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

[Zumeng Chen]

On 12/05/2017 12:06 AM, Claudiu Manoil wrote:
> > -----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;

En, seems more reasonable and clear, feel free to merge it if 
convenience, thanks :-)

> Do you have any performance numbers for this change?

No performance testing since just padding issue but with
the clear sky of /proc/cpu/alignment in arm side.

Cheers,
Zumeng

> Thanks,
> Claudiu