Re: [dpdk-dev] [PATCH v2] ethdev: fix device info getting

["Lu, Wenzhuo" <wenzhuo.lu@intel.com>]

Hi Ferruh, Andrew,


> -----Original Message-----
> From: Thomas Monjalon [mailto:thomas@monjalon.net]
> Sent: Tuesday, October 23, 2018 3:29 PM
> To: Lu, Wenzhuo <wenzhuo.lu@intel.com>
> Cc: dev@dpdk.org; Yigit, Ferruh <ferruh.yigit@intel.com>; Andrew
> Rybchenko <arybchenko@solarflare.com>
> Subject: Re: [dpdk-dev] [PATCH v2] ethdev: fix device info getting
> 
> 23/10/2018 03:25, Lu, Wenzhuo:
> > Hi Thomas, Ferruh, Andrew,
> >
> > From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > > 22/10/2018 14:01, Ferruh Yigit:
> > > > On 8/23/2018 9:58 AM, Andrew Rybchenko wrote:
> > > > > On 22.08.2018 19:55, Ferruh Yigit wrote:
> > > > >> On 8/14/2018 1:57 AM, Lu, Wenzhuo wrote:
> > > > >>> Hi Andrew,
> > > > >>> From: Andrew Rybchenko [mailto:arybchenko@solarflare.com]
> > > > >>>> On 13.08.2018 05:50, Lu, Wenzhuo wrote:
> > > > >>>>> From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > > > >>>>>> 16/07/2018 03:58, Lu, Wenzhuo:
> > > > >>>>>>> Hi Andrew,
> > > > >>>>>>> From: Lu, Wenzhuo
> > > > >>>>>>>> From: Andrew Rybchenko
> [mailto:arybchenko@solarflare.com]
> > > > >>>>>>>>>
> > > > >>>>>>>>> Hi, Wenzhuo,
> > > > >>>>>>>>>
> > > > >>>>>>>>> I'm sorry, but I have more even harder questions than
> > > > >>>>>>>>> the previous
> > > > >>>> one.
> > > > >>>>>>>>> This questions are rather generic and mainly to ethdev
> > > maintainers.
> > > > >>>>>>>>>
> > > > >>>>>>>>> On 13.07.2018 05:42, Wenzhuo Lu wrote:
> > > > >>>>>>>>>> The device information cannot be gotten correctly
> > > > >>>>>>>>>> before the configuration is set. Because on some NICs
> > > > >>>>>>>>>> the information has dependence on the configuration.
> > > > >>>>>>>>> Thinking about it I have the following question. Is it
> > > > >>>>>>>>> valid behaviour of the dev_info if it changes after
> configuration?
> > > > >>>>>>>>> I always thought that the primary goal of the dev_info
> > > > >>>>>>>>> is to provide information to app about device
> > > > >>>>>>>>> capabilities to allow app configure device and queues
> > > > >>>>>>>>> correctly. Now we see the case when dev_info changes on
> > > > >>>>>>>>> configure. May be it is acceptable, but it is really
> > > > >>>>>>>>> suspicious. If we accept it, it should
> > > be documented.
> > > > >>>>>>>>> May be dev_info should be split into parts: part which
> > > > >>>>>>>>> is persistent and part which may depend on device
> configuration.
> > > > >>>>>>>> As I remember, the similar discussion has happened :)
> > > > >>>>>>>> I've raised the similar suggestion like this. But we
> > > > >>>>>>>> don’t make it
> > > happen.
> > > > >>>>>>>> The reason is, you see, this is the rte layer's behavior.
> > > > >>>>>>>> So the user doesn't have to know it. From APP's PoV, it
> > > > >>>>>>>> inputs the configuration, it calls this API
> "rte_eth_dev_configure".
> > > > >>>>>>>> It doesn't know  the configuration is copied before
> > > > >>>>>>>> getting the
> > > info or not.
> > > > >>>>>>>> So, to my opinion, we can still keep the behavior. We
> > > > >>>>>>>> only need to split it into parts when we do see the case
> > > > >>>>>>>> that cannot
> > > make it.
> > > > >>>>>>> Maybe I talked too much about the patch. Think about it again.
> > > > >>>>>>> Your comments is about how to use the APIs,
> > > > >>>>>>> rte_eth_dev_info_get,
> > > > >>>>>> rte_eth_dev_configure. To my opinion, rte_eth_dev_info_get
> > > > >>>>>> is just to get the info. It can be called anywhere, before
> > > > >>>>>> configuration or after. It's reasonable the info changes
> > > > >>>>>> with the
> > > configuration changing.
> > > > >>>>>>> But we do have something missing, like,
> > > > >>>>>>> rte_eth_dev_capability_get which
> > > > >>>>>> should be stable. APP can use this API to get the necessary
> > > > >>>>>> info before configuration.
> > > > >>>>>>> A question, maybe a little divergent thinking, that APP
> > > > >>>>>>> should have some
> > > > >>>>>> intelligence to handle the capability automatically. So
> > > > >>>>>> getting the capability is not so good and effective, looks
> > > > >>>>>> like we still need the human
> > > > >>>> involvement.
> > > > >>>>>> Maybe that the reason currently we suppose APP know the
> > > > >>>>>> capability from the paper copies, examples...
> > > > >>>>>>
> > > > >>>>>> I am not sure to understand all the sentences.
> > > > >>>>>> But I agree that we should take a decision about the
> > > > >>>>>> stability of these
> > > > >>>> infos.
> > > > >>>>>> Either infos cannot change after probing, or we must
> > > > >>>>>> document that the app must request infos regularly (when?).
> > > > >>>>> Sorry, I missed this mail.
