[PATCH V6 3/4] net: xilinx: axienet: Derive RX frame length from residue in dmaengine path

[Neeli, Srinivas]

Hi Jakub,

I would like to follow up on this patch. The dependent DMA patches (1/4, 
2/4, and 4/4) from this series have already been applied through the 
DMAEngine framework.
Could you please advise if there are any remaining concerns with this 
patch or how you would prefer it to be handled on the netdev side?
Also, I noticed that you were inadvertently included in the CC list 
rather than the To list in the original posting. Apologies for that.

Thanks
Srinivas neeli


On 8/18/2026 11:11 PM, Srinivas Neeli wrote:
> The dmaengine RX path derived the received frame length from the descriptor
> APP metadata. That only works when the optional AXI4-Stream status/control
> interface is present, because the hardware populates the APP fields solely
> when that interface is enabled. On designs without it the length read back
> is invalid.
>
> The AXI DMA engine already reports how many bytes it wrote into the buffer
> through the standard dmaengine residue mechanism. Compute the RX frame
> length as the posted buffer length minus result->residue, which is
> independent of the status/control interface and correct across all designs,
> including multi-descriptor frames where the residue is summed over the
> chain.
>
> Drop the descriptor metadata lookup, which was only used for this purpose.
> Detect a failed transfer from dmaengine_result.result instead of the
> metadata pointer return value, and remove the now unused LEN_APP macro.
>
> The transmit path is unaffected. It still passes APP metadata for checksum
> offload and derives its length from the skb.
>
> Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey at amd.com>
> Signed-off-by: Srinivas Neeli <srinivas.neeli at amd.com>
> ---
> Changes in V6:
>   - No change.
>
> Changes in V5:
>   - No change.
>
> Changes in V4:
>   - Renamed subject to "Derive RX frame length from residue in dmaengine
>     path".
>   - Condensed the commit message.
>   - Dropped the Fixes tag.
>
> Changes in V3:
>   - New patch in this series.
>   - This patch enables axienet to work on designs where the AXI4-Stream
>     status/control interface is not present. By using the standard
>     dmaengine residue mechanism, the driver no longer depends on APP
>     fields being populated by hardware.
>   - This approach replaces the V2 xferred_bytes mechanism (V2 patch 5/5),
>     making the dt-bindings patch (V2 patch 4/5) for xlnx,include-stscntrl-strm
>     also unnecessary. Both V2 patches are dropped in this series.
> ---
>   drivers/net/ethernet/xilinx/xilinx_axienet_main.c | 14 +++++---------
>   1 file changed, 5 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> index 1722b7038f34..f9252880f411 100644
> --- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> +++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> @@ -53,7 +53,6 @@
>   #define TX_BD_NUM_MAX			4096
>   #define RX_BD_NUM_MAX			4096
>   #define DMA_NUM_APP_WORDS		5
> -#define LEN_APP				4
>   #define RX_BUF_NUM_DEFAULT		128
>   
>   /* Must be shorter than length of ethtool_drvinfo.driver field to fit */
> @@ -1159,29 +1158,26 @@ axienet_start_xmit(struct sk_buff *skb, struct net_device *ndev)
>   static void axienet_dma_rx_cb(void *data, const struct dmaengine_result *result)
>   {
>   	struct skbuf_dma_descriptor *skbuf_dma;
> -	size_t meta_len, meta_max_len, rx_len;
>   	struct axienet_local *lp = data;
>   	struct sk_buff *skb;
> -	u32 *app_metadata;
> +	size_t rx_len;
>   	int i;
>   
>   	skbuf_dma = axienet_get_rx_desc(lp, lp->rx_ring_tail++);
>   	skb = skbuf_dma->skb;
> -	app_metadata = dmaengine_desc_get_metadata_ptr(skbuf_dma->desc, &meta_len,
> -						       &meta_max_len);
>   	dma_unmap_single(lp->dev, skbuf_dma->dma_address, lp->max_frm_size,
>   			 DMA_FROM_DEVICE);
>   
> -	if (IS_ERR(app_metadata)) {
> +	if (result->result != DMA_TRANS_NOERROR) {
>   		if (net_ratelimit())
> -			netdev_err(lp->ndev, "Failed to get RX metadata pointer\n");
> +			netdev_err(lp->ndev, "RX DMA transfer failed\n");
>   		dev_kfree_skb_any(skb);
>   		lp->ndev->stats.rx_dropped++;
>   		goto rx_submit;
>   	}
>   
> -	/* TODO: Derive app word index programmatically */
> -	rx_len = (app_metadata[LEN_APP] & 0xFFFF);
> +	/* Actual length = posted buffer length - residue. */
> +	rx_len = lp->max_frm_size - result->residue;
>   	skb_put(skb, rx_len);
>   	skb->protocol = eth_type_trans(skb, lp->ndev);
>   	skb->ip_summed = CHECKSUM_NONE;