Add helpers to initialize, iterate, and clean up a tso_dma_map:

tso_dma_map_init(): DMA-maps the linear payload region and all frags
upfront into the tso_dma_map struct. Returns 0 on success, cleans up
partial mappings on failure.

tso_dma_map_cleanup(): unmaps all DMA regions. Used on error paths.

tso_dma_map_count(): counts how many descriptors the next N bytes of
payload will need, without advancing the iterator.

tso_dma_map_next(): yields the next (dma_addr, chunk_len) pair.
Indicates when a chunk starts a new DMA mapping so the driver can set
dma_unmap_len on that BD for completion-time unmapping.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Joe Damato <joe@dama.to>
---
 include/net/tso.h |   8 +++
 net/core/tso.c    | 165 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 173 insertions(+)

diff --git a/include/net/tso.h b/include/net/tso.h
index cd4b98dbea71..a1fa605f26b4 100644
--- a/include/net/tso.h
+++ b/include/net/tso.h
@@ -62,4 +62,12 @@ struct tso_dma_map {
 	} frags[MAX_SKB_FRAGS];
 };
 
+int tso_dma_map_init(struct tso_dma_map *map, struct device *dev,
+		     const struct sk_buff *skb, unsigned int hdr_len);
+void tso_dma_map_cleanup(struct tso_dma_map *map);
+unsigned int tso_dma_map_count(const struct tso_dma_map *map, unsigned int len);
+bool tso_dma_map_next(struct tso_dma_map *map, dma_addr_t *addr,
+		      unsigned int *chunk_len, unsigned int *mapping_len,
+		      unsigned int seg_remaining);
+
 #endif	/* _TSO_H */
diff --git a/net/core/tso.c b/net/core/tso.c
index 6df997b9076e..fdbef4ca840d 100644
--- a/net/core/tso.c
+++ b/net/core/tso.c
@@ -3,6 +3,7 @@
 #include <linux/if_vlan.h>
 #include <net/ip.h>
 #include <net/tso.h>
+#include <linux/dma-mapping.h>
 #include <linux/unaligned.h>
 