> > > > >>>>>
> > > > >>>>> I have the concern that different NICs have different behavior.
> > > > >>>>> One info
> > > > >>>> can be stable on a NIC but dynamic on another. Considering
> > > > >>>> this, we may better not splitting the rte_eth_dev_info_get to 2
> APIs.
> > > > >>>> And comparing with handling this in rte layer, maybe we can
> > > > >>>> let every
> > > NIC has its own decision.
> > > > >>>>> I have an idea. Maybe we can add a parameter for potential
> > > > >>>>> dynamic fields. Like, Changing uint16_t nb_rx_queues; to
> > > > >>>>> struct nb_rx_queues { uint16_t value; bool stable; }
> > > > >>>> May be it is just very bad example, but as I understand
> > > > >>>> nb_rx_queues is mainly required to configure the device properly.
> > > > >>>> Or should app configure, get new value, reconfigure again,
> > > > >>>> get new value and so on and stop when previous is equal to the
> new one.
> > > Yes, I dramatise and it sounds really bad.
> > > > >>>> In any case it would over-complicate interface and no single
> > > > >>>> app will do it correctly.
> > > > >>> I  think you're talking about max_rx_queues. APP can get that
> > > > >>> info
> > > before configuration. Then configure rx queue number which is not
> > > larger than it. That's enough.
> > > > >>> nb_rx_queues should be the number which is configured by APP
> > > > >>> and
> > > how many queues are actually used. To my opinion, it's mainly used
> > > by the GUI to show the value to human being.
> > > > >>>
> > > > >>> BTW, max_rx_queues could be an good example that shows that
> > > some parameters are stable on some NICs but not on other NICs.
> > > > >>> Take Intel NICs for example (I don’t familiar with others.),
> > > > >>> normally
> > > max_rx_queues is stable on PF. But on VF, as the max number is
> > > decided by PF, it could be dynamic. When VF starts, it can get an default
> value from PF.
> > > If it not enough, it can request a larger one from PF. If the number
> > > works, VF can get a new number.
> > > > >> "struct rte_eth_dev_info" is a little overloaded, it has:
> > > > >> - static info, like *device
> > > > >> - device limitations, max_*, *_lim
> > > > >> - device capabilities, *_capa
> > > > >> - suggested configurations, default_*conf
> > > > >> - device configuration, nb_[r/t]x_queues
> > > > >> - other, switch_info
> > > > >>
> > > > >> There is a concern that some values are dynamic, but this is
> > > > >> not new, for example nb_rx/tx_queues can be changed by
> > > > >> rte_eth_dev_rx/tx_queue_config() API and rte_eth_dev_info()
> > > > >> output
> > > will be changed.
> > > > >
> > > > > The example looks different to me. It is explicit changes
> > > > > directly requested by the application. So, it is not a surprise that it
> changes.
> > > > >
> > > > >> For this patch suggested configuration changes based on some
> > > > >> other config values looks ok as concept.
> > > > >> So I think we can say after every configuration related API dev
> > > > >> info can be changed.
> > > > >
> > > > > I think that saying that any configuration changes may result in
> > > > > any changes in dev_info is hardly helpful. I'd suggest to be more
> specific.
> > > > > Yes, it is harder and will have bugs, but at least it is helpful.
> > > >
> > > > Hi Andrew, Wenzhuo,
> > > >
> > > > Back to this patch, which fixes an actual defect,
> > > >
> > > > What do you think about:
> > > > 1- Keep existing patch but extend it as, save the original "dev->data"
> > > > and revert it back to this original data on all error path.
> > > > 2- Update rte_eth_dev_info() API document and say default
> > > > configuration can be changed based on other config fields. So this
> > > > reduces the scope of things can change in dev_info.
> > >
> > > I think we are doing too much juggling with data in ethdev layer.
> > > All these things should be the responsibility of the PMD.
> > > My radical proposal would be to remove rte_eth_dev_info and
> > > integrate all the data into rte_eth_dev_data.
> > >
> > Sorry for missing this discussion. It's a good discussion about how to
> optimize the rte_eth.
> > But I have to say that above discussion can reach a huge reconsitution of
> the rte_eth and impact every PMD. Is that fair?
> > This patch is only try to revert a bad commit as we already find bug. As I
> remember, at the beginning, Andrew said the discussion may not about the
> patch but generic. So could we just tell if this patch itself OK at first?  Thanks.
> 
> I have no strong opinion about your patch.
> Sorry, I am more interested by a possible future big clean-up.
> About a temporary fix, I am OK if Ferruh and Andrew are OK.
About this patch itself, is it OK? If so, would you like ack it? Thanks.