openamp: add ns_unbind_cb to rpmsg_device

[xiaoxiang781216]

it's useful to notify the port layer and also symmetry with ns_bind_cb callback.

[arnopo]

Could you detail your need? in which use case you need to notify the port layer in addition of the endpoint?

[xiaoxiang781216]

Could you detail your need? in which use case you need to notify the port layer in addition of the endpoint?

In some case, the service/apps may start later than open-amp framework, and then lose the creation notification from remote peer. To fix this issue, the porting layer could catch the notification and forward it to the client/service at the launch.
Since the client/server may create and destroy the endpointer dynamically and unbound, the porting layer need some mechanism(unbind callback here) to reclaim the resource from cache.

[arnopo]

Could you detail your need? in which use case you need to notify the port layer in addition of the endpoint?

In some case, the service/apps may start later than open-amp framework, and then lose the creation notification from remote peer. To fix this issue, the porting layer could catch the notification and forward it to the client/service at the launch.
Since the client/server may create and destroy the endpointer dynamically and unbound, the porting layer need some mechanism(unbind callback here) to reclaim the resource from cache.

In such case, is the endpoint created by the porting layer?
Seems quite tricky to address:

• your port layer could free resource that are still used by the application.
• you have to manage error case where an unbind is receive after the bind, while the application is not ready.

What about having the endpoint->ns_unbind_cb calling a porting layer function that would forward to the application?

[xiaoxiang781216]

Could you detail your need? in which use case you need to notify the port layer in addition of the endpoint?

In some case, the service/apps may start later than open-amp framework, and then lose the creation notification from remote peer. To fix this issue, the porting layer could catch the notification and forward it to the client/service at the launch.
Since the client/server may create and destroy the endpointer dynamically and unbound, the porting layer need some mechanism(unbind callback here) to reclaim the resource from cache.

In such case, is the endpoint created by the porting layer?

No, the endpoint is still created by service/application. The resource I mean here is simple channel name and port number:
https://github.com/apache/incubator-nuttx/blob/master/drivers/rptun/rptun.c#L581-L582

Seems quite tricky to address:

• your port layer could free resource that are still used by the application.

No, the house keeping data is managed by the porting layer internally, not shared with application at all.

• you have to manage error case where an unbind is receive after the bind, while the application is not ready.

Of course, the application doesn't get any notification in this case since the channel name and port number is removed from cache in ns_unbind_cb callback.

What about having the endpoint->ns_unbind_cb calling a porting layer function that would forward to the application?

As I explain before, the porting layer doesn't create the endpoint behalf the application, so it can't hook the notification from endpoint->ns_unbind_cb.

BTW, it's also wrong to create the endpoint in porting layer because:

1. The service represented by the endpoint mayn't exist at all. It's hard to know inside the porting layer in a dynamical environment.
2. Even the service exist, it's wrong to create the endpoint before the service is ready. e.g. How to handle the message send from client before the service start?

[arnopo]

Thanks for the link to the code, easier to understand your need.
The addition of this callback seems reasonable to me.
just documentation to address

[edmooring]

This seems like a sensible addition.