[PATCH V3 3/4] net: xilinx: axienet: Derive RX frame length from DMA residue

[Pandey, Radhey Shyam]

rephrase to - derive RX frame length from residue in dmaengine path

> The dmaengine RX path determined the received frame length by reading APP
> word 4 of the DMA descriptor metadata, masking the lower 16 bits of
> app_metadata[LEN_APP].

Avoid above explanation and make commit description concise.>
> This relies on the optional AXI4-Stream status/control interface being
> present in the design. The descriptor APP fields are only populated by the
> hardware when that interface is enabled. On designs without it the APP
> fields are not updated, so 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
> (dmaengine_result.residue). The received frame length is therefore the
> posted buffer length minus the 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.
> 
> Use result->residue to compute the RX frame length and drop the descriptor
> metadata lookup, which was only used for this purpose. The error path now
> uses the standard dmaengine_result.result status instead of the metadata
> pointer return value, and the now-unused LEN_APP macro is removed.

now unused>
> The transmit path is unaffected. It still passes APP metadata for checksum
> offload and derives its length from the skb.
> 

Switching to dmaengine residue is better alternative but consider it as 
an enhancement. Drop the fixes tag. The non-dmaengine axienet RX path 
still derives frame length from APP field and it's a design assumption.

> Fixes: 6a91b846af85 ("net: axienet: Introduce dmaengine support")
> Signed-off-by: Srinivas Neeli <srinivas.neeli at amd.com>
> ---
> 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;