net: spacemit: clear TX descriptor on fragment mapping failure

emac_tx_mem_map() writes TX_DESC_0_OWN into the ring descriptor for
every slot beyond old_head as soon as that slot's memset()'d local
copy is committed with "*tx_desc_addr = tx_desc", i.e. before the
buffers for that slot have necessarily all been mapped successfully.
If emac_tx_map_frag() then fails on a later fragment, the err_free_skb
path calls emac_free_tx_buf() to unmap and drop the skb, but leaves
the already-written descriptor memory untouched, and tx_ring->head is
never advanced past old_head (the "tx_ring->head = head" store is
skipped by the goto).

So a slot between old_head and the rolled-back head can be left with
TX_DESC_0_OWN set and buffer_addr_{1,2} pointing at DMA mappings that
emac_free_tx_buf() just tore down, while software considers that slot
free again. The next successful emac_tx_mem_map() call only rebuilds
old_head itself; if the DMA engine auto-advances into the following
descriptor once it finishes old_head's packet, it will fetch that
stale, already-unmapped address.

emac_tx_clean_desc() already treats emac_free_tx_buf() and clearing
the descriptor as a pair when reclaiming completed descriptors; do
the same in the mapping failure path.

Fixes: bfec6d7f20 ("net: spacemit: Add K1 Ethernet MAC")
Signed-off-by: Muhammad Bilal <meatuni001@gmail.com>
Reviewed-by: Vivian Wang <wangruikang@iscas.ac.cn>
Reviewed-by: Troy Mitchell <troy.mitchell@linux.spacemit.com>
Link: https://patch.msgid.link/20260919191937.271202-1-meatuni001@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/ethernet/spacemit/k1_emac.c

@@ -803,6 +803,9 @@ static void emac_tx_mem_map(struct emac_priv *priv, struct sk_buff *skb)
 	while (i != head) {
 		emac_free_tx_buf(priv, i);
 
+		tx_desc_addr = &((struct emac_desc *)tx_ring->desc_addr)[i];
+		memset(tx_desc_addr, 0, sizeof(*tx_desc_addr));
+
 		if (++i == tx_ring->total_cnt)
 			i = 0;
 	}