linux-kernel - Re: [RFC PATCH devicetree 00/10] Do something about ls-extirq interrupt-map breakage

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Date:   Thu, 24 Mar 2022 17:21:50 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Rob Herring <robh+dt@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>, Leo Li <leoyang.li@....com>,
        Biwen Li <biwen.li@....com>, "Z.Q. Hou" <zhiqiang.hou@....com>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [RFC PATCH devicetree 00/10] Do something about ls-extirq interrupt-map breakage

On Thu, 24 Mar 2022 17:10:42 +0000,
Vladimir Oltean <vladimir.oltean@....com> wrote:
> 
> Hello Marc,
> 
> On Tue, Dec 14, 2021 at 10:20:36AM +0000, Marc Zyngier wrote:
> > On Tue, 14 Dec 2021 09:58:54 +0000,
> > Vladimir Oltean <vladimir.oltean@....com> wrote:
> > > 
> > > Hi Marc (with a c),
> > > 
> > > I wish the firmware for these SoCs was smart enough to be compatible
> > > with the bindings that are in the kernel and provide a blob that the
> > > kernel could actually use. Some work has been started there and this is
> > > work in progress. True, I don't know what other OF-based firmware some
> > > other customers may use, but I trust it isn't a lot more advanced than
> > > what U-Boot currently has :)
> > > 
> > > Also, the machines may have been in the wild for years, but the
> > > ls-extirq driver was added in November 2019. So not with the
> > > introduction of the SoC device trees themselves. That isn't so long ago.
> > > 
> > > 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.
> > 
> > 	M.
> > 
> > -- 
> > Without deviation from the norm, progress is not possible.
> 
> I was just debugging an interesting issue with an old kernel not working
> with a new DT blob, and after figuring out what the problem was (is),
> I remembered this message and I'm curious what you have to say about it.
> 
> I have this DT layout:
> 
> 	ethernet-phy@1 {
> 		reg = <0x1>;
> 		interrupts-extended = <&extirq 2 IRQ_TYPE_LEVEL_LOW>;
> 	};
> 
> 	extirq: interrupt-controller@1ac {
> 		compatible = "fsl,ls1021a-extirq";
> 		<bla bla>
> 	};
> 
> I booted the new DT blob (which has "interrupts-extended") on a kernel
> where the ls-extirq driver did not exist. This had the result of
> of_mdiobus_phy_device_register() -> of_irq_get() returning -EPROBE_DEFER
> forever and ever. So the PHY driver in turn never probed, and Ethernet
> was broken. So I had to delete the interrupts OF property to let the PHY
> at least work in poll mode.
> 
> What went wrong here in your opinion?

I'm not sure what you expect me to say here. You have a device that
references an interrupt. The DT seems sound (I don't get why you think
"interrupt-extended" is a problem here, but hey...).

If your kernel doesn't have a driver for the interrupt controller
referenced here, what do you expect, other than things not working?

	M.

-- 
Without deviation from the norm, progress is not possible.