RE: [PATCH 1/1] gianfar: fix a flooded alignment reports because of padding issue.

[Claudiu Manoil]

>-----Original Message-----
>From: Zumeng Chen [mailto:zumeng.chen@xxxxxxxxx]
>Sent: Monday, December 04, 2017 5:22 AM
>To: netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
>Cc: Claudiu Manoil <claudiu.manoil@xxxxxxx>; davem@xxxxxxxxxxxxx
>Subject: [PATCH 1/1] gianfar: fix a flooded alignment reports because of padding
>issue.
>
>According to LS1021A RM, the value of PAL can be set so that the start of the
>IP header in the receive data buffer is aligned to a 32-bit boundary. Normally,
>setting PAL = 2 provides minimal padding to ensure such alignment of the IP
>header.
>
>However every incoming packet's 8-byte time stamp will be inserted into the
>packet data buffer as padding alignment bytes when hardware time stamping is
>enabled.
>
>So we set the padding 8+2 here to avoid the flooded alignment faults:
>
>root@128:~# cat /proc/cpu/alignment
>User:           0
>System:         17539 (inet_gro_receive+0x114/0x2c0)
>Skipped:        0
>Half:           0
>Word:           0
>DWord:          0
>Multi:          17539
>User faults:    2 (fixup)
>
[...]
>
> drivers/net/ethernet/freescale/gianfar.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
>diff --git a/drivers/net/ethernet/freescale/gianfar.c
>b/drivers/net/ethernet/freescale/gianfar.c
>index e616b71..e47945f 100644
>--- a/drivers/net/ethernet/freescale/gianfar.c
>+++ b/drivers/net/ethernet/freescale/gianfar.c
>@@ -1413,9 +1413,11 @@ static int gfar_probe(struct platform_device *ofdev)
>
> 	gfar_init_addr_hash_table(priv);
>
>-	/* Insert receive time stamps into padding alignment bytes */
>+	/* Insert receive time stamps into padding alignment bytes, and
>+	 * plus 2 bytes padding to ensure the cpu alignment.
>+	 */
> 	if (priv->device_flags & FSL_GIANFAR_DEV_HAS_TIMER)
>-		priv->padding = 8;
>+		priv->padding = 8 + DEFAULT_PADDING;
>
> 	if (dev->features & NETIF_F_IP_CSUM ||
> 	    priv->device_flags & FSL_GIANFAR_DEV_HAS_TIMER)
>--
>2.5.0

Why handle only the rx timestamp path (HAS_TIMER) when the issue, as presented 
by you, should be applicable to the default path as well?

The code change according to the patch description should be more likely:

+       priv->padding = DEFAULT_PADDING;
        /* Insert receive time stamps into padding alignment bytes */
        if (priv->device_flags & FSL_GIANFAR_DEV_HAS_TIMER)
-               priv->padding = 8;
+               priv->padding += 8;


Do you have any performance numbers for this change?

Thanks,
Claudiu