net/macb: add scatter-gather hw feature

The scatter-gather feature will allow to enable the Generic Segmentation Offload.
Generic Segmentation Offload can be enabled/disabled using ethtool -K DEVNAME gso on|off.

e.g:
ethtool -K eth0 gso off

When enabled, the driver may be provided with socket buffers splitted into many fragments.
These fragments need to be queued into the TX ring in reverse order, starting from to the
last one down to the first one, to avoid a race condition with the MAC.
Especially the 'TX_USED' bit in word 1 of the transmit buffer descriptor of the
first fragment should be cleared at the very final step of the queueing algorithm.
This will tell the hardware that fragments are ready to be sent.

Also since the MAC only update the status word of the first buffer descriptor of the
ethernet frame, the queueing algorithm can no longer expect a 'TX_USED' bit to be set by
the MAC into the buffer descriptor following the one for last fragment of the skb.
This is why the driver sets the 'TX_USED' bit before queueing any fragment, so the end of
queue position is well defined for the MAC.
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/cadence/macb.h

@@ -335,6 +335,7 @@
 #define MACB_CAPS_ISR_CLEAR_ON_WRITE		0x00000001
 #define MACB_CAPS_FIFO_MODE			0x10000000
 #define MACB_CAPS_GIGABIT_MODE_AVAILABLE	0x20000000
+#define MACB_CAPS_SG_DISABLED			0x40000000
 #define MACB_CAPS_MACB_IS_GEM			0x80000000
 
 /* Bit manipulation macros */
@@ -468,14 +469,23 @@ struct macb_dma_desc {
 #define MACB_TX_USED_OFFSET			31
 #define MACB_TX_USED_SIZE			1
 
+#define GEM_TX_FRMLEN_OFFSET			0
+#define GEM_TX_FRMLEN_SIZE			14
+
 /**
  * struct macb_tx_skb - data about an skb which is being transmitted
- * @skb: skb currently being transmitted
- * @mapping: DMA address of the skb's data buffer
+ * @skb: skb currently being transmitted, only set for the last buffer
+ *       of the frame
+ * @mapping: DMA address of the skb's fragment buffer
+ * @size: size of the DMA mapped buffer
+ * @mapped_as_page: true when buffer was mapped with skb_frag_dma_map(),
+ *                  false when buffer was mapped with dma_map_single()
  */
 struct macb_tx_skb {
 	struct sk_buff		*skb;
 	dma_addr_t		mapping;
+	size_t			size;
+	bool			mapped_as_page;
 };
 
 /*
@@ -617,6 +627,7 @@ struct macb {
 	struct sk_buff *skb;			/* holds skb until xmit interrupt completes */
 	dma_addr_t skb_physaddr;		/* phys addr from pci_map_single */
 	int skb_length;				/* saved skb length for pci_unmap_single */
+	unsigned int		max_tx_length;
 };
 
 extern const struct ethtool_ops macb_ethtool_ops;