netdev - Re: [PATCH v5 5/9] net: Define enum for net device features.

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Date:	Thu, 17 Nov 2011 00:00:24 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Michał Mirosław <mirq-linux@...e.qmqm.pl>
CC:	<netdev@...r.kernel.org>, "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v5 5/9] net: Define enum for net device features.

On Thu, 2011-11-17 at 00:34 +0100, Michał Mirosław wrote:
> On Wed, Nov 16, 2011 at 10:39:37PM +0000, Ben Hutchings wrote:
> > On Wed, 2011-11-16 at 02:29 +0100, Michał Mirosław wrote:
> > > Define feature values by bit position instead of direct 2**i values
> > > and force the values to be of type netdev_features_t.
> > > 
> > > Cleaned and extended from patch by Mahesh Bandewar <maheshb@...gle.com>:
> > > + added netdev_features_t casts
> > > + included bits under NETIF_F_GSO_MASK
> > > + moved feature #defines out of struct net_device definition
> > > 
> > > Signed-off-by: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> > > ---
> > >  include/linux/netdev_features.h |  129 +++++++++++++++++++++++++++-----------
> > >  1 files changed, 91 insertions(+), 38 deletions(-)
> > > 
> > > diff --git a/include/linux/netdev_features.h b/include/linux/netdev_features.h
> > > index af52381..04ac8f8 100644
> > > --- a/include/linux/netdev_features.h
> > > +++ b/include/linux/netdev_features.h
> > [...]
> > > +	/**/NETIF_F_GSO_SHIFT,		/* keep the order of SKB_GSO_* bits */
> > > +	NETIF_F_TSO_BIT			/* ... TCPv4 segmentation */
> > > +		= NETIF_F_GSO_SHIFT,
> > > +	NETIF_F_UFO_BIT,		/* ... UDPv4 fragmentation */
> > > +	NETIF_F_GSO_ROBUST_BIT,		/* ... ->SKB_GSO_DODGY */
> > > +	NETIF_F_TSO_ECN_BIT,		/* ... TCP ECN support */
> > > +	NETIF_F_TSO6_BIT,		/* ... TCPv6 segmentation */
> > > +	NETIF_F_FSO_BIT,		/* ... FCoE segmentation */
> > > +	NETIF_F_GSO_RESERVED1,		/* ... free (fill GSO_MASK to 8 bits) */
> > > +	/**/NETIF_F_GSO_LAST,		/* [can't be last bit, see GSO_MASK] */
> > > +	NETIF_F_GSO_RESERVED2		/* ... free (fill GSO_MASK to 8 bits) */
> > > +		= NETIF_F_GSO_LAST,
> > >  
> > > -/* Segmentation offload features */
> > > -#define NETIF_F_GSO_SHIFT	16
> > > -#define NETIF_F_GSO_MASK	0x00ff0000
> > > -#define NETIF_F_TSO		(SKB_GSO_TCPV4 << NETIF_F_GSO_SHIFT)
> > > -#define NETIF_F_UFO		(SKB_GSO_UDP << NETIF_F_GSO_SHIFT)
> > > -#define NETIF_F_GSO_ROBUST	(SKB_GSO_DODGY << NETIF_F_GSO_SHIFT)
> > > -#define NETIF_F_TSO_ECN		(SKB_GSO_TCP_ECN << NETIF_F_GSO_SHIFT)
> > > -#define NETIF_F_TSO6		(SKB_GSO_TCPV6 << NETIF_F_GSO_SHIFT)
> > > -#define NETIF_F_FSO		(SKB_GSO_FCOE << NETIF_F_GSO_SHIFT)
> > [...]
> > 
> > You should either add BUILD_BUG_ON()s somewhere to ensure that the
> > netdev feature and skb GSO flags remain in sync, or redefine the skb GSO
> > flags using the netdev feature flags (which I thought was the reason for
> > moving features to their own header).
> 
> Main motiviation for the moving out of features to separate headers was
> to avoid circular inclusion of linux/netdevice.h from linux/skbuff.h.
> We get readability for free.
> 
> BUILD_BUG_ON() is a good idea. It should go to skbuff.h and as separate
> patch as it's something new and independent.

It may be functionally independent, but this change is unsafe without
the addition of such assertions.

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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