Re: [PATCH v2 16/16] net: Add support for networking over Thunderbolt cable

On Tue, Sep 26, 2017 at 09:47:21PM -0700, David Miller wrote:

From: Mika Westerberg <mika.westerberg@linux.intel.com>
Date: Mon, 25 Sep 2017 14:07:38 +0300

quoted

+struct thunderbolt_ip_header {
+	u32 route_hi;
+	u32 route_lo;
+	u32 length_sn;
+	uuid_t uuid;
+	uuid_t initiator_uuid;
+	uuid_t target_uuid;
+	u32 type;
+	u32 command_id;
+} __packed;

Again, the __packed attribute should not be necessary and needs to be
removed.

OK, will do.

quoted

+static void tbnet_pull_tail(struct sk_buff *skb)
+{
+	skb_frag_t *frag = &skb_shinfo(skb)->frags[0];
+	unsigned int pull_len;
+	void *hdr;
+
+	hdr = skb_frag_address(frag);
+	pull_len = eth_get_headlen(hdr, TBNET_RX_HDR_SIZE);
+
+	/* Align pull length to size of long to optimize memcpy performance */
+	skb_copy_to_linear_data(skb, hdr, ALIGN(pull_len, sizeof(long)));

You do not need to copy here, instead you can build SKB's where the
skb->data points directly at the head of your first frag page memory.

See build_skb().

quoted

+		skb = net->skb;
+		if (!skb) {
+			skb = netdev_alloc_skb_ip_align(net->dev,
+							TBNET_RX_HDR_SIZE);
+			net->skb = skb;
+		}
+		if (!skb)
+			break;
+
+		/* Single small buffer we can copy directly to the
+		 * header part of the skb.
+		 */
+		if (hdr->frame_count == 1 && frame_size <= TBNET_RX_HDR_SIZE) {

Here you would use build_skb() instead of netdev_alloc_skb*() for the first
frag, and keep the existing code tacking on subsequent frags using
skb_add_Rx_frag().

I'm reading kernel-doc of build_skb() (or rather __build_skb()) and it
says caller needs to reserve head room of NET_SKB_PAD and then make sure
there is space for SKB_DATA_ALIGN(skb_shared_info).

Now, in case of ThunderboltIP frames, they look like this:

  +---------+
  | hdr     | 12 bytes
  +---------+
  | data    | 4096 - 12 = 4084 bytes
  |         |
  |         |
  +---------+

A packet can consist of multiple frames where each have the 12-byte
header and 4084 bytes of TSO/LRO payload except the last one which can
be smaller than 4084.

Using build_skb() then would require to allocate larger buffer, that
includes NET_SKB_PAD + SKB_DATA_ALIGN(skb_shared_info) and that exceeds
page size. Is this something supported by build_skb()? It was not clear
to me based on the code and other users of build_skb() but I may be
missing something.

quoted

+	ret = register_netdev(dev);
+	if (ret) {
+		free_netdev(dev);
+		return ret;
+	}
+
+	net->handler.uuid = &tbnet_svc_uuid;
+	net->handler.callback = tbnet_handle_packet,
+	net->handler.data = net;
+	tb_register_protocol_handler(&net->handler);
+
+	tb_service_set_drvdata(svc, net);

There could be races here.

At the exact moment you call register_netdev(), your device can be
brought UP, packets transmitted, etc.  You entire set of driver code
paths can be executed.

The rest of those initializations after register_netdev() probably
are needed by the rest of the driver to function properly, so may
need to happen before register_netdev() publishes the device to the
entire world.

You're right. I'll change the ordering so that register_netdev() happens
last.

Thanks!