video: fbdev: ssd1307fb: Simplify ssd1307fb_update_display()

Simplify the nested loops to handle conversion from linear frame buffer
to ssd1307 page layout:
  1. Move last page handling one level up, as the value of "m" is the
     same inside a page,
  2. array->data[] is filled linearly, so there is no need to
     recalculate array_idx over and over again; a simple increment is
     sufficient.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210727134730.3765898-3-geert@linux-m68k.org

drivers/video/fbdev/ssd1307fb.c

@@ -158,6 +158,7 @@ static int ssd1307fb_update_display(struct ssd1307fb_par *par)
 	u8 *vmem = par->info->screen_buffer;
 	unsigned int line_length = par->info->fix.line_length;
 	unsigned int pages = DIV_ROUND_UP(par->height, 8);
+	u32 array_idx = 0;
 	int ret, i, j, k;
 
 	array = ssd1307fb_alloc_array(par->width * pages, SSD1307FB_DATA);
@@ -194,19 +195,21 @@ static int ssd1307fb_update_display(struct ssd1307fb_par *par)
 	 */
 
 	for (i = 0; i < pages; i++) {
+		int m = 8;
+
+		/* Last page may be partial */
+		if (i + 1 == pages && par->height % 8)
+			m = par->height % 8;
 		for (j = 0; j < par->width; j++) {
-			int m = 8;
-			u32 array_idx = i * par->width + j;
-			array->data[array_idx] = 0;
-			/* Last page may be partial */
-			if (i + 1 == pages && par->height % 8)
-				m = par->height % 8;
+			u8 data = 0;
+
 			for (k = 0; k < m; k++) {
 				u8 byte = vmem[(8 * i + k) * line_length +
 					       j / 8];
 				u8 bit = (byte >> (j % 8)) & 1;
-				array->data[array_idx] |= bit << k;
+				data |= bit << k;
 			}
+			array->data[array_idx++] = data;
 		}
 	}