Re: [PATCH v6 1/9] librte_mbuf:the rte_mbuf structure changes

[Thomas Monjalon <thomas.monjalon-pdR9zngts4EAvxtiuMwx3w@public.gmane.org>]

Hi guys,

We still have some problems with the mbuf changes introduced for VXLAN.
I want to raise the packet type issue here.

2014-10-23 02:23, Zhang, Helin:
> From: dev [mailto:dev-bounces-VfR2kkLFssw@public.gmane.org] On Behalf Of Thomas Monjalon
> > 2014-10-21 14:14, Liu, Jijiang:
> > > From: Thomas Monjalon [mailto:thomas.monjalon-pdR9zngts4EAvxtiuMwx3w@public.gmane.org]
> > > > 2014-10-21 16:46, Jijiang Liu:
> > > > > -	uint16_t reserved2;       /**< Unused field. Required for padding */
> > > > > +
> > > > > +	/**
> > > > > +	 * Packet type, which is used to indicate ordinary L2 packet format and
> > > > > +	 * also tunneled packet format such as IP in IP, IP in GRE, MAC in GRE
> > > > > +	 * and MAC in UDP.
> > > > > +	 */
> > > > > +	uint16_t packet_type;
> > > >
> > > > Why not name it "l2_type"?
> 
> 'packet_type' is for storing the hardware identified packet type upon different layers
> of protocols (l2, l3, l4, ...).
> It is quite useful for user application or middle layer software stacks, it can know
> what the packet type is without checking the packet too much by software.
> Actually ixgbe already has packet types (less than 10), which is transcoded into 'ol_flags'.
> For i40e, the packet type can represent about 256 types of packet, 'ol_flags' does not
> have enough bits for it anymore. So put the i40e packet types into mbuf would be better.
> Also this field can be used for NON-Intel NICs, I think there must be the similar concepts
> of other NICs. And 16 bits 'packet_type' has severl reserved bits for future and NON-Intel NICs.

Thanks Helin, that's the best description of packet_type I've seen so far.
It's not so clear in the commit log:
	http://dpdk.org/browse/dpdk/commit/?id=73b7d59cf4f6faf

> > > In datasheet, this term is called packet type(s).
> > 
> > That's exactly the point I want you really understand!
> > This is a field in generic mbuf structure, so your datasheet has no value here.
> > 
> > > Personally , I think packet type is  more clear what meaning of this field is .
> > 
> > You cannot add an API field without knowing what will be its generic meaning.
> > Please think about it and describe its scope.

I integrated this patch with the VXLAN patchset in the hope that you'll
improve the situation afterwards.
This is the answer you recently gave to Olivier:
	http://dpdk.org/ml/archives/dev/2014-November/007599.html
"
	Regarding adding a packet_type in mbuf, we ever had a lot of discussions as follows:
	http://dpdk.org/ml/archives/dev/2014-October/007027.html
	http://dpdk.org/ml/archives/dev/2014-September/005240.html
	http://dpdk.org/ml/archives/dev/2014-September/005241.html
	http://dpdk.org/ml/archives/dev/2014-September/005274.html
"

To sum up the situation:
- We don't know what are the possible values of packet_type
- It's only filled by i40e, while other drivers use ol_flags
- There is no special value "unknown" which should be set by drivers
  not supporting this feature.
- Its only usage is to print a decimal value in app/test-pmd/rxonly.c

It's now clear that nobody cares about this part of the API.
So I'm going to remove packet_type from mbuf.
I don't want to keep something that we don't know how to use, that is
not consistent across drivers, and that overlap another API part (ol_flags).

-- 
Thomas