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

[Srinivas Neeli <srinivas.neeli@amd.com>]

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].

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.

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

Fixes: 6a91b846af85 ("net: axienet: Introduce dmaengine support")
Signed-off-by: Srinivas Neeli <srinivas.neeli@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;
-- 
2.25.1