netdev - Re: [PATCH 11/11] net: xilinx: Show csum in bootlog

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Date:	Tue, 9 Oct 2012 17:56:40 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	<monstr@...str.eu>
CC:	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Anirudha Sarangi <anirudh@...inx.com>,
	John Linn <John.Linn@...inx.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>,
	"David S. Miller" <davem@...emloft.net>,
	John Linn <linnj@...inx.com>
Subject: Re: [PATCH 11/11] net: xilinx: Show csum in bootlog

On Tue, 2012-10-09 at 11:08 +0200, Michal Simek wrote:
> On 10/05/2012 03:51 PM, Ben Hutchings wrote:
> > On Fri, 2012-10-05 at 11:35 +0200, Michal Simek wrote:
> >> On 10/04/2012 09:15 PM, Ben Hutchings wrote:
> >>> On Thu, 2012-10-04 at 20:14 +0200, Michal Simek wrote:
> >>>> Just show current setting in bootlog.
> >>> [...]
> >>>> --- a/drivers/net/ethernet/xilinx/ll_temac_main.c
> >>>> +++ b/drivers/net/ethernet/xilinx/ll_temac_main.c
> >>>> @@ -1052,12 +1052,14 @@ static int __devinit temac_of_probe(struct platform_device *op)
> >>>>    	/* Setup checksum offload, but default to off if not specified */
> >>>>    	lp->temac_features = 0;
> >>>>    	p = (__be32 *)of_get_property(op->dev.of_node, "xlnx,txcsum", NULL);
> >>>> +	dev_info(&op->dev, "TX_CSUM %d\n", be32_to_cpup(p));
> >>>>    	if (p && be32_to_cpu(*p)) {
> >>>>    		lp->temac_features |= TEMAC_FEATURE_TX_CSUM;
> >>>>    		/* Can checksum TCP/UDP over IPv4. */
> >>>>    		ndev->features |= NETIF_F_IP_CSUM;
> >>>>    	}
> >>>>    	p = (__be32 *)of_get_property(op->dev.of_node, "xlnx,rxcsum", NULL);
> >>>> +	dev_info(&op->dev, "RX_CSUM %d\n", be32_to_cpup(p));
> >>> [...]
> >>>
> >>> Is there any particular reason you think this needs to be logged by
> >>> default, rather than letting users run ethtool -k?  I suggest using
> >>> dev_dbg() instead.
> >>
> >> Ok. I have looked at it and there are missing some bits in ndev->features.
> >>
> >> Can you please check that my setting is correct?
> >>
> >> It is SG DMA ip/driver.
> >> ndev->features = NETIF_F_FRAGLIST | NETIF_F_SG
> >
> > NETIF_F_SG only; NETIF_F_FRAGLIST means you can handle skbs chained
> > through the frag_list pointer.
> 
> The driver is able to handle skb fragments too. temac_start_xmit

"git grep -w -E 'frag_list|skb_walk_frags|skb_to_sgvec' drivers/net/ethernet/xilinx"
returns nothing in net-next.

[...]
> >> rx Full csum -> NETIF_F_RXCSUM
> >>
> >> Is there any option to support partial csum?
> >
> > There is no need to differentiate these in the device features.  For TX
> > the stack needs to know whether to use a software fallback before
> > passing the skb to you, but on RX it looks at the ip_summed field of
> > each skb you pass up.
> 
> Hardware can be setup asymmetrically. It means enable CSUM only on RX or TX.
> All combination are valid.
> The point here is if Linux is not able to handle this then we have to
> create logic in the driver to support these options too.

Linux handles this just fine.  The point is you don't have to tell the
stack in advance whether or what kind of RX checksum validation your
devices will do.  (In fact the only reason that feature flag exists at
all is so that it can be generically exposed through the ethtool API.)

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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