netdev - Re: [PATCH] Net-ethtool : Allow ethtool to set interface in loopback mode.

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Date:	Tue, 30 Nov 2010 13:22:06 -0800
From:	Mahesh Bandewar <maheshb@...gle.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	David Miller <davem@...emloft.net>,
	linux-netdev <netdev@...r.kernel.org>,
	laurent chavey <chavey@...gle.com>
Subject: Re: [PATCH] Net-ethtool : Allow ethtool to set interface in loopback mode.

On Tue, Nov 30, 2010 at 11:15 AM, Ben Hutchings
<bhutchings@...arflare.com> wrote:
> On Tue, 2010-11-30 at 11:05 -0800, Mahesh Bandewar wrote:
>> On Tue, Nov 30, 2010 at 7:01 AM, Ben Hutchings
>> <bhutchings@...arflare.com> wrote:
> [...]
>> > > diff --git a/include/linux/ethtool.h b/include/linux/ethtool.h
>> > > index 6628a50..7523d45 100644
>> > > --- a/include/linux/ethtool.h
>> > > +++ b/include/linux/ethtool.h
>> > > @@ -678,6 +678,8 @@ struct ethtool_ops {
>> > >                                   struct ethtool_rxfh_indir *);
>> > >         int     (*set_rxfh_indir)(struct net_device *,
>> > >                                   const struct ethtool_rxfh_indir *);
>> > > +       int     (*get_loopback)(struct net_device *, u32 *);
>> > > +       int     (*set_loopback)(struct net_device *, u32);
>> > >  };
>> > >  #endif /* __KERNEL__ */
>> > >
>> > > @@ -741,6 +743,8 @@ struct ethtool_ops {
>> > >  #define ETHTOOL_GSSET_INFO     0x00000037 /* Get string set info */
>> > >  #define ETHTOOL_GRXFHINDIR     0x00000038 /* Get RX flow hash indir'n table */
>> > >  #define ETHTOOL_SRXFHINDIR     0x00000039 /* Set RX flow hash indir'n table */
>> > > +#define ETHTOOL_SLOOPBACK      0x0000003a /* Enable / Disable Loopback */
>> > [...]
>> >
>> > Where should loopback be done, when enabled?  As near as possible to the
>> > host, so it only covers the DMA engines, or as far away as possible, so
>> > it covers most of the MAC/PHY hardware?
>> >
>> Putting it very close to the HW would stress the maximum soft path and
>> would make it
>> beneficial to more people / developers. I understand that capabilities
>> offered by different
>> NIC vendors vary so it's little difficult to draw the line as to where
>> it  should be done.
>
> Of course, that's why I suggest 'as near/far as possible' rather than
> trying to specify the exact point at which loopback would be enabled.
>
>> So
>> if the theme to "maximize the soft path" is maintained, we can leave
>> it to the individual
>> driver(s) to maximize the value in offering.
>
> I think this should be specified, otherwise measurements on different
> types of NIC will not be comparable.  The ethtool API suffers greatly
> from losse specification and resulting inconsistency between drivers.
> So please add the comment that loopback should be enabled as near to the
> host as possible.
>

This could be seen more as a stress test tool than a performance tool. Also
I think having this feature available for developers is more important than
the place it is done. But I see your point of having more
inconsistencies, so I will
add the comment as guidance for implementation.

--mahesh..
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