RE: [PATCH net-next 1/8] qed: LL2 to use packed information for tx

From: Mintz, Yuval

Sent: 09 June 2017 08:52

quoted

From: David Laight [mailto:David.Laight-ZS65k/vG3HxXrIkS9f7CXA@public.gmane.org]
Sent: Friday, June 09, 2017 10:28 AM
To: 'David Miller' <davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org>; Mintz, Yuval
[off-list ref]
Cc: netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org; linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org; Kalderon, Michal
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Subject: RE: [PATCH net-next 1/8] qed: LL2 to use packed information for tx

From: David Miller

quoted

Sent: 09 June 2017 00:24

From: Yuval Mintz <Yuval.Mintz-YGCgFSpz5w/QT0dZR+AlfA@public.gmane.org>
Date: Thu, 8 Jun 2017 19:13:16 +0300

quoted

@@ -67,6 +79,21 @@ struct qed_ll2_stats {
 	u64 sent_bcast_pkts;
 };

+struct qed_ll2_tx_pkt_info {
+	u8 num_of_bds;
+	u16 vlan;
+	u8 bd_flags;
+	u16 l4_hdr_offset_w;	/* from start of packet */
+	enum qed_ll2_tx_dest tx_dest;
+	enum qed_ll2_roce_flavor_type qed_roce_flavor;
+	dma_addr_t first_frag;
+	u16 first_frag_len;
+	bool enable_ip_cksum;
+	bool enable_l4_cksum;
+	bool calc_ip_len;
+	void *cookie;
+};
+

This layout is extremely inefficient, with lots of padding in between
struct members.

Group small u8 members and u16 members together so that they consume
full 32-bit areas so you can eliminate all of the padding.

I'd also query the use of u16 sizes/lengths, any arithmetic on u16 (and u8)
variables is likely to generate extra code (on non-x86).
You are using 32 bits for the 'enum' - I bet the values fit in 8 bits, so aren't
really worried about size.

If size did matter you can easily get the above down to 32 bytes.

You're right, and that's exactly the point -  since this is not data-path critical
I don't see why the size/efficiency should matter [greatly].

It is just good practise so that it happens automatically when it does matter.
Just swapping 'vlan' and 'bd_flags' would make it look much better.

	David

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