Re: [dpdk-dev] [PATCH] pmd: Add generic support for TCP TSO (Transmit Segmentation Offload)

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2014-10-20 12:45, Walukiewicz, Miroslaw:
> > >  	/* fields to support TX offloads */
> > > -	union {
> > > -		uint16_t l2_l3_len; /**< combined l2/l3 lengths as single var
> > */
> > > -		struct {
> > > -			uint16_t l3_len:9;      /**< L3 (IP) Header Length. */
> > > -			uint16_t l2_len:7;      /**< L2 (MAC) Header Length.
> > */
> > > +	/* two bytes - l2/l3 len for compatibility (endian issues)
> > > +	 * two bytes - reseved for alignment
> > > +	 * two bytes - l4 len (TCP/UDP) header len
> > > +	 * two bytes - TCP tso segment size
> > > + 	 */
> > > +	struct {
> > > +		union {
> > > +			uint16_t l2_l3_len; /**< combined l2/l3 len */
> > > +			struct {
> > > +				uint16_t l3_len:9; /**< L3 (IP) Header */
> > > +				uint16_t l2_len:7; /**< L2 (MAC) Header */
> > > +			};
> > >  		};
> > 
> > Why nesting these fields in an anonymous structure?
> 
> I want to keep a source compatibility with non-TSO applications using that
> field for example IP checksum computing by NIC. 
> Keeping this structure anonymous I do not require changes in old
> applications that do not need TSO support.
> 
> The second argument is that in original patch extending the rte_mbuf to 128
> bytes made by Bruce the author made this structure anonymous and I follow
> this assumption too.

Excuse me, maybe I missed something, but I still don't understand why you are
embedding the union into a struct?

-- 
Thomas