Re: [PATCH] net: spacemit: clear TX descriptor on fragment mapping failure

["Troy Mitchell" <troy.mitchell@linux.spacemit.com>]

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On Sun Sep 20, 2026 at 3:19 AM +08, Muhammad Bilal wrote:
> 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: bfec6d7f2001 ("net: spacemit: Add K1 Ethernet MAC")
> Signed-off-by: Muhammad Bilal <meatuni001@gmail.com>
Reviewed-by: Troy Mitchell <troy.mitchell@linux.spacemit.com>


-- 
Troy Mitchell


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