Re: [PATCH net-next v1 1/5] net: lan743x: reset DMA channels that fail to stop

[Nicolai Buchwitz <nb@tipi-net.de>]

On 15.10.2026 17:00, Chris Babroski wrote:
> lan743x_tx_close() and lan743x_rx_close() ask the DMA controller to 
> stop
> the channel and wait for it, but ignore the result. If the channel is
> still stop-pending when the wait gives up, the descriptor ring, the 
> head
> write-back buffer and the packet buffers are unmapped and freed while
> the channel may still be fetching from or writing to them.
> 
> A TX channel has been seen to stop making progress, with its head index
> no longer advancing while DMAC_CMD still reports it as started. A
> channel in that state may not honour the stop request either, and
> closing the interface would then free memory the engine can still
> access.
> 
> If the channel is still stop-pending after the wait, or still reads as
> started, issue a channel soft reset and wait for it before anything is
> freed, and log when that happens.
> 
> Use the same helpers for the channel resets that lan743x_tx_open() and
> lan743x_rx_open() already open-code, and log there too if the reset
> times out.
> 
> Fixes: 23f0703c125b ("lan743x: Add main source files for new lan743x 
> driver")
> Assisted-by: LLM
> Signed-off-by: Chris Babroski <cbabroski@nvidia.com>
> ---
>  drivers/net/ethernet/microchip/lan743x_main.c | 64 +++++++++++++++----
>  1 file changed, 52 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/net/ethernet/microchip/lan743x_main.c 
> b/drivers/net/ethernet/microchip/lan743x_main.c
> index 82d3ec20ba25..9e1276cf313c 100644
> --- a/drivers/net/ethernet/microchip/lan743x_main.c
> +++ b/drivers/net/ethernet/microchip/lan743x_main.c

> [...]

> @@ -2221,11 +2239,22 @@ static int lan743x_tx_ring_init(struct 
> lan743x_tx *tx)
>  static void lan743x_tx_close(struct lan743x_tx *tx)
>  {
>  	struct lan743x_adapter *adapter = tx->adapter;
> +	int ret;
> 
>  	lan743x_csr_write(adapter,
>  			  DMAC_CMD,
>  			  DMAC_CMD_STOP_T_(tx->channel_number));
> -	lan743x_dmac_tx_wait_till_stopped(adapter, tx->channel_number);
> +	ret = lan743x_dmac_tx_wait_till_stopped(adapter, tx->channel_number);
> +	if (ret < 0 || ret == DMAC_CHANNEL_STATE_STARTED) {
> +		netdev_warn(adapter->netdev,
> +			    "TX channel %d failed to stop, resetting it\n",
> +			    tx->channel_number);
> +		ret = lan743x_dmac_tx_reset(adapter, tx->channel_number);
> +		if (ret)
> +			netdev_err(adapter->netdev,
> +				   "TX channel %d reset timed out\n",
> +				   tx->channel_number);
> +	}

While looking something up in the registers for the other thread I
stumbled over this in AN2948 about TX_SWR_x:

"This is an immediate reset and will cause any, in process, PCIe request
to [be] aborted. This may likely result in a corrupt PCIe packet."

A channel that did not stop is the one case where a request is still in
flight, so this resets right when the datasheet says not to. Is there a
sanctioned way to recover a channel that ignores STOP_T?

Woojung, maybe you can say?

> [...]

Thanks,
Nicolai