Re: How to replace rte_eth_dev_attach with rte_eal_hotplug_add

[Hideyuki Yamashita <yamashita.hideyuki@po.ntt-tx.co.jp>]

Dear Thomas,

Thanks for your answer.
It took me a little time to digest answer.
Please see inline.


> 21/09/2018 09:19, Hideyuki Yamashita:
> > Dear Gaetan and Thomas, 
> > 
> > Thanks for your answer.
> > Please see inline.
> > 
> > > 20/09/2018 11:09, Ga?an Rivet:
> > > > On Thu, Sep 20, 2018 at 05:46:37PM +0900, Hideyuki Yamashita wrote:
> > > > > Hello,
> > > > > 
> > > > > From dpdk 18.08 release rte_eth_dev_attach and 
> > > > > rte_eth_dev_detach becom deprecated API and 
> > > > > it is recommended to replace with rte_eal_hotplug_add
> > > > > and rte_eal_hotplug_remove.
> > > > > 
> > > > > My program uses  above mentioned deprecated APIs
> > > > > and have to replace those.
> > > > > Note that my program uses attach to attach vhost, pcap pmd.
> > > > > 
> > > > > My question is whether it is correct to replace those as following:
> > > > > find rte_eth_dev_attach function in rte_ethdev.c and
> > > > > migrate those content into my program.
> > > > > 
> > > > > e.g. 
> > > > > lib/librte_ethdev/rte_ethdev.c line 643-686 for attach
> > > > > lib/librte_ethdev/rte_ethdev.c line 690-720 for detach
> > > > > 
> > > > > Your advice/guidance are much appreciated.
> > > > > Thanks!
> > > > 
> > > > Hello Hideyuki,
> > > > 
> > > > You could use this code for guidance, while leaving the ethdev
> > > > specificities such as verifying the eth_dev_count_total(). The hotplug
> > > > function would already return an error if the PMD was not able to create
> > > > the necessary devices.
> > > > 
> > > > The main issue might be to find the port_id of your new port.
> > > > You won't be able to use eth_dev_last_created_port, so you would have to
> > > > iterate over the ethdev using RTE_ETH_FOREACH_DEV and find the one
> > > > matching your parameters (you might for example match the rte_device
> > > > name with the name you used in hotplug_add, as there is no standard
> > > > naming scheme at the ethdev level).
> > First of all, thank for your answering to my question.
> > But I have questions.
> > (Sorry, I have poor knowledge about dpdk and have many basic questions)
> > 
> > Q1. 
> > Why eth_dev_last_created_port can not be used?
> > When I look into rte_eth_dev_atthach in 18.08, it calls 
> > 
> > *port_id = eth_dev_last_created_port;
> > 
> > at the end of the function.
> 
> You can have a race condition.
Please elaborate me a bit more.

Is it correct understanding that race condition 
includes
- read information before port is available
- other device may be plugged (or unplugged)
and so using "eth_dev_last_created_port" is 
NOT reliable?


> > Q2. 
> > Is it possible to use rte_eth_dev_get_port_by name 
> > instead of calling RTE_ETH_FOREACH_DEV or using
> > eth_dev_last_created_port?	
> 
> This function works only if you know the ethdev name generated by the PMD.
Thanks
 
> > Q3. 
> > If answer to Q2 is no, then how can I get device name from each device?
> > For example, rte_eth_dev_info_get takes port_id as its
> > argument.But what I want to know is the port id of the specified device
> > name.
> 
> If you want the ethdev port ids created after probing (based on devargs),
> you probably want to request it with the same devargs.
> I will try to work on something for this need.
> For now, the most reliable solution is to use the notifications.

I think I have two alternatives:
Alt1:
call "rte_eal_hotplug_add"
and retrieve port_id for newly plugged device
using RTE_ETH_FOREACH_DEV.
(as Gaetan said)

Alt2:
call "rte_eth_dev_callback_register"  to subscribe "port probe" event
and "rte_eal_hotplug_add"
and finally get notification including port id.
(As you said)

My question is which is better alternative?
Are there any bad point on Alt1 like using
eth_dev_last_created_port?
(e.g. port not available when called RTE_ETH_FOREACH_DEV)

BR,
Hideyuki Yamashita
NTT TechnoCross

> > > It is recommended to register a callback to receive the notifications
> > > of new ethdev ports.
> > > So it may be a change of programming style: sync vs async.
> > Thanks for your advice.
> > In general, I agree.
> > 
> > But what is the example of notification that program can receive
> > after device hotplugged?
> > Device unplugged notification like specified in rte_eth_event_type?
> 
> Yes, look at these ones:
>     RTE_ETH_EVENT_NEW,      /**< port is probed */
>     RTE_ETH_EVENT_DESTROY,  /**< port is released */
> 
> > If there are code in example, it is highly appreciated.
> 
> You can register a callback with
> 	rte_eth_dev_callback_register(RTE_ETH_ALL, RTE_ETH_EVENT_NEW, ...)
> like done in drivers/net/failsafe/failsafe.c
> 
> 
>