From: Morten Brørup <mb@smartsharesystems.com> To: "Ferruh Yigit" <ferruh.yigit@intel.com>, "Bruce Richardson" <bruce.richardson@intel.com> Cc: <thomas@monjalon.net>, <arybchenko@solarflare.com>, <jia.guo@intel.com>, <dev@dpdk.org> Subject: Re: [dpdk-dev] [PATCH] ethdev: rte_eth_rx_burst()nb_pktsrequirements Date: Wed, 14 Oct 2020 10:26:33 +0200 Message-ID: <98CBD80474FA8B44BF855DF32C47DC35C6136A@smartserver.smartshare.dk> (raw) In-Reply-To: <60d87dbf-167e-d0a6-781a-5f54ee5cb5ac@intel.com> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ferruh Yigit > Sent: Tuesday, October 13, 2020 12:56 PM > > On 9/14/2020 1:42 PM, Morten Brørup wrote: > >> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Bruce Richardson > >> Sent: Monday, September 14, 2020 1:27 PM > >> > >> On Mon, Sep 14, 2020 at 01:05:11PM +0200, Morten Brørup wrote: > >>> Updated description of rte_eth_rx_burst() to reflect what drivers, > >>> when using vector instructions, expect from nb_pkts. > >>> > >>> Also discussed on the mailing list here: > >>> > >> http://inbox.dpdk.org/dev/98CBD80474FA8B44BF855DF32C47DC35C61257@smarts > >> erver.smartshare.dk/ > >>> > >>> Signed-off-by: Morten Brørup <mb@smartsharesystems.com> > >>> --- > >>> lib/librte_ethdev/rte_ethdev.h | 5 +++++ > >>> 1 file changed, 5 insertions(+) > >>> > >>> diff --git a/lib/librte_ethdev/rte_ethdev.h > >> b/lib/librte_ethdev/rte_ethdev.h > >>> index 70295d7ab..41f8ba4ef 100644 > >>> --- a/lib/librte_ethdev/rte_ethdev.h > >>> +++ b/lib/librte_ethdev/rte_ethdev.h > >>> @@ -4469,6 +4469,10 @@ int > >> rte_eth_dev_hairpin_capability_get(uint16_t port_id, > >>> * burst-oriented optimizations in both synchronous and asynchronous > >>> * packet processing environments with no overhead in both cases. > >>> * > >>> + * @note > >>> + * Some drivers using vector instructions require that *nb_pkts* > >> is > >>> + * divisible by 4 or 8, depending on the driver implementation. > >>> + * > >> > >> Not technically true, in that the drivers will round the value down to > >> the > >> nearest multiple of 4 or 8. So how about rewording as: > >> > >> "Some drivers using vector instructions may round the *nb_pkts* driver > >> to > >> a multiple of 4 or 8 depending upon the driver implementation." > >> > > > > You are correct about the driver behavior. > > > > However, if you pass nb_pkts=9, the driver will return 8 packets, > > and thus it does not conform to the API behavior of returning nb_pkts > > if they are there. > > > > This is why the description in this patch differs from the description we > reached in the RFC discussion. > > > > Hi Morten, Bruce, > > +1 to document the this behavior. > > But in the patch the wording is more strict: > "... require that *nb_pkts* is divisible by 4 or 8 ..." > "... The value must be divisible by 8 in order to work with any driver." > > I am not sure the requirement is that strict. Application still provide any > value for 'nb_pkts', so the value doesn't "have to" be divisible 8/4. > > But for vector PMD case it will return number of packets round down to 8/4. > Perhaps can add for vector PMD it must be at least 4/8? > > Bruce's explanation sound more accurate to me, what do you think? > I aim to keep the explanation in the documentation relatively simple. Keep the parameter description short, and add the details about vector driver behavior as a note to the function. The reason for all this is the existing documentation describing how to use the rte_eth_rx_burst() function at high level: The rte_eth_rx_burst() function returns the number of packets actually retrieved [...]. A return value equal to nb_pkts indicates [...] that other received packets remain in the input queue. Applications implementing a "retrieve as much received packets as possible" policy can check this specific case and keep invoking the rte_eth_rx_burst() function until a value less than nb_pkts is returned. As an alternative to my proposed solution, we could add that vector drivers round down to 4 or 8, and the application's comparison of the nb_pkts and return value must consider this. But that would make the above description strangely complex, rather than just requiring that nb_pkts for vector drivers must be divisible by 4 or 8. And as a minor detail, keeping my proposed restriction would also eliminate the vector drivers' need to round down. I don't see a need to be able to call rte_eth_rx_burst() with a value not divisible by 4 or 8 for a vector driver, so my proposed restriction is a tradeoff favoring simplicity over unnecessary flexibility. > > >>> * The rte_eth_rx_burst() function does not provide any error > >>> * notification to avoid the corresponding overhead. As a hint, the > >>> * upper-level application might check the status of the device link > >> once > >>> @@ -4485,6 +4489,7 @@ int rte_eth_dev_hairpin_capability_get(uint16_t > >> port_id, > >>> * must be large enough to store *nb_pkts* pointers in it. > >>> * @param nb_pkts > >>> * The maximum number of packets to retrieve. > >>> + * The value must be divisible by 8 in order to work with any > >> driver. > >> > >> Similarly here, I think it's better to state that it should be at least > >> 8, > >> and any values not divisible by 8 may be rounded down. > >> > >>> * @return > >>> * The number of packets actually retrieved, which is the number > >>> * of pointers to *rte_mbuf* structures effectively supplied to > >> the > >>> -- > >>> 2.17.1 > >>> > > >
next prev parent reply other threads:[~2020-10-14 8:27 UTC|newest] Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-09-14 11:05 [dpdk-dev] [PATCH] ethdev: rte_eth_rx_burst() nb_pkts requirements Morten Brørup 2020-09-14 11:26 ` Bruce Richardson 2020-09-14 12:42 ` [dpdk-dev] [PATCH] ethdev: rte_eth_rx_burst() nb_pktsrequirements Morten Brørup 2020-10-13 10:55 ` Ferruh Yigit 2020-10-14 8:26 ` Morten Brørup [this message] 2020-10-14 8:53 ` [dpdk-dev] [PATCH] ethdev: rte_eth_rx_burst()nb_pktsrequirements Thomas Monjalon 2020-10-14 9:29 ` Bruce Richardson 2020-10-14 11:14 ` Morten Brørup 2020-10-14 11:38 ` Bruce Richardson 2020-10-15 7:36 ` Morten Brørup 2020-10-26 15:06 ` [dpdk-dev] [PATCH] ethdev: rte_eth_rx_burst() nb_pkts requirements Morten Brørup 2020-10-26 15:15 ` Ajit Khaparde 2020-10-27 10:24 ` Bruce Richardson 2020-10-30 9:27 ` Ferruh Yigit
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