netdev - [PATCH v3 2/2] r8169: Avoid pointer aliasing

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Date:   Wed,  6 Feb 2019 13:30:18 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     "David S. Miller" <davem@...emloft.net>
Cc:     Heiner Kallweit <hkallweit1@...il.com>,
        Andrew Lunn <andrew@...n.ch>, Joe Perches <joe@...ches.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Paul Zimmerman <pauldzim@...il.com>,
        Michal Kubecek <mkubecek@...e.cz>,
        Realtek linux nic maintainers <nic_swsd@...ltek.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v3 2/2] r8169: Avoid pointer aliasing

From: Thierry Reding <treding@...dia.com>

Read MAC address 32-bit at a time and manually extract the individual
bytes. This avoids pointer aliasing and gives the compiler a better
chance of optimizing the operation.

Suggested-by: Andrew Lunn <andrew@...n.ch>
Signed-off-by: Thierry Reding <treding@...dia.com>
---
Applies to net-next.

I tested this on a Jetson TX2 with an add-in Realtek ethernet card that
has a properly programmed OTP to verify that I got the endianess right.
Seems like everything works and the device behaves the same with or
without this patch.

Changes in v3:
- align MAC address to u16 for is_valid_ether_addr()

 drivers/net/ethernet/realtek/r8169.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/realtek/r8169.c b/drivers/net/ethernet/realtek/r8169.c
index 501891be7c56..1dd72137fd53 100644
--- a/drivers/net/ethernet/realtek/r8169.c
+++ b/drivers/net/ethernet/realtek/r8169.c
@@ -7113,12 +7113,21 @@ static int rtl_alloc_irq(struct rtl8169_private *tp)
 static void rtl_read_mac_address(struct rtl8169_private *tp,
 				 u8 mac_addr[ETH_ALEN])
 {
+	u32 value;
+
 	/* Get MAC address */
 	switch (tp->mac_version) {
 	case RTL_GIGA_MAC_VER_35 ... RTL_GIGA_MAC_VER_38:
 	case RTL_GIGA_MAC_VER_40 ... RTL_GIGA_MAC_VER_51:
-		*(u32 *)&mac_addr[0] = rtl_eri_read(tp, 0xe0, ERIAR_EXGMAC);
-		*(u16 *)&mac_addr[4] = rtl_eri_read(tp, 0xe4, ERIAR_EXGMAC);
+		value = rtl_eri_read(tp, 0xe0, ERIAR_EXGMAC);
+		mac_addr[0] = (value >>  0) & 0xff;
+		mac_addr[1] = (value >>  8) & 0xff;
+		mac_addr[2] = (value >> 16) & 0xff;
+		mac_addr[3] = (value >> 24) & 0xff;
+
+		value = rtl_eri_read(tp, 0xe4, ERIAR_EXGMAC);
+		mac_addr[4] = (value >>  0) & 0xff;
+		mac_addr[5] = (value >>  8) & 0xff;
 		break;
 	default:
 		break;
@@ -7316,7 +7325,8 @@ static int rtl_get_ether_clk(struct rtl8169_private *tp)
 static int rtl_init_one(struct pci_dev *pdev, const struct pci_device_id *ent)
 {
 	const struct rtl_cfg_info *cfg = rtl_cfg_infos + ent->driver_data;
-	u8 mac_addr[ETH_ALEN] __aligned(4) = {};
+	/* align to u16 for is_valid_ether_addr() */
+	u8 mac_addr[ETH_ALEN] __aligned(2) = {};
 	struct rtl8169_private *tp;
 	struct net_device *dev;
 	int chipset, region, i;
-- 
2.19.1