From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jakub Kicinski <jakub.kicinski@netronome.com>
Subject: Re: [PATCH 2/2] net: add driver for Netronome NFP4000/NFP6000 NIC
 VFs
Date: Tue, 27 Oct 2015 11:16:03 +0000
Message-ID: <20151027111603.77111ce4@jkicinski-Precision-T1700>
References: <1445626691-19819-1-git-send-email-jakub.kicinski@netronome.com>
	<1445626691-19819-3-git-send-email-jakub.kicinski@netronome.com>
	<20151026.182307.873190227567325622.davem@davemloft.net>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Cc: netdev@vger.kernel.org, rolf.neugebauer@netronome.com,
	jason.mcmullan@netronome.com, simon.horman@netronome.com
To: David Miller <davem@davemloft.net>
Return-path: <netdev-owner@vger.kernel.org>
Received: from mail-qg0-f49.google.com ([209.85.192.49]:32920 "EHLO
	mail-qg0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751170AbbJ0LQJ (ORCPT
	<rfc822;netdev@vger.kernel.org>); Tue, 27 Oct 2015 07:16:09 -0400
Received: by qgeo38 with SMTP id o38so143313261qge.0
        for <netdev@vger.kernel.org>; Tue, 27 Oct 2015 04:16:08 -0700 (PDT)
In-Reply-To: <20151026.182307.873190227567325622.davem@davemloft.net>
Sender: netdev-owner@vger.kernel.org
List-ID: <netdev.vger.kernel.org>

On Mon, 26 Oct 2015 18:23:07 -0700 (PDT), David Miller wrote:
> From: Jakub Kicinski <jakub.kicinski@netronome.com>
> Date: Fri, 23 Oct 2015 19:58:11 +0100
> 
> > +struct nfp_net_tx_buf {
> > +	struct sk_buff *skb;
> > +	dma_addr_t dma_addr;
> > +	short int fidx;
> > +	u16 pkt_cnt;
> > +	u32 real_len;
> > +};
> 
> This packs very poorly, and has a lot of padding holes.  Better ordering
> would be:
> 
> struct nfp_net_tx_buf {
> 	struct sk_buff *skb;
> 	dma_addr_t dma_addr;
> 	u32 real_len;
> 	short int fidx;
> 	u16 pkt_cnt;
> };

Seems to pack fine on x86:

struct nfp_net_tx_buf {
        struct sk_buff *           skb;                  /*     0     8 */
        dma_addr_t                 dma_addr;             /*     8     8 */
        short int                  fidx;                 /*    16     2 */
        u16                        pkt_cnt;              /*    18     2 */
        u32                        real_len;             /*    20     4 */

        /* size: 24, cachelines: 1, members: 5 */
        /* last cacheline: 24 bytes */
};

Are my packing skills deceiving me?  Maybe I'll try to build the driver
for more esoteric architectures.