linux-kernel - Re: [PATCH 1/2] 8139too: Allow setting MTU larger than 1500

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Message-ID: <20141121195738.038d2cc2@tock>
Date:	Fri, 21 Nov 2014 19:57:38 +0100
From:	Alban <albeu@...e.fr>
To:	Ben Hutchings <ben@...adent.org.uk>
Cc:	Aban Bedel <albeu@...e.fr>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
	Benoit Taine <benoit.taine@...6.fr>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 1/2] 8139too: Allow setting MTU larger than 1500

On Fri, 21 Nov 2014 18:51:51 +0000
Ben Hutchings <ben@...adent.org.uk> wrote:

> On Fri, 2014-11-21 at 14:58 +0100, Alban wrote:
> > On Fri, 21 Nov 2014 00:34:34 +0000
> > Ben Hutchings <ben@...adent.org.uk> wrote:
> > 
> > > On Sat, 2014-11-08 at 12:48 +0100, Alban Bedel wrote:
> > > > Replace the default ndo_change_mtu callback with one that allow
> > > > setting MTU that the driver can handle.
> > > > 
> > > > Signed-off-by: Alban Bedel <albeu@...e.fr>
> > > > ---
> > > >  drivers/net/ethernet/realtek/8139too.c | 13 ++++++++++++-
> > > >  1 file changed, 12 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/net/ethernet/realtek/8139too.c
> > > > b/drivers/net/ethernet/realtek/8139too.c index 007b38c..8387de9
> > > > 100644 --- a/drivers/net/ethernet/realtek/8139too.c
> > > > +++ b/drivers/net/ethernet/realtek/8139too.c
> > > > @@ -185,6 +185,9 @@ static int debug = -1;
> > > >  /* max supported ethernet frame size -- must be at least
> > > > (dev->mtu+14+4).*/ #define MAX_ETH_FRAME_SIZE	1536
> > > >  
> > > > +/* max supported payload size */
> > > > +#define MAX_ETH_DATA_SIZE	(MAX_ETH_FRAME_SIZE -
> > > > ETH_HLEN - ETH_FCS_LEN)
> > > [...]
> > > 
> > > Does this maximum still allow for VLAN tags, or should it use
> > > VLAN_ETH_HLEN instead of ETH_HLEN?
> > 
> > That might well be as the VLAN code seems to assume that the
> > physical device can handle frames of MTU + VLAN_HLEN bytes. I can
> > fix it, but to me it seems like the VLAN code should be fixed to
> > respect the physical device MTU.
> 
> Drivers that support VLANs have to allow for at least one VLAN tag
> when validating the MTU.  This is obviously broken for multiple
> layers of VLAN tags, but those are the semantics we're stuck with.

Ok, I see. Should a I send a fix patch or redo a new version of this
patch?

Alban


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