Re: [dpdk-dev] [PATCH v5 2/3] mbuf:add three TX ol_flags and repalce PKT_TX_VXLAN_CKSUM

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

> -----Original Message-----
> From: Olivier MATZ [mailto:olivier.matz@6wind.com]
> Sent: Thursday, December 04, 2014 1:51 PM
> To: Ananyev, Konstantin; Thomas Monjalon
> Cc: dev@dpdk.org; Liu, Jijiang
> Subject: Re: [dpdk-dev] [PATCH v5 2/3] mbuf:add three TX ol_flags and repalce PKT_TX_VXLAN_CKSUM
> 
> Hi,
> 
> On 12/04/2014 12:03 PM, Ananyev, Konstantin wrote:
> >>>>> 1/ (Jijiang's patch)
> >>>>> PKT_TX_IP_CKSUM  /* packet is IPv4, and we want hw cksum */
> >>>>> PKT_TX_IPV6      /* packet is IPv6 */
> >>>>> PKT_TX_IPV4      /* packet is IPv4, and we don't want hw cksum */
> >>>>>
> >>>>> with PKT_TX_IP_CKSUM and PKT_TX_IPV4 exclusive
> >>>>>
> >>>>> and
> >>>>>
> >>>>> 2/
> >>>>> PKT_TX_IP_CKSUM  /* we want hw IP cksum */
> >>>>> PKT_TX_IPV6      /* packet is IPv6 */
> >>>>> PKT_TX_IPV4      /* packet is IPv4 */
> >>>>>
> >>>>> with PKT_TX_IP_CKSUM implies PKT_TX_IPV4
> >>>>>
> >>>>>
> >>>>> Solution 2/ looks better from a user point of view. Anyone else has an opinion?
> >>>>
> >>>> Let's think about these IPv4/6 flags in terms of checksum and IP version/type,
> >>>>
> >>>> 1. For IPv6
> >>>> IP checksum is meaningful only for IPv4,  so we define 'PKT_TX_IPV6      /* packet is IPv6 */' to tell driver/HW that this is IPV6
> >> packet,
> >>>> here we don't talk about the checksum for IPv6 as it is meaningless. Right?
> >>>>
> >>>> PKT_TX_IPV6      /* packet is IPv6 */         ------ IP type: v6;  HW checksum: meaningless
> >>>>
> >>>> 2. For IPv4,
> >>>> My patch:
> >>>>
> >>>> PKT_TX_IP_CKSUM  /* packet is IPv4, and we want hw cksum */--------------------------IP type: v4;  HW checksum: Yes
> >>>> PKT_TX_IPV4      /* packet is IPv4, and we don't want hw cksum */ ----------------------- IP type: v4;  HW checksum: No
> >>>>
> >>>> You want:
> >>>> PKT_TX_IP_CKSUM  /* we want hw IP cksum */-------------------------- IP type: v4;  HW checksum: Yes
> >>>> PKT_TX_IPV4      /* packet is IPv4*/ ------------------------  IP type: v4; HW checksum: yes or no?
> >>>>                                                                                                         driver/HW don't know, just know this is packet with IPv4 header.
> >>>>                                                                                                         HW checksum: meaningless??
> >>>
> >>> Yep, that's why I also don't like that suggestion: PKT_TX_IPV4 itself doesn't contain all information.
> >>> PMD will have to check PKT_TX_IP_CKSUM  anyway.
> >>
> >> I prefer solution 2 because a flag should bring only 1 information.
> >
> > Why is that? For example in mbuf we already have a flag that brings 2 things:
> > PKT_TX_IP_CKSUM  /* packet is IPv4, and we want hw cksum */
> 
> For the user, it's clearer to have one information in a flag.
> If you just look at the name of the flag, the natural meaning is 2/,
> else we would need to rename them in:
>    PKT_TX_IPV4_CKSUM
>    PKT_TX_IPV4_NO_CKSUM
> 
> > If it would be possible to compress 10 meanings into 1 bit, I would happily do that.
> > Unfortunately, it is rarely possible :)
> >
> >> It's simply saner and could fit to more situations in the future.
> >
> > Could you give an example of such situation?
> > I personally couldn't come up with the case where #2 would have any real advantage.
> 
> in solution 2/, PKT_TX_IP_CKSUM implies PKT_TX_IPV4 so checking
> PKT_TX_IP_CKSUM is still enough in drivers.

Both 1 and 2 seems backward compatible.

> 
> In the driver, it is also simpler. With solution 1/:
> 
> /* check if we need ipcsum */
> if (flags & PKT_TX_IP_CKSUM)
> 
> /* check if packet is ipv4, may be needed to set a hw field */
> if (flags & (PKT_TX_IP_CKSUM|PKT_TX_IPV4))

Do you really mean 1 here? When all 3 flags are mutually exclusive?
If so, it doesn't look right. For 1 both (PKT_TX_IP_CKSUM|PKT_TX_IPV4) should never be up.  

> 
> 
> With solution 2/
> 
> /* check if we need ipcsum */
> if (flags & PKT_TX_IP_CKSUM)
> 
> /* check if packet is ipv4, may be needed to set a hw field */
> if (flags & PKT_TX_IPV4)

The thing is that it wouldn't be possible with FVL driver - it has to setup mutually exclusive fields for these 2 cases: 
PKT_TX_IP_CKSUM - ipv4 with HW checksum
PKT_TX_IPV4 - ipv4 without HW checksum

So with #2, driver has either:
if (flags & PKT_TX_IP_CKSUM) {...} else if (flags & PKT_TX_IPV4) {...}
And always keep condition for PKT_TX_IP_CKSUM first.
Or do:
if (flags & PKT_TX_IPV4) {...} if (flags & PKT_TX_IP_CKSUM) {...}
and in that case always keep condition for PKT_TX_IP_CKSUM last, so it always overwrite PKT_TX_IPV4 settings.

Basically with #2 PKT_TX_IPV4 is not enough to make a decision, even if it is set, we'll have to check for PKT_TX_IP_CKSUM anyway.

While with 1 we can put them in any order, both:
If (flags & PKT_TX_IP_CKSUM) {...} else if (flags & PKT_TX_IPV4) {...}
And
If (flags & PKT_TX_IPV4) {...} else if (flags & PKT_TX_IP_CKSUM) {...}
Will work.

Konstantin

> 
> 
> I agree it can looks like a detail, but I really think it's important
> to have the most logical and straightforward api for mbuf, as it's
> the core of DPDK.
> 
> Regards,
> Olivier