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

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Date:   Fri, 9 Jun 2017 07:28:07 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'David Miller' <davem@...emloft.net>,
        "Yuval.Mintz@...ium.com" <Yuval.Mintz@...ium.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "Michal.Kalderon@...ium.com" <Michal.Kalderon@...ium.com>
Subject: RE: [PATCH net-next 1/8] qed: LL2 to use packed information for tx

From: David Miller
> Sent: 09 June 2017 00:24
> 
> From: Yuval Mintz <Yuval.Mintz@...ium.com>
> Date: Thu, 8 Jun 2017 19:13:16 +0300
> 
> > @@ -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.

	David