drm/fb-helper: Clip damage area horizontally

Clip the damage area horizontally if only a single scanline has been
changed. This is helpful to reduce the memcpy overhead for small writes.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220209161617.3553-6-tzimmermann@suse.de

drivers/gpu/drm/drm_fb_helper.c

@@ -690,6 +690,18 @@ static void drm_fb_helper_memory_range_to_clip(struct fb_info *info, off_t off,
 	u32 x2 = info->var.xres;
 	u32 y2 = DIV_ROUND_UP(end, info->fix.line_length);
 
+	if ((y2 - y1) == 1) {
+		/*
+		 * We've only written to a single scanline. Try to reduce
+		 * the number of horizontal pixels that need an update.
+		 */
+		off_t bit_off = (off % info->fix.line_length) * 8;
+		off_t bit_end = (end % info->fix.line_length) * 8;
+
+		x1 = bit_off / info->var.bits_per_pixel;
+		x2 = DIV_ROUND_UP(bit_end, info->var.bits_per_pixel);
+	}
+
 	drm_rect_init(clip, x1, y1, x2 - x1, y2 - y1);
 }