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

[Vivian Wang]

On 9/20/26 03:19, 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@xxxxxxxxx>
> ---
>  drivers/net/ethernet/spacemit/k1_emac.c | 3 +++
>  1 file changed, 3 insertions(+)

Reviewed-by: Vivian Wang <wangruikang@xxxxxxxxxxx>

Much appreciated. I indeed did not consider this.

Also, a minor nit, but it should probably be:

Fixes: 86292155bea5 ("net: spacemit: Fix error handling in emac_tx_mem_map()")

Thanks,
Vivian "dramforeve" Wang