Re: [dpdk-dev] [PATCH 2/5] ethdev: add capability to keep shared objects on restart

[Dmitry Kozlyuk <dkozlyuk@oss.nvidia.com>]

> -----Original Message-----
> From: Ferruh Yigit <ferruh.yigit@intel.com>
> [...]
> > Introducing UNKNOWN state seems wrong to me.
> > What should an application do when it is reported?
> > Now there's just no way to learn how the PMD behaves,
> > but if it provides a response, it can't be "I don't know what I do".
> >
> 
> I agree 'unknown' state is not ideal, but my intentions is prevent
> drivers that not implemented this new feature report wrong capability.
> 
> Without capability, application already doesn't know how underlying
> PMD behaves, so this is by default 'unknown' state.
> I suggest keeping that state until driver explicitly updates its state
> to the correct value.

My concern is that when all the drivers are changed to report a proper
capability, UNKNOWN remains in the API meaning "there's a bug in DPDK".

Instead of UNKNOWN response we can declare that rte_flow_flush()
must be called unless the application wants to keep the rules
and has made sure it's possible, or the behavior is undefined.
(Can be viewed as "UNKNOWN by default", but is simpler.)
This way neither UNKNOWN state is needed,
nor the bit saying the flow rules are flushed.
Here is why, let's consider KEEP and FLUSH combinations:

(1) FLUSH=0, KEEP=0 is equivalent to UNKNOWN, i.e. the application
                    must explicitly flush the rules itself
                    in order to get deterministic behavior.
(2) FLUSH=1, KEEP=0 means PMD flushes all rules on the device stop.
(3) FLUSH=0, KEEP=1 means PMD can keep at least some rules,
                    exact support must be checked with rte_flow_create()
                    when the device is stopped.
(4) FLUSH=1, KEEP=1 is forbidden.

If the application doesn't need the PMD to keep flow rules,
it can as well flush them always before the device stop
regardless of whether the driver does it automatically or not.
It's even simpler and probably as efficient. Testpmd does this.
If the application wants to take advantage of rule-keeping ability,
it just tests the KEEP bit. If it is unset that's the previous case,
application should call rte_flow_flush() before the device stop to be sure.
Otherwise, the application can test capability to keep flow rule kinds
it is interested in (see my reply to Andrew).

Result: no changes to PMDs are _immediately_ needed when such behavior
is documented. They can start advertising it whenever they like,
it's not even an RC2 task. Currently applications that relied on certain
behavior are non-portable anyway.

> But having below list is good, if you will update all drivers than
> no need to have the 'unknown' state, but updating drivers may require
> driver maintainers ack which can take some time.

If you agree with what I suggest above, there will be no urgency.
The list can be used to notify maintainers that they can enhance
their PMD user experience whenever they like.

> Can you please clarify what is you plan according PMDs, will you update
> them all, or will you only update mlx5 in -rc2?
> And what is the exact plan for the -rc2 that you mention?

mlx5 PMD will be updated with the patches from this series.
Regarding indirect actions: no other PMD needs an update.
Regarding flow rules: if the above suggestion is accepted,
no PMDs need to be updated urgently.