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

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 8/14/2018 1:57 AM, Lu, Wenzhuo wrote:
> Hi Andrew,
> 
>> -----Original Message-----
>> From: Andrew Rybchenko [mailto:arybchenko@solarflare.com]
>> Sent: Monday, August 13, 2018 4:39 PM
>> To: Lu, Wenzhuo <wenzhuo.lu@intel.com>; Thomas Monjalon
>> <thomas@monjalon.net>; Yigit, Ferruh <ferruh.yigit@intel.com>
>> Cc: dev@dpdk.org
>> Subject: Re: [dpdk-dev] [PATCH v2] ethdev: fix device info getting
>>
>> On 13.08.2018 05:50, Lu, Wenzhuo wrote:
>>> Hi Thomas,
>>>
>>>
>>>> -----Original Message-----
>>>> From: Thomas Monjalon [mailto:thomas@monjalon.net]
>>>> Sent: Wednesday, August 1, 2018 11:37 PM
>>>> To: Lu, Wenzhuo <wenzhuo.lu@intel.com>; Andrew Rybchenko
>>>> <arybchenko@solarflare.com>; Yigit, Ferruh <ferruh.yigit@intel.com>
>>>> Cc: dev@dpdk.org
>>>> Subject: Re: [dpdk-dev] [PATCH v2] ethdev: fix device info getting
>>>>
>>>> 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?).
>>> 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.
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.

But rte_eth_dev_info_get() has been called within rte_eth_dev_configure()
creating a cyclic dependency, this is forcing copy the user provided config
before rte_eth_dev_info().

This case the concern of copying user provided config to device config in early
stages cause inconsistent data in error case, this is valid concern and
unfortunately already an issue with the current implementation.

What would you think keep the logic in this patch but improve it with save and
restore existing device config for error cases?

> 
>>
>> Stable dev_info is simple. If there are real cases when something can't be
>> stable (and may be recommended Rx/Tx ring sizes is good example, it should
>> at least documented in dev_info structure description or may be moved to
>> separate API.
>>
>>> By default, the stable is false. Then every NIC can maintain its own
>> behavior.
>>>
>>> Some fileds that must be stable can be left unchanged, like, driver_name,
>> max_rx_queues.
>>>
>>> As this patch is just reversing a bad commit to fix a bug, if my idea sounds
>> good or worth discussing, I can send another RFC mail for it.
>>>
>