netdev - Re: No traffic with Marvell switch and latest linux-next

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Message-ID: <d638a284-186f-d5b7-4f49-3df82e6db7fd@gmail.com>
Date:   Sun, 24 Feb 2019 22:42:03 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>
Cc:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: No traffic with Marvell switch and latest linux-next

On 24.02.2019 22:26, Florian Fainelli wrote:
> 
> 
> On February 24, 2019 9:04:55 AM PST, Andrew Lunn <andrew@...n.ch> wrote:
>>> The added difficulty here and the reason why Andrew went with the
>>> approach that is used by the code currently is because neither do the
>>> CPU or DSA ports are backed by a net_device. It is somewhere on my
>> TODO
>>> to permit the use of PHYLINK without the need of a net_device to
>> cover
>>> those specific DSA cases unless we just brute force the whole thing
>> and
>>> allocate a net_device structure but not register that net_device? Yes
>> in
>>> fact, why don't we do that?
>>
>> Hi Florian
>>
>> At the moment, we are using a phydev which is not connected to a
>> MAC. That is rather odd, but the phylib maintainers mostly know about
>> this, and keep an eye out for changes which might break any
>> assumptions. And the phylib API is quite small.
> 
> I would argue that this very thread is a proof against your argument since we all failed to predict that Heiner's changes would change those assumptions. Having a certain of assumptions is fine but given all the recent PHYLIB helpers that have been added I am not sure how well that will scale.
> 
>>
>> How many assumptions are going to break with a netdev which is not
>> registered? The API is much bigger, more people hack on it, and it is
>> going to be much harder to review changes to make sure assumptions are
>> not changed.
> 
> A non registered net_device appears easier to manage and debug since there is state tracking all over the network stack for those cases.
> 
>>
>> If we are going to do something odd, we should keep the scope as small
>> as possible.
> 
> Hence my suggestion to allocate a dummy net_device object just so calls to netif_carrier_{on,off} (and possibly more in the future) do nothing. I don't think that teaching either PHYLIB or PHYLINK about a NULL net_device is going to scale really well over time nor make it easier  for respective maintainers. If we make the net_device optional, it will be harder to review changes as well as make sure that we do not create locking/object interactions  issues.
> 
> Another approach could be to define a minimal network port object (struct devlink, maybe?) which could be used independently from a net_device, or a lightweight net_device with no visibility into existing namespaces. None of these ideas are new though and would probably require several cycles to get done right.
> 
> Heiner, Russell, which approach would you take?
> 
Given my 2 days of experience with DSA (feels like 3 already) I would like to spend few more minutes on thinking before I answer.

Heiner