Message-ID: <20220701025012.5dd38c81@xps-13>
Date:   Fri, 1 Jul 2022 02:50:12 +0200
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     Alexander Aring <aahringo@...hat.com>
Cc:     Alexander Aring <alex.aring@...il.com>,
        Stefan Schmidt <stefan@...enfreihafen.org>,
        linux-wpan - ML <linux-wpan@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Network Development <netdev@...r.kernel.org>,
        David Girault <david.girault@...vo.com>,
        Romuald Despres <romuald.despres@...vo.com>,
        Frederic Blain <frederic.blain@...vo.com>,
        Nicolas Schodet <nico@...fr.eu.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH wpan-next v3 2/4] net: ieee802154: Add support for inter
 PAN management

Hi Alexander,

aahringo@...hat.com wrote on Thu, 30 Jun 2022 19:27:49 -0400:

> Hi,
> 
> On Thu, Jun 30, 2022 at 4:14 AM Miquel Raynal <miquel.raynal@...tlin.com> wrote:
> >
> > Hi Alexander,
> >
> > aahringo@...hat.com wrote on Wed, 29 Jun 2022 21:40:14 -0400:
> >  
> > > Hi,
> > >
> > > On Tue, Jun 28, 2022 at 3:58 AM Miquel Raynal <miquel.raynal@...tlin.com> wrote:  
> > > >
> > > > Hi Alexander,
> > > >
> > > > aahringo@...hat.com wrote on Mon, 27 Jun 2022 21:32:08 -0400:
> > > >  
> > > > > Hi,
> > > > >
> > > > > On Mon, Jun 27, 2022 at 4:43 AM Miquel Raynal <miquel.raynal@...tlin.com> wrote:  
> > > > > >
> > > > > > Hi Alexander,
> > > > > >
> > > > > > aahringo@...hat.com wrote on Sat, 25 Jun 2022 22:29:08 -0400:
> > > > > >  
> > > > > > > Hi,
> > > > > > >
> > > > > > > On Mon, Jun 20, 2022 at 10:26 AM Miquel Raynal
> > > > > > > <miquel.raynal@...tlin.com> wrote:  
> > > > > > > >
> > > > > > > > Let's introduce the basics for defining PANs:
> > > > > > > > - structures defining a PAN
> > > > > > > > - helpers for PAN registration
> > > > > > > > - helpers discarding old PANs
> > > > > > > >  
> > > > > > >
> > > > > > > I think the whole pan management can/should be stored in user space by
> > > > > > > a daemon running in background.  
> > > > > >
> > > > > > We need both, and currently:
> > > > > > - while the scan is happening, the kernel saves all the discovered PANs
> > > > > > - the kernel PAN list can be dumped (and also flushed) asynchronously by
> > > > > >   the userspace
> > > > > >
> > > > > > IOW the userspace is responsible of keeping its own list of PANs in
> > > > > > sync with what the kernel discovers, so at any moment it can ask the
> > > > > > kernel what it has in memory, it can be done during a scan or after. It
> > > > > > can request a new scan to update the entries, or flush the kernel list.
> > > > > > The scan operation is always requested by the user anyway, it's not
> > > > > > something happening in the background.
> > > > > >  
> > > > >
> > > > > I don't see what advantage it has to keep the discovered pan in the
> > > > > kernel. You can do everything with a start/stop/pan discovered event.  
> > > >
> > > > I think the main reason is to be much more user friendly. Keeping track
> > > > of the known PANs in the kernel matters because when you start working
> > > > with 802.15.4 you won't blindly use a daemon (if there is any) and will
> > > > use test apps like iwpan which are stateless. Re-doing a scan on demand
> > > > just takes ages (from seconds to minutes, depending on the beacon
> > > > order).
> > > >  
> > >
> > > I can see that things should work "out-of the box" and we are already
> > > doing it by manual setting pan_id, etc. However, doing it in an
> > > automatic way there exists a lot of "interpretation" about how you
> > > want to handle it (doesn't matter if this is what the spec says or
> > > not)... moving it to user space will offload it to the user.
> > >  
> > > > Aside from this non technical reason, I also had in mind to retrieve
> > > > values gathered from the beacons (and stored in the PAN descriptors) to
> > > > know more about the devices when eg. listing associations, like
> > > > registering the short address of a coordinator. I don't yet know how
> > > > useful this is TBH.
> > > >  
> > > > > It also has more advantages as you can look for a specific pan and
> > > > > stop afterwards. At the end the daemon has everything that the kernel
> > > > > also has, as you said it's in sync.
> > > > >  
> > > > > > > This can be a network manager as it
> > > > > > > listens to netlink events as "detect PAN xy" and stores it and
> > > > > > > offers it in their list to associate with it.  
> > > > > >
> > > > > > There are events produced, yes. But really, this is not something we
> > > > > > actually need. The user requests a scan over a given range, when the
> > > > > > scan is over it looks at the list and decides which PAN it
> > > > > > wants to associate with, and through which coordinator (95% of the
> > > > > > scenarii).
> > > > > >  
> > > > >
> > > > > This isn't either a kernel job to decide which pan it will be
> > > > > associated with.  
> > > >
> > > > Yes, "it looks at the list and decides" referred to "the user".
> > > >  
> > > > > > > We need somewhere to draw a line and I guess the line is "Is this
> > > > > > > information used e.g. as any lookup or something in the hot path", I
> > > > > > > don't see this currently...  
> > > > > >
> > > > > > Each PAN descriptor is like 20 bytes, so that's why I don't feel back
> > > > > > keeping them, I think it's easier to be able to serve the list of PANs
> > > > > > upon request rather than only forwarding events and not being able to
> > > > > > retrieve the list a second time (at least during the development).
> > > > > >  
> > > > >
> > > > > This has nothing to do with memory.
> > > > >  
> > > > > > Overall I feel like this part is still a little bit blurry because it
> > > > > > has currently no user, perhaps I should send the next series which
> > > > > > actually makes the current series useful.
> > > > > >  
> > > > >
> > > > > Will it get more used than caching entries in the kernel for user
> > > > > space? Please also no in-kernel association feature.  
> > > >
> > > > I am aligned on this.
> > > >  
> > >
> > > I am sorry I am not sure what that means.  
> >
> > I was referring to the "no in-kernel association feature".
> >
> > There is however one situation which I _had_ to be handled in the
> > kernel: other devices asking for being associated or disassociated. In
> > the case of the disassociation, the receiving device is only notified
> > and cannot refuse the disassociation. For the association however,
> > the device receiving the association request has to make a decision.
> > There are three possible outcomes:
> > - accepting
> > - refusing because the PAN is at capacity
> > - refusing because the device is blacklisted  
> 
> Why not move this decision to the user as well? The kernel will wait
> for the reason? This isn't required to be fast and the decision may
> depend on the current pan management...

