i++)
 		efi_earlycon_scroll_up();