net: tso: Introduce tso_dma_map and helpers

Add struct tso_dma_map to tso.h for tracking DMA addresses of mapped
GSO payload data and tso_dma_map_completion_state.

The tso_dma_map combines DMA mapping storage with iterator state, allowing
drivers to walk pre-mapped DMA regions linearly. Includes fields for
the DMA IOVA path (iova_state, iova_offset, total_len) and a fallback
per-region path (linear_dma, frags[], frag_idx, offset).

The tso_dma_map_completion_state makes the IOVA completion state opaque
for drivers. Drivers are expected to allocate this and use the added
helpers to update the completion state.

Adds skb_frag_phys() to skbuff.h, returning the physical address
of a paged fragment's data, which is used by the tso_dma_map helpers
introduced in this commit described below.

The added TSO DMA map helpers are:

tso_dma_map_init(): DMA-maps the linear payload region and all frags
upfront. Prefers the DMA IOVA API for a single contiguous mapping with
one IOTLB sync; falls back to per-region dma_map_phys() otherwise.
Returns 0 on success, cleans up partial mappings on failure.

tso_dma_map_cleanup(): Handles both IOVA and fallback teardown paths.

tso_dma_map_count(): counts how many descriptors the next N bytes of
payload will need. Returns 1 if IOVA is used since the mapping is
contiguous.

tso_dma_map_next(): yields the next (dma_addr, chunk_len) pair.
On the IOVA path, each segment is a single contiguous chunk. On the
fallback path, indicates when a chunk starts a new DMA mapping so the
driver can set dma_unmap_len on that descriptor for completion-time
unmapping.

tso_dma_map_completion_save(): updates the completion state. Drivers
will call this at xmit time.

tso_dma_map_complete(): tears down the mapping at completion time and
returns true if the IOVA path was used. If it was not used, this is a
no-op and returns false.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Joe Damato <joe@dama.to>
Link: https://patch.msgid.link/20260408230607.2019402-2-joe@dama.to
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 26fe18bcfad8..2bcf78a4de7b 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3764,6 +3764,17 @@ static inline void *skb_frag_address_safe(const skb_frag_t *frag)
 }
 
 /**
+ * skb_frag_phys - gets the physical address of the data in a paged fragment
+ * @frag: the paged fragment buffer
+ *
+ * Returns: the physical address of the data within @frag.
+ */
+static inline phys_addr_t skb_frag_phys(const skb_frag_t *frag)
+{
+	return page_to_phys(skb_frag_page(frag)) + skb_frag_off(frag);
+}
+
+/**
  * skb_frag_page_copy() - sets the page in a fragment from another fragment
  * @fragto: skb fragment where page is set
  * @fragfrom: skb fragment page is copied from