static int __btrfs_map_block_for_discard(struct btrfs_fs_info *fs_info,
					 u64 logical, u64 length,
					 struct btrfs_bio **bbio_ret)
{
	struct extent_map *em;
	struct map_lookup *map;
	struct btrfs_bio *bbio;
	u64 offset;
	u64 stripe_nr;
	u64 stripe_nr_end;
	u64 stripe_end_offset;
	u64 stripe_cnt;
	u64 stripe_len;
	u64 stripe_offset;
	u64 num_stripes;
	u32 stripe_index;
	u32 factor = 0;
	u32 sub_stripes = 0;
	u64 stripes_per_dev = 0;
	u32 remaining_stripes = 0;
	u32 last_stripe = 0;
	int ret = 0;
	int i;

	/* discard always return a bbio */
	ASSERT(bbio_ret);

	em = btrfs_get_chunk_map(fs_info, logical, length);
	if (IS_ERR(em))
		return PTR_ERR(em);

	map = em->map_lookup;
	/* we don't discard raid56 yet */
	if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK) {
		ret = -EOPNOTSUPP;
		goto out;
	}

	offset = logical - em->start;
	length = min_t(u64, em->len - offset, length);

	stripe_len = map->stripe_len;
	/*
	 * stripe_nr counts the total number of stripes we have to stride
	 * to get to this block
	 */
	stripe_nr = div64_u64(offset, stripe_len);

	/* stripe_offset is the offset of this block in its stripe */
	stripe_offset = offset - stripe_nr * stripe_len;

	stripe_nr_end = round_up(offset + length, map->stripe_len);
	stripe_nr_end = div64_u64(stripe_nr_end, map->stripe_len);
	stripe_cnt = stripe_nr_end - stripe_nr;
	stripe_end_offset = stripe_nr_end * map->stripe_len -
			    (offset + length);
	/*
	 * after this, stripe_nr is the number of stripes on this
	 * device we have to walk to find the data, and stripe_index is
	 * the number of our device in the stripe array
	 */
	num_stripes = 1;
	stripe_index = 0;
	if (map->type & (BTRFS_BLOCK_GROUP_RAID0 |
			 BTRFS_BLOCK_GROUP_RAID10)) {
		if (map->type & BTRFS_BLOCK_GROUP_RAID0)
			sub_stripes = 1;
		else
			sub_stripes = map->sub_stripes;

		factor = map->num_stripes / sub_stripes;
		num_stripes = min_t(u64, map->num_stripes,
				    sub_stripes * stripe_cnt);
		stripe_nr = div_u64_rem(stripe_nr, factor, &stripe_index);
		stripe_index *= sub_stripes;
		stripes_per_dev = div_u64_rem(stripe_cnt, factor,
					      &remaining_stripes);
		div_u64_rem(stripe_nr_end - 1, factor, &last_stripe);
		last_stripe *= sub_stripes;
	} else if (map->type & (BTRFS_BLOCK_GROUP_RAID1 |
				BTRFS_BLOCK_GROUP_DUP)) {
		num_stripes = map->num_stripes;
	} else {
		stripe_nr = div_u64_rem(stripe_nr, map->num_stripes,
					&stripe_index);
	}

	bbio = alloc_btrfs_bio(num_stripes, 0);
	if (!bbio) {
		ret = -ENOMEM;
		goto out;
	}

	for (i = 0; i < num_stripes; i++) {
		bbio->stripes[i].physical =
			map->stripes[stripe_index].physical +
			stripe_offset + stripe_nr * map->stripe_len;
		bbio->stripes[i].dev = map->stripes[stripe_index].dev;

		if (map->type & (BTRFS_BLOCK_GROUP_RAID0 |
				 BTRFS_BLOCK_GROUP_RAID10)) {
			bbio->stripes[i].length = stripes_per_dev *
				map->stripe_len;

			if (i / sub_stripes < remaining_stripes)
				bbio->stripes[i].length +=
					map->stripe_len;

			/*
			 * Special for the first stripe and
			 * the last stripe:
			 *
			 * |-------|...|-------|
			 *     |----------|
			 *    off     end_off
			 */
			if (i < sub_stripes)
				bbio->stripes[i].length -=
					stripe_offset;

			if (stripe_index >= last_stripe &&
			    stripe_index <= (last_stripe +
					     sub_stripes - 1))
				bbio->stripes[i].length -=
					stripe_end_offset;

			if (i == sub_stripes - 1)
				stripe_offset = 0;
		} else {
			bbio->stripes[i].length = length;
		}

		stripe_index++;
		if (stripe_index == map->num_stripes) {
			stripe_index = 0;
			stripe_nr++;
		}
	}

	*bbio_ret = bbio;
	bbio->map_type = map->type;
	bbio->num_stripes = num_stripes;
out:
	free_extent_map(em);
	return ret;
}