 void tso_build_hdr(const struct sk_buff *skb, char *hdr, struct tso_t *tso,
@@ -87,3 +88,167 @@ int tso_start(struct sk_buff *skb, struct tso_t *tso)
 	return hdr_len;
 }
 EXPORT_SYMBOL(tso_start);
+
+/**
+ * tso_dma_map_init - DMA-map GSO payload regions
+ * @map: map struct to initialize
+ * @dev: device for DMA mapping
+ * @skb: the GSO skb
+ * @hdr_len: per-segment header length in bytes
+ *
+ * DMA-maps the linear payload (after headers) and all frags.
+ * Positions the iterator at byte 0 of the payload.
+ *
+ * Returns 0 on success, -ENOMEM on DMA mapping failure (partial mappings
+ * are cleaned up internally).
+ */
+int tso_dma_map_init(struct tso_dma_map *map, struct device *dev,
+		     const struct sk_buff *skb, unsigned int hdr_len)
+{
+	unsigned int linear_len = skb_headlen(skb) - hdr_len;
+	unsigned int nr_frags = skb_shinfo(skb)->nr_frags;
+	int i;
+
+	map->dev = dev;
+	map->skb = skb;
+	map->hdr_len = hdr_len;
+	map->frag_idx = -1;
+	map->offset = 0;
+	map->linear_len = 0;
+	map->nr_frags = 0;
+
+	if (linear_len > 0) {
+		map->linear_dma = dma_map_single(dev, skb->data + hdr_len,
+						 linear_len, DMA_TO_DEVICE);
+		if (dma_mapping_error(dev, map->linear_dma))
+			return -ENOMEM;
+		map->linear_len = linear_len;
+	}
+
+	for (i = 0; i < nr_frags; i++) {
+		skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
+
+		map->frags[i].len = skb_frag_size(frag);
+		map->frags[i].dma = skb_frag_dma_map(dev, frag, 0,
+						     map->frags[i].len,
+						     DMA_TO_DEVICE);
+		if (dma_mapping_error(dev, map->frags[i].dma)) {
+			tso_dma_map_cleanup(map);
+			return -ENOMEM;
+		}
+		map->nr_frags = i + 1;
+	}
+
+	if (linear_len == 0 && nr_frags > 0)
+		map->frag_idx = 0;
+
+	return 0;
+}
+EXPORT_SYMBOL(tso_dma_map_init);
+
+/**
+ * tso_dma_map_cleanup - unmap all DMA regions in a tso_dma_map
+ * @map: the map to clean up
+ *
+ * Unmaps linear payload and all mapped frags. Used on error paths.
+ * Success paths use the driver's completion path to handle unmapping.
+ */
+void tso_dma_map_cleanup(struct tso_dma_map *map)
+{
+	int i;
+
+	if (map->linear_len)
+		dma_unmap_single(map->dev, map->linear_dma, map->linear_len,
+				 DMA_TO_DEVICE);
+
+	for (i = 0; i < map->nr_frags; i++)
+		dma_unmap_page(map->dev, map->frags[i].dma, map->frags[i].len,
+			       DMA_TO_DEVICE);
+
+	map->linear_len = 0;
+	map->nr_frags = 0;
+}
+EXPORT_SYMBOL(tso_dma_map_cleanup);
+
+/**
+ * tso_dma_map_count - count descriptors for a payload range
+ * @map: the payload map
+ * @len: number of payload bytes in this segment
+ *
+ * Counts how many contiguous DMA region chunks the next @len bytes
+ * will span, without advancing the iterator. Uses region sizes from
+ * the current position.
+ *
+ * Returns the number of descriptors needed for @len bytes of payload.
+ */
+unsigned int tso_dma_map_count(const struct tso_dma_map *map, unsigned int len)
+{
+	unsigned int offset = map->offset;
+	int idx = map->frag_idx;
+	unsigned int count = 0;
+
+	while (len > 0) {
+		unsigned int region_len, chunk;
+
+		if (idx == -1)
+			region_len = map->linear_len;
+		else
+			region_len = map->frags[idx].len;
+
+		chunk = min(len, region_len - offset);
+		len -= chunk;
+		count++;
+		offset = 0;
+		idx++;
+	}
+
+	return count;
+}
+EXPORT_SYMBOL(tso_dma_map_count);
+
+/**
+ * tso_dma_map_next - yield the next DMA address range
+ * @map: the payload map
+ * @addr: output DMA address
+ * @chunk_len: output chunk length
+ * @mapping_len: full DMA mapping length when this chunk starts a new
+ *               mapping region, or 0 when continuing a previous one.
+ *               Driver can assign this to the last descriptor.
+ * @seg_remaining: bytes left in current segment
+ *
+ * Yields the next (dma_addr, chunk_len) pair and advances the iterator.
+ *
+ * Returns true if a chunk was yielded, false when @seg_remaining is 0.
+ */
+bool tso_dma_map_next(struct tso_dma_map *map, dma_addr_t *addr,
+		      unsigned int *chunk_len, unsigned int *mapping_len,
+		      unsigned int seg_remaining)
+{
+	unsigned int region_len, chunk;
+
+	if (!seg_remaining)
+		return false;
+
+	if (map->frag_idx == -1) {
+		region_len = map->linear_len;
+		chunk = min(seg_remaining, region_len - map->offset);
+		*addr = map->linear_dma + map->offset;
+		*mapping_len = (map->offset == 0) ? region_len : 0;
+	} else {
+		region_len = map->frags[map->frag_idx].len;
+		chunk = min(seg_remaining, region_len - map->offset);
+		*addr = map->frags[map->frag_idx].dma + map->offset;
+		*mapping_len = (map->offset == 0) ? region_len : 0;
+	}
+
+	*chunk_len = chunk;
+	map->offset += chunk;
+
+	if (map->offset >= region_len) {
+		map->frag_idx++;
+		map->offset = 0;
+	}
+
+	return true;
+}
+EXPORT_SYMBOL(tso_dma_map_next);
-- 
2.52.0