I've opted out for the simplest option, which is allowing X devices
being associated, X being manageable by the user. For now I'll keep
this very simple approach, I propose we add this filtering feature
later?

> > For now I've only implemented the first reason, because it's much
> > easier and only requires a maximum device number variable, set by the
> > user. For the second reason, it requires handling a
> > whitelist/blacklist, I don't plan to implement this for now, but that
> > should not impact the rest of the code. I'll let that to other
> > developers, or future-me, perhaps :-). Anyhow, you can kick-out devices
> > at any time anyway if needed with a disassociation notification
> > controlled by the user.
> >  
> > > > > We can maybe agree to that point to put it under
> > > > > IEEE802154_NL802154_EXPERIMENTAL config, as soon as we have some
> > > > > _open_ user space program ready we will drop this feature again...
> > > > > this program will show that there is no magic about it.  
> > > >
> > > > Yeah, do you want to move all the code scan/beacon/pan/association code
> > > > under EXPERIMENTAL sections? Or is it just the PAN management logic?  
> > >
> > > Yes, why not. But as I can see there exists two categories of
> > > introducing your netlink api:
> > >
> > > 1. API candidates which are very likely to become stable
> > > 2. API candidates which we want to remove when we have a user
> > > replacement for it (will probably never go stable)
> > >
> > > The 2. should be defining _after_ the 1. In the "big" netlink API
> > > enums of EXPERIMENTAL sections.  
> >
> > Yeah, got it.
> >  
> > > Also you should provide for 2. some kind of ifdef/functions dummy/etc.
> > > that it's easy to remove from the kernel when we have a user
> > > replacement for it.
> > > I hope that is fine for everybody.
> > >
> > > I try to find solutions here, I don't see a reason for putting this
> > > pan management into the kernel... whereas I appreciate the effort
> > > which is done here and will not force you to write some user space
> > > software that does this job. From my point of view I can't accept this
> > > functionality in the kernel "yet".  
> >
> > I've already spent a couple of days reworking all that part, I've
> > dropped most of the in-kernel PAN management, which means:
> > - when a new coordinator gets discovered (beacon received), if the mac
> >   was scanning then it calls a generic function from the cfg layer to
> >   advertise this pan.
> > - the cfg layer will send a NL message to the user with all the
> >   important information
> > - BUT the cfg layer will also keep in memory the beacon information for
> >   the time of the scan (only), to avoid polluting the user with the same
> >   information over and over again, this seems a necessary step to me,
> >   because otherwise if you track on the same channel two coordinators
> >   not emitting at the same pace, you might end up with 100 user
> >   notifications, for just 2 devices. I think this is the kernel duty to
> >   filter out identical beacons.
> >  
> 
> Okay, I am sure if somebody complains about such kernel behaviour and
> has a good argument to switch back... we still can do it.

Great!

> 
> > I _will_ send a v4, including the scanning part this time by the end of
> > the week, I need to settle everything down, ensure it still works and
> > clean the branch.
> >  
> 
> ok.
> 
> - Alex
> 


Thanks,
Miquèl

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