Re: [dpdk-dev] [PATCH v4 1/4] ethdev: add the API for getting burst mode information

[Thomas Monjalon <thomas@monjalon.net>]

26/10/2019 06:40, Wang, Haiyue:
> From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > 25/10/2019 18:02, Jerin Jacob:
> > > On Fri, Oct 25, 2019 at 9:15 PM Thomas Monjalon <thomas@monjalon.net> wrote:
> > > > 25/10/2019 16:08, Ferruh Yigit:
> > > > > On 10/25/2019 10:36 AM, Thomas Monjalon wrote:
> > > > > > 15/10/2019 09:51, Haiyue Wang:
> > > > > >> Some PMDs have more than one RX/TX burst paths, add the ethdev API
> > > > > >> that allows an application to retrieve the mode information about
> > > > > >> Rx/Tx packet burst such as Scalar or Vector, and Vector technology
> > > > > >> like AVX2.
> > > > > >
> > > > > > I missed this patch. I and Andrew, maintainers of ethdev, were not CC'ed.
> > > > > > Ferruh, I would expect to be Cc'ed and/or get a notification before merging.
> > > > >
> > > > > It has been discussed in the mail list and went through multiple discussions,
> > > > > patch is out since the August, +1 to cc all maintainers I missed that part,
> > > > > but when the patch is reviewed and there is no objection, why block the merge?
> > > >
> > > > I'm not saying blocking the merge.
> > > > My bad is that I missed the patch and I am asking for help with a notification
> > > > in this case. Same for Andrew I guess.
> > > > Note: it is merged in master and I am looking to improve this feature.
> > >
> > > > > >> +/**
> > > > > >> + * Ethernet device RX/TX queue packet burst mode information structure.
> > > > > >> + * Used to retrieve information about packet burst mode setting.
> > > > > >> + */
> > > > > >> +struct rte_eth_burst_mode {
> > > > > >> +  uint64_t options;
> > > > > >> +};
> > > > > >
> > > > > > Why a struct for an integer?
> > > > >
> > > > > Again by a request from me, to not need to break the API if we need to add more
> > > > > thing in the future.
> > > >
> > > > I would replace it with a string. This is the most flexible API.
> > >
> > > IMO, Probably, best of both worlds make a good option here,
> > > as Haiyue suggested if we have an additional dev_specific[1] in structure.
> > > and when a pass to the application, let common code make final string as
> > > (options flags to string + dev_specific)
> > >
> > > options flag can be zero if PMD does not have any generic flags nor
> > > interested in such a scheme.
> > > Generic flags will help at least to have some common code.
> > >
> > > [1]
> > > struct rte_eth_burst_mode {
> > >         uint64_t options;
> > >         char dev_specific[128]; /* PMD has specific burst mode information */
> > > };
> > 
> > I really don't see how we can have generic flags.
> > The flags which are proposed are just matching
> > the functions implemented in Intel PMDs.
> > And this is a complicate solution.
> > Why not just returning a name for the selected Rx/Tx mode?
> 
> Intel PMDs use the *generic* methods like x86 SSE, AVX2, ARM NEON, PPC ALTIVEC,
> 'dev->data->scattered_rx' etc for the target : "DPDK is the Data Plane Development Kit
> that consists of libraries to accelerate packet processing workloads running on a wide
> variety of CPU architectures."

How RTE_ETH_BURST_SCATTERED and RTE_ETH_BURST_BULK_ALLOC are generic?
They just match some features of the Intel PMDs.
Why not exposing other optimizations of the Rx/Tx implementations?
You totally missed the point of generic burst mode description.

> If understand these new experimental APIs from above, then bit options is the best,
> and we didn't invent new words to describe them, just from the CPU & other *generic*
> technology. And the application can loop to check which kind of burst is running by
> just simple bit test.
> 
> If PMDs missed these, they can update them in future roadmaps to enhance their PMDs,
> like MLX5 supports ARM NEON, x86 SSE.

I have no word!
You really think other PMDs should learn from Intel how to "enhance" their PMD?
You talk about mlx5, did you look at its code? Did you see the burst modes
depending on which specific hardware path is used (MPRQ, EMPW, inline)?
Or depending on which offloads are handled?

Again, the instruction set used by the function is a small part
of the burst mode optimization.

So you did not reply to my question:
Why not just returning a name for the selected Rx/Tx mode?