staging: fsl-dpaa2/eth: Make Tx buffer alignment optional

Aligning the Tx buffers at 64B is a performance optimization
recommendation, not a hard requirement.

Make optional the alignment of Tx FD buffers, without enforcing
a reallocation in case there is not enough headroom for it.

On Rx, we keep allocating buffers with enough headroom to allow
Tx alignment of forwarded frames.

Signed-off-by: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/fsl-dpaa2/ethernet/dpaa2-eth.c

@@ -433,13 +433,19 @@ static int build_single_fd(struct dpaa2_eth_priv *priv,
 			   struct dpaa2_fd *fd)
 {
 	struct device *dev = priv->net_dev->dev.parent;
-	u8 *buffer_start;
+	u8 *buffer_start, *aligned_start;
 	struct sk_buff **skbh;
 	dma_addr_t addr;
 
-	buffer_start = PTR_ALIGN(skb->data -
-				 dpaa2_eth_needed_headroom(priv, skb),
-				 DPAA2_ETH_TX_BUF_ALIGN);
+	buffer_start = skb->data - dpaa2_eth_needed_headroom(priv, skb);
+
+	/* If there's enough room to align the FD address, do it.
+	 * It will help hardware optimize accesses.
+	 */
+	aligned_start = PTR_ALIGN(buffer_start - DPAA2_ETH_TX_BUF_ALIGN,
+				  DPAA2_ETH_TX_BUF_ALIGN);
+	if (aligned_start >= skb->head)
+		buffer_start = aligned_start;
 
 	/* Store a backpointer to the skb at the beginning of the buffer
 	 * (in the private data area) such that we can release it

drivers/staging/fsl-dpaa2/ethernet/dpaa2-eth.h

@@ -370,7 +370,7 @@ unsigned int dpaa2_eth_needed_headroom(struct dpaa2_eth_priv *priv,
 	if (skb_is_nonlinear(skb))
 		return 0;
 
-	return DPAA2_ETH_SWA_SIZE + DPAA2_ETH_TX_BUF_ALIGN;
+	return DPAA2_ETH_SWA_SIZE;
 }
 
 /* Extra headroom space requested to hardware, in order to make sure there's