[PATCH v1 2/2] efi/earlycon: Speed up scrolling by skiping moving empty space

[Andy Shevchenko]

Currently the scroll copies the full screen which is slow on
the hi-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@xxxxxxxxxxxxxxx>
---
 drivers/firmware/efi/earlycon.c | 28 ++++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/drivers/firmware/efi/earlycon.c b/drivers/firmware/efi/earlycon.c
index be7c83b6cd10..f54e6fdf08e2 100644
--- a/drivers/firmware/efi/earlycon.c
+++ b/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();
 
-- 
2.35.1