linux-kernel - Re: [RFC PATCH net 0/2] Make phylink and DSA wait for PHY driver that defers probe

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Date:   Thu, 19 May 2022 15:38:13 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Andrew Lunn <andrew@...n.ch>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Saravana Kannan <saravanak@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        John Stultz <jstultz@...gle.com>,
        Alvin ┼áipraga <alsi@...g-olufsen.dk>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Heiner Kallweit <hkallweit1@...il.com>
Subject: Re: [RFC PATCH net 0/2] Make phylink and DSA wait for PHY driver that
 defers probe

On Thu, May 19, 2022 at 05:32:35PM +0200, Andrew Lunn wrote:
> > > There is a very different approach, which might be simpler.
> > > 
> > > We know polling will always work. And it should be possible to
> > > transition between polling and interrupt at any point, so long as the
> > > phylock is held. So if you get -EPROBE_DEFFER during probe, mark some
> > > state in phydev that there should be an irq, but it is not around yet.
> > > When the phy is started, and phylib starts polling, look for the state
> > > and try getting the IRQ again. If successful, swap to interrupts, if
> > > not, keep polling. Maybe after 60 seconds of polling and trying, give
> > > up trying to find the irq and stick with polling.
> > 
> > That doesn't sound like something that I'd backport to stable kernels.
> 
> > What motivates me to make these changes in the first place is the idea
> > that current kernels should work with updated device trees.
> 
> By current, you mean old kernels, LTS etc. You want an LTS kernel to
> work with a new DT blob? You want forward compatibility with a DT
> blob. Do the stable rules say anything about that?
> 
>       Andrew

Hmm, not sure about stable rules, but at least Marc Zyngier has
suggested in the past that this is something which should work:
https://lore.kernel.org/linux-arm-kernel/87czlzjxmz.wl-maz@kernel.org/

To quote:

| > As for compatibility between old kernel and new DT: I guess you'll hear
| > various opinions on this one.
| > https://www.spinics.net/lists/linux-mips/msg07778.html
| > 
| > | > Are we okay with the new device tree blobs breaking the old kernel?
| > |
| > | From my point of view, newer device trees are not required to work on
| > | older kernel, this would impose an unreasonable limitation and the use
| > | case is very limited.
| 
| My views are on the opposite side. DT is an ABI, full stop. If you
| change something, you *must* guarantee forward *and* backward
| compatibility. That's because:
| 
| - you don't control how updatable the firmware is
| 
| - people may need to revert to other versions of the kernel because
|   the new one is broken
| 
| - there are plenty of DT users beyond Linux, and we are not creating
|   bindings for Linux only.
| 
| You may disagree with this, but for the subsystems I maintain, this is
| the rule I intent to stick to.