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

[Thomas Monjalon <thomas@monjalon.net>]

16/07/2018 03:58, Lu, Wenzhuo:
> Hi Andrew,
> 
> > -----Original Message-----
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Lu, Wenzhuo
> > Sent: Monday, July 16, 2018 9:08 AM
> > To: Andrew Rybchenko <arybchenko@solarflare.com>; dev@dpdk.org
> > Cc: Yigit, Ferruh <ferruh.yigit@intel.com>; Thomas Monjalon
> > <thomas@monjalon.net>
> > Subject: Re: [dpdk-dev] [PATCH v2] ethdev: fix device info getting
> > 
> > Hi Andrew,
> > 
> > > -----Original Message-----
> > > From: Andrew Rybchenko [mailto:arybchenko@solarflare.com]
> > > Sent: Friday, July 13, 2018 4:03 PM
> > > To: Lu, Wenzhuo <wenzhuo.lu@intel.com>; dev@dpdk.org
> > > Cc: Yigit, Ferruh <ferruh.yigit@intel.com>; Thomas Monjalon
> > > <thomas@monjalon.net>
> > > Subject: Re: [dpdk-dev] [PATCH v2] ethdev: fix device info getting
> > >
> > > 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?).