Re: [dpdk-dev] [PATCH v6 1/4] lib/librte_ether: support device reset

[Jerin Jacob <jerin.jacob@caviumnetworks.com>]

On Tue, Jun 21, 2016 at 01:10:40PM +0000, Ananyev, Konstantin wrote:
> 
> 
> > 
> > Hi Konstantin,
> > 
> > > Hi Jerin,
> > >
> > > > -----Original Message-----
> > > > From: Jerin Jacob [mailto:jerin.jacob@caviumnetworks.com]
> > > > Sent: Tuesday, June 21, 2016 9:56 AM
> > > > To: Lu, Wenzhuo
> > > > Cc: Stephen Hemminger; dev@dpdk.org; Ananyev, Konstantin; Richardson, Bruce; Chen, Jing D; Liang, Cunming; Wu, Jingjing;
> > Zhang,
> > > > Helin; thomas.monjalon@6wind.com
> > > > Subject: Re: [dpdk-dev] [PATCH v6 1/4] lib/librte_ether: support device reset
> > > >
> > > > On Tue, Jun 21, 2016 at 08:24:36AM +0000, Lu, Wenzhuo wrote:
> > > > > Hi Jerin,
> > > >
> > > > Hi Wenzhuo,
> > > >
> > > > > > > > > > On Mon, Jun 20, 2016 at 02:24:27PM +0800, Wenzhuo Lu wrote:
> > > > > > > > > > > Add an API to reset the device.
> > > > > > > > > > > It's for VF device in this scenario, kernel PF + DPDK VF.
> > > > > > > > > > > When the PF port down->up, APP should call this API to reset
> > > > > > > > > > > VF port. Most likely, APP should call it in its management
> > > > > > > > > > > thread and guarantee the thread safe. It means APP should stop
> > > > > > > > > > > the rx/tx and the device, then reset the device, then recover
> > > > > > > > > > > the device and rx/tx.
> > > > > > > > > >
> > > > > > > > > > Following is _a_ use-case for Device reset. But may be not be
> > > > > > > > > > _the_ use case. IMO, We need to first say expected behavior of
> > > > > > > > > > this API and add a use-case later.
> > > > > > > > > >
> > > > > > > > > > Other use-case would be, PCIe VF with functional level reset for
> > > > > > > > > > SRIOV migration.
> > > > > > > > > > Are we on same page?
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > In my experience with Linux devices, this is normally handled by
> > > > > > > > > the device driver in the start routine.  Since any use case which
> > > > > > > > > needs this is going to do a stop/reset/start sequence, why not
> > > > > > > > > just have the VF device driver do this in the start routine?.
> > > > > > > > >
> > > > > > > > > Adding yet another API and state transistion if not necessary
> > > > > > > > > increases the complexity and required test cases for all devices.
> > > > > > > >
> > > > > > > > I agree with Stephen here.I think if application needs to call start
> > > > > > > > after the device reset then we could add this logic in start itself
> > > > > > > > rather exposing a yet another API
> > > > > > > Do you mean changing the device_start to include all these actions, stop
> > > > > > device -> stop queue -> re-setup queue -> start queue -> start device ?
> > > > > >
> > > > > > What was the expected API call sequence when you were introduced this API?
> > > > > >
> > > > > > Point was to have implicit device reset in the API call sequence(Wherever make
> > > > > > sense for specific PMD)
> > > > > I think the API call sequence depends on the implementation of the APP. Let's say if there's not this reset API, APP can use this
> > API
> > > > call sequence to handle the PF link down/up event, rte_eth_dev_close -> rte_eth_rx_queue_setup -> rte_eth_tx_queue_setup -
> > >
> > > > rte_eth_dev_start.
> > > > > Actually our purpose is to use this reset API instead of the API call sequence. You can see the reset API is not necessary. The
> > benefit
> > > > is to save the code for APP.
> > > >
> > > > Then I am bit confused with original commit log description.
> > > > |
> > > > |It means APP should stop the rx/tx and the device, then reset the
> > > > |device, then recover the device and rx/tx.
> > > > |
> > > > I was under impression that it a low level reset API for this device? Is
> > > > n't it?
> > > >
> > > > The other issue is generalized outlook of the API, Certain PMD will not
> > > > have PF link down/up event? Link down/up and only connected to VF and PF
> > > > only for configuration.
> > > >
> > > > How about fixing it more transparently in PMD driver itself as
> > > > PMD driver knows the PF link up/down event, Is it possible to
> > > > recover the VF on that event if its only matter of resetting it?
> > >
> > > I think we already went through that discussion on the list.
> > > Unfortunately with current dpdk design it is hardly possible.
> > > To achieve that we need to introduce some sort of synchronisation
> > > between IO and control APIs (locking or so).
> > > Actually I am not sure why having a special reset function will be a problem.
> > 
> > |
> > |It means APP should stop the rx/tx and the device, then reset the
> > |device, then recover the device and rx/tx.
> > |
> > Just to understand, If application still need  to do the stop then what
> > value addtion reset API brings on the table?
> 
> If application calls dev_reset() it doesn't need to call dev_stop() before it.
> dev_reset() will take care of it. 
> But it needs to make sure that no other thread will try to modify that device state
> (either dev_stop/start, or eth_rx_busrst/eth_tx_burst) while the reset op is in place.

OK. This description looks different than commit log and API doxygen comment. Please fix it.
How about a different name for this API. Device reset is too generic?

> 
> > 
> > 
> > > Yes, it would exist only for VFs, for PF it could be left unimplemented.
> > > Though it definitely seems more convenient from user point of view,
> > > they would know: to handle VF reset event, they just need to call that
> > > particular function, not to re-implement their own.
> > What if driver returns "not implemented" then application will have do
> > generic rte_eth_dev_stop/rte_eth_dev_start.
> >That way in application  perspective we are NOT solving any problem.
> 
> True, but as I said for PF application would just never receive such event.
What is this event ? Is it VF Link up/down event?

No I was referring to VF itself, Other VF PMD drivers in drivers/net
where this callback is not implemented.

Jerin


> I suppose it is possible to implement one for PF too, I just don't see
> much point - as probably no-one will ever use it.
> 
> Konstantin