block: introduce dma map backed bio type

@@ include/linux/blk_types.h
 	struct bio {
 		...
 		/* The actual vec list, preserved by bio_reset() */
-		struct bio_vec		*bi_io_vec;
+		union {
+			struct bio_vec		*bi_io_vec;
+			/* Driver specific dma map, valid IFF REQ_DMABUF is set */
+			struct dma_buf_io_map	*bi_dmabuf_map;
+		};
 		struct bvec_iter	bi_iter;
 		...
 	};

 	enum req_flag_bits {
 		...
 		__REQ_ATOMIC,		/* for atomic write operations */
+		__REQ_DMABUF,		/* Using premmaped dma buffers */
 		...
 	};
+	#define REQ_DMABUF	(__force blk_opf_t)(1ULL << __REQ_DMABUF)

+	static inline bool op_is_dmabuf(blk_opf_t op)
+	{
+		return op & REQ_DMABUF;
+	}

@@ block/bio.c
 	bool bio_iov_iter_set(struct bio *bio, const struct iov_iter *iter)
 	{
-		if (!iov_iter_is_bvec(iter))
+		if (!iov_iter_is_bvec(iter) && !iov_iter_is_dmabuf_map(iter))
 			return false;

 		WARN_ON_ONCE(bio->bi_max_vecs);

+		static_assert(offsetof(struct bio, bi_io_vec) ==
+			      offsetof(struct bio, bi_dmabuf_map));
+		static_assert(offsetof(struct iov_iter, bvec) ==
+			      offsetof(struct iov_iter, dmabuf_map));
+
 		bio->bi_io_vec = (struct bio_vec *)iter->bvec;
 		bio->bi_iter.bi_idx = 0;
 		bio->bi_iter.bi_offset = iter->iov_offset;
 		bio->bi_iter.bi_size = iov_iter_count(iter);
 		bio_set_flag(bio, BIO_CLONED);
+		if (iov_iter_is_dmabuf_map(iter))
+			bio->bi_opf |= REQ_NOMERGE | REQ_DMABUF;
 		return true;
 	}

@@ block/blk-merge.c
+static inline int bio_split_io_at_dmabuf(struct bio *bio,
+		const struct queue_limits *lim, unsigned *segs,
+		unsigned max_bytes, unsigned len_align_mask,
+		unsigned start_align_mask)
+{
+	unsigned bytes = min(bio->bi_iter.bi_size, max_bytes);
+	unsigned seg_shift = bio->bi_dmabuf_map->seg_shift;
+	unsigned offset = bio->bi_iter.bi_offset & ((1U << seg_shift) - 1);
+
+	if ((bio->bi_iter.bi_offset & start_align_mask) ||
+	    (bio->bi_iter.bi_size & len_align_mask))
+		return -EINVAL;
+
+	/* single contiguous range into the dma-buf */
+	*segs = 1;
+
+	bytes = min(bytes, ((unsigned)lim->max_segments << seg_shift) - offset);
+	if (bytes != bio->bi_iter.bi_size)
+		return bytes;
+	return 0;
+}
+
 	int bio_split_io_at(struct bio *bio, const struct queue_limits *lim, ...)
 	{
 		...
+		if (op_is_dmabuf(bio->bi_opf)) {
+			ret = bio_split_io_at_dmabuf(bio, lim, &nsegs, ...);
+			if (ret < 0)
+				return ret;
+			if (!ret)
+				goto out;
+			bytes = ret;
+			goto split;
+		}
 		bio_for_each_bvec(bv, bio, iter) { ... }