Re: [PATCH v4 3/4] net: xilinx: axienet: Derive RX frame length from residue in dmaengine path

["Pandey, Radhey Shyam" <radheys@amd.com>]

On 7/13/2026 12:51 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.
> 
> Signed-off-by: Srinivas Neeli <srinivas.neeli@amd.com>

Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
Thanks!
> ---
> 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 fcf517069d16..67d1b8e91d68 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;