netdev - Re: [PATCH v2 2/2] r8169: Avoid pointer aliasing

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Date:   Mon, 4 Feb 2019 19:44:41 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Andrew Lunn <andrew@...n.ch>
Cc:     Realtek linux nic maintainers <nic_swsd@...ltek.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] r8169: Avoid pointer aliasing

On 04.02.2019 17:42, Thierry Reding wrote:
> 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.
> 
>  drivers/net/ethernet/realtek/r8169.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/ethernet/realtek/r8169.c b/drivers/net/ethernet/realtek/r8169.c
> index 501891be7c56..192fbb36bc9f 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 +7325,7 @@ 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) = {};
> +	u8 mac_addr[ETH_ALEN] = {};
>  	struct rtl8169_private *tp;
>  	struct net_device *dev;
>  	int chipset, region, i;
> 
I just have one concern / question:

After this there's a call to is_valid_ether_addr(mac_addr) and kernel-doc of
is_valid_ether_addr() states that argument must be u16-aligned.
AFAIK that's not guaranteed for a byte array.

Heiner