netdev - [RFC PATCH net-next 01/11] net: enetc: optimize struct enetc_rx

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Date:   Mon,  6 Feb 2023 12:08:27 +0200
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Björn Töpel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [RFC PATCH net-next 01/11] net: enetc: optimize struct enetc_rx_swbd layout

Eliminate a 4 byte hole on arm64, to be able to introduce a new member
to this structure in a future patch without increasing the overall
structure size.

Before:

struct enetc_rx_swbd {
        struct page *              page;                 /*     0     8 */
        enum dma_data_direction    dir;                  /*     8     4 */

        /* XXX 4 bytes hole, try to pack */

        dma_addr_t                 dma;                  /*    16     8 */
        u16                        page_offset;          /*    24     2 */
        u16                        len;                  /*    26     2 */

        /* size: 32, cachelines: 1, members: 5 */
        /* sum members: 24, holes: 1, sum holes: 4 */
        /* padding: 4 */
        /* last cacheline: 32 bytes */
};

After:

struct enetc_rx_swbd {
        struct page *              page;                 /*     0     8 */
        dma_addr_t                 dma;                  /*     8     8 */
        enum dma_data_direction    dir;                  /*    16     4 */
        u16                        page_offset;          /*    20     2 */
        u16                        len;                  /*    22     2 */

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

Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
---
 drivers/net/ethernet/freescale/enetc/enetc.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/freescale/enetc/enetc.h b/drivers/net/ethernet/freescale/enetc/enetc.h
index e21d096c5a90..704aa1f9dfa3 100644
--- a/drivers/net/ethernet/freescale/enetc/enetc.h
+++ b/drivers/net/ethernet/freescale/enetc/enetc.h
@@ -48,10 +48,10 @@ struct enetc_tx_swbd {
 	(SKB_WITH_OVERHEAD(ENETC_RXB_TRUESIZE) - XDP_PACKET_HEADROOM)
 
 struct enetc_rx_swbd {
-	dma_addr_t dma;
 	struct page *page;
-	u16 page_offset;
+	dma_addr_t dma;
 	enum dma_data_direction dir;
+	u16 page_offset;
 	u16 len;
 };
 
-- 
2.34.1