Re: [dpdk-dev] [PATCH v2 6/7] net/e1000: set min and max MTU for igb devices

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi Qi,

> >
> > On 3/22/2019 1:01 PM, Ian Stokes wrote:
> > > This commit sets the min and max supported MTU values for igb devices
> > > via the eth_igb_info_get() function. Min MTU supported is set to
> > > ETHER_MIN_MTU and max mtu is calculated as the max packet length
> > > supported minus the transport overhead. To aid in these calculations a
> > > new MACRO 'E1000_ETH_OVERHEAD' has been introduced to consolidate
> > > overhead calculation and avoid duplication.
> > >
> > > Signed-off-by: Ian Stokes <ian.stokes@intel.com>
> > > ---
> > >  drivers/net/e1000/e1000_ethdev.h | 6 ++++++
> > >  drivers/net/e1000/igb_ethdev.c   | 7 +++++--
> > >  2 files changed, 11 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/net/e1000/e1000_ethdev.h
> > > b/drivers/net/e1000/e1000_ethdev.h
> > > index 94edff08e..3e74cd8fe 100644
> > > --- a/drivers/net/e1000/e1000_ethdev.h
> > > +++ b/drivers/net/e1000/e1000_ethdev.h
> > > @@ -89,6 +89,12 @@
> > >  	ETH_RSS_IPV6_UDP_EX)
> > >
> > >  /*
> > > + * The overhead from MTU to max frame size.
> > > + * Considering VLAN so a tag needs to be counted.
> > > + */
> > > +#define E1000_ETH_OVERHEAD (ETHER_HDR_LEN + ETHER_CRC_LEN +
> > > +VLAN_TAG_SIZE)
> >
> > As an overhead, following drivers set:
> > i40e: HDR + CRC + 2 * VLAN
> > ixgbe: HDR + CRC
> > e1000: HDR + CRC + VLAN
> >
> > I wonder if this difference is HW limitation, or driver limitation or just
> > implementation inconsistency.
> 
> I think this is implementation inconsistency
> 
> The NIC only accept Max Frame Size.
> 
> The problem here is seems all of three setup are not perfect.
> 
> HDR + CRC + 2 * VLAN - it may allow non vlan or single vlan packet that exceed mtu.

Hmm, wonder how?

> HDR + CRC - it may reject vlan or double vlan packet that follow mtu.
> HDR + CRC + VLAN ,  it may reject double vlan packet that follow mtu
> 
> I agree it's better to keep consistent on all drivers, but before this, we may need to decide which one we should take :)
> 
> Regards
> Qi
> 
> >
> > Better to confirm it that it is not implementation inconsistency.
> >
> > Wenzhuo, Konstantin, Beilei, Qi,
> >
> > Can you please comment?
> >
> > Thanks,
> > ferruh