Re: [dpdk-dev] DPDK and HW offloads

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, Mar 22, 2016 at 05:50:28AM +0000, Qiu, Michael wrote:
> On 3/21/2016 11:27 PM, Kyle Larose wrote:
> > On Mon, Mar 21, 2016 at 10:52 AM, Bruce Richardson
> > <bruce.richardson@intel.com> wrote:
> >> On Sun, Mar 20, 2016 at 08:18:57PM +0100, Thomas Monjalon wrote:
> >>> 2016-03-20 14:17, Zhang, Helin:
> >>>> From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> >>>>> 2016-03-18 10:16, Stephen Hemminger:
> >>>>>> Right now, all those offload features are pretty much unusable in a
> >>>>>> real product without lots and lots of extra codes and huge bug
> >>>>>> surface. It bothers me enough that I would recommend removing much of the
> >>>>> filter/offload/ptype stuff from DPDK!
> >>>>>
> >>>>> One of the biggest challenge is to think about a good filtering API.
> >>>>> The offloading has some interaction with the mbuf struct.
> >>>>>
> >>>>> I would like to suggest rewriting ethdev API by keeping it as is for some time for
> >>>>> compatibility while creating a new one. What about the prefix dpdk_netdev_ to
> >>>>> progressively replace rte_eth_dev?
> >>>> I totally agree with to add new and generic APIs for user applications. But I don't
> >>>> think we need to remove all current APIs. Generic APIs may not support all advanced
> >>>> hardware features, while specific APIs can. Why not support all? One generic APIs for
> >>>> common users, and others APIs for advanced users.
> >>> Yes we cannot access to every features of a device through generic API.
> >>> Until now we were trying to add an ethdev API for every features even if it
> >>> is used by only one driver.
> >>> I think we should allow a direct access to the driver by the applications and
> >>> work on generic API only for common features.
> >> Definite +1.
> >> I think that we need to start pushing driver-specific functionality to get exposed
> >> via a driver's header files. That allow users who want to extract the max
> >> functionality from a particular NIC to do so via those APIs calls, while not
> >> polluting the generic ethdev layer.
> >>
> > What sort of requirements on ABI/API compatibility would this place on
> > the drivers? I would hope that it would be treated like any other
> > public API within DPDK. I don't think this would be too onerous, but
> > it would require that the drivers be designed to deal with it. (I.e.
> > don't just expose any old internal driver function).
> 
> Why not to implement one simple API with variable arguments, just like
> syscall ioctl() does. And drivers implement it's specific hardware
> features with a feature bit param, and other needed variable arguments.
> 
> Thanks,
> Michael

A very much dislike that idea. 
* It makes the code much harder to read as you have to closely examine all the
  parameters to work out what a function call is actually meant to do.
* It makes it much harder to see that you have an implicit dependency on a
  specific device. Having to include a driver specific header file e.g. i40e.h,
  and call a function named e.g. i40e_do_magic_stuff(), makes it pretty explicit
  that you have a dependency on i40e-based hardware
* It prevents the compiler from doing type-checking on parameters and informing
  you of little inconsistencies.

For all these reasons, I prefer the device-specific functions option. However,
at the same time, we also need to ensure we have a reasonable set of generic
APIs so that the cases where users are forced to drop down to the lower-level
device-specific primitives are reduced.

Regards,
/Bruce

> >> On the other hand, I don't like the idea of dpdk_netdev. I think we can work
> >> within the existing rte_eth_dev framework.
> >>
> >> /Bruce
> >>
> 
>