Date:   Thu, 25 Aug 2016 09:38:39 +0200
From:   Ralph Sennhauser <ralph.sennhauser@...il.com>
To:     Jason Cooper <jason@...edaemon.net>
Cc:     Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Gregory CLEMENT <gregory.clement@...e-electrons.com>,
        Lennart Sorensen <lsorense@...lub.uwaterloo.ca>
Subject: Re: [Regression?] Commit cb4f71c429 deliberately changes order of
 network interfaces

Hi Jason.

On Wed, 24 Aug 2016 21:48:36 +0000
Jason Cooper <jason@...edaemon.net> wrote:

> All,
> 
> On Wed, Aug 24, 2016 at 10:41:02PM +0200, Ralph Sennhauser wrote:
> > On Wed, 24 Aug 2016 20:15:31 +0200
> > Thomas Petazzoni <thomas.petazzoni@...e-electrons.com> wrote:  
> > > On Wed, 24 Aug 2016 19:10:04 +0200, Ralph Sennhauser wrote:
> > > 
> > > The people who can take this decision are rather the maintainers
> > > of the platform itself, or possibly the arm-soc maintainers if
> > > you still don't like what the platform maintainers decided.  
> > 
> > We both have a conflict of interest here, so your offer in an other
> > message to let the platform maintainers decide is appreciated.  
> 
> Ok, I'm going to jump in here with the caveats that a) I don't mind
> being overruled, and b) I'm not going to participate in a never-ending
> thread on this topic.  ;-)

I'm also not interested in a never ending thread. It's moot that udev
can't rename to kernel names sanely and we were sold ep34aj17asz98 as
the replacement. Or that tearing apart the casing to replace the wifi
modules with an ethernet one when there are already 5 Gbit ports is not
a case I care about.

Relying on the order might in theory have flaws but works just fine in
practice. Thomas Petazzoni, I, OpenWrt / Lede / etc all do so with
success.

It's also not a side effect from major changes, so it didn't break by
accident but as the subject says deliberately.

> So, I'm just a back-seat co-maintainer for mvebu nowadays, my opinion
> should be taken with a grain of salt.
> 
> From the kernel-side, there is no guarantee of device naming.  The
> kernel simply doesn't have sufficient information at boot time.  This
> is why udev, systemd-udev-ntpd-syslog-kitchensink, and others were
> created. To read immutable attributes of a device and apply
> consistent naming to them based on configuration files.  That's why
> userspace frequently uses uuids to locate partitions, rather
> than /dev/sdX[0-9] nodes.
> 
> The devicetree "ordered by address" rule is, in my opinion, an
> arbitrary CDO-rule [1].  It doesn't describe the hardware, or a
> relationship between them.  As such, it's just as arbitrary as probe
> order.  It just happens to be something we can twiddle.  It also
> depends on dtc preserving that order.
> 

How exceptional is this exception we are talking about? I mean if it's
the only place you might want to change it even in 2 years but you
can't because of the very same rule which was broken here.

> From the user side, without udev and friends, shit changed from one
> kernel to the next.  That's not good.  I can certainly see the point
> that it should have been left alone to begin with.  But we aren't
> there today.
> 

I think if we were at 4.6-rcX reverting would be an easy call. I know
it's more difficult to make this call now.

Ltsi is 4.1, longterm is 4.4, so penetration is probably marginal at
best at this time. From my view the damage caused by a revert would be
less than the damage that will be caused by the commit if left in but we
can't wait much much longer until this changes.

> So what happens if we revert now?  Right or wrong, in a couple of
> months, someone else will complain, asking to revert the revert.
> And what will our answer be?  "We did it last time, but not this
> time." or "Ok, but gosh-darnit, this is the last time."
> 

If you use the ordering by address as main argument for the revert there
will be nothing to argue about.

> To be blunt, I think our best path forward is to just hold our noses
> and let it stand as is.  Some will fix their userspace to adapt,
> others will carry a patch.  It's more important at this point to be
> consistent moving forward.  It's better to hear "Yeah, it fucking
> changed once." rather than "I don't know what to expect, it changes
> every few releases."
> 
> thx,
> 
> Jason.
> 
> 
> [1] CDO: OCD with the letters neatly arranged in alphabetical order.

Thanks for sharing your thoughts

Regards
Ralph

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