netdev - Re: [PATCH 1/2] igb: Allow extra 4 bytes on RX for vlan tags.

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Date:	Wed, 20 Jul 2011 18:21:34 -0700
From:	Jeff Kirsher <jeffrey.t.kirsher@...el.com>
To:	Ben Greear <greearb@...delatech.com>
Cc:	Jesse Gross <jesse@...ira.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"Duyck, Alexander H" <alexander.h.duyck@...el.com>
Subject: Re: [PATCH 1/2] igb: Allow extra 4 bytes on RX for vlan tags.

On Wed, 2011-07-20 at 17:27 -0700, Ben Greear wrote:
> On 07/20/2011 05:18 PM, Jesse Gross wrote:
> > On Thu, Feb 17, 2011 at 9:28 AM, Ben Greear<greearb@...delatech.com>  wrote:
> >> On 02/17/2011 03:04 AM, Jeff Kirsher wrote:
> >>>
> >>> On Thu, Feb 10, 2011 at 13:59,<greearb@...delatech.com>    wrote:
> >>>>
> >>>> From: Ben Greear<greearb@...delatech.com>
> >>>>
> >>>> This allows the NIC to receive 1518 byte (not counting
> >>>> FCS) packets when MTU is 1500, thus allowing 1500 MTU
> >>>> VLAN frames to be received.  Please note that no VLANs
> >>>> were actually configured on the NIC...it was just acting
> >>>> as pass-through device.
> >>>>
> >>>> Signed-off-by: Ben Greear<greearb@...delatech.com>
> >>>> ---
> >>>> :100644 100644 58c665b... 30c9cc6... M  drivers/net/igb/igb_main.c
> >>>>   drivers/net/igb/igb_main.c |    5 +++--
> >>>>   1 files changed, 3 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/drivers/net/igb/igb_main.c b/drivers/net/igb/igb_main.c
> >>>> index 58c665b..30c9cc6 100644
> >>>> --- a/drivers/net/igb/igb_main.c
> >>>> +++ b/drivers/net/igb/igb_main.c
> >>>> @@ -2281,7 +2281,8 @@ static int __devinit igb_sw_init(struct igb_adapter
> >>>> *adapter)
> >>>>         adapter->rx_itr_setting = IGB_DEFAULT_ITR;
> >>>>         adapter->tx_itr_setting = IGB_DEFAULT_ITR;
> >>>>
> >>>> -       adapter->max_frame_size = netdev->mtu + ETH_HLEN + ETH_FCS_LEN;
> >>>> +       adapter->max_frame_size = (netdev->mtu + ETH_HLEN + ETH_FCS_LEN
> >>>> +                                  + VLAN_HLEN);
> >>>>         adapter->min_frame_size = ETH_ZLEN + ETH_FCS_LEN;
> >>>>
> >>>>         spin_lock_init(&adapter->stats64_lock);
> >>>> @@ -4303,7 +4304,7 @@ static int igb_change_mtu(struct net_device
> >>>> *netdev, int new_mtu)
> >>>>   {
> >>>>         struct igb_adapter *adapter = netdev_priv(netdev);
> >>>>         struct pci_dev *pdev = adapter->pdev;
> >>>> -       int max_frame = new_mtu + ETH_HLEN + ETH_FCS_LEN;
> >>>> +       int max_frame = new_mtu + ETH_HLEN + ETH_FCS_LEN + VLAN_HLEN;
> >>>>         u32 rx_buffer_len, i;
> >>>>
> >>>>         if ((new_mtu<    68) || (max_frame>    MAX_JUMBO_FRAME_SIZE)) {
> >>>
> >>> While testing this patch, validation found that the patch reduces the
> >>> maximum mtu size
> >>> by 4 bytes (reduces it from 9216 to 9212).  This is not a desired side
> >>> effect of this patch.
> >>
> >> You could add handling for that case and have it act as it used to when
> >> new_mtu is greater than 9212?
> >>
> >> I tested e1000e and it worked w/out hacking at 1500 MTU, so maybe
> >> check how it does it?
> >
> > I just wanted to bring this up again to see if any progress had been
> > made.  We were looking at this driver and trying to figure out the
> > best way to convert it to use the new vlan model but I'm not familiar
> 
> I've been watching :)
> 
> > enough with the hardware to know.  It seems that all of the other
> > Intel drivers unconditionally add space for the vlan tag to the
> > receive buffer (and would therefore have similar effects as this
> > patch), is there something different about this card?
> >
> > I believe that Alex was working on something in this area (in the
> > context of one of my patches from a long time ago) but I'm not sure
> > what came of that.
> 
> Truth is, I don't really see why it's a problem to decrease the
> maximum MTU slightly in order to make it work with VLANs.
> 
> I'm not sure if there is some way to make it work with VLANs
> and not decrease the maximum MTU.

This was the reason this did not get accepted.  I was looking into what
could be done so that we did not decease the maximum MTU, but I got
side-tracked and have not done anything on it in several months.

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