Re: [PATCH] checks: Warn about missing #address-cells for interrupt parents

On Tue, Dec 17, 2024 at 8:01 PM Brad Griffis <[email protected]> wrote:
>
> > From: Thierry Reding <[email protected]>
> >
> > The device tree specification (v0.4) suggests that #address-cells is
> > mandatory for interrupt parent nodes. If this property is missing, Linux
> > will default to the value of 0.
>
> Just to clarify, this relates to interrupt-map specifically.

Yes. That is the only case that really cares and needs it. We used to
warn on all interrupt-controller nodes if #address-cells was missing.
But that turns out to be way too many cases to fix, so we dropped that
part.

> In that
> scenario the device tree spec requires that both the child node and
> parent node specify #address-cells and #interrupt-cells. It further
> specifies if a unit address component is not needed then it must be
> explicitly defined as zero. In other words, this does not seem to be
> just a suggestion, but more of a firm requirement.

unit-address being 0 and #address-cells being 0 are 2 different things.

> > A number of device tree files rely on Linux' fallback and don't specify
> > an explicit #address-cells as suggested by the specification. This can
> > cause issues when these device trees are passed to software with a more
> > pedantic interpretation of the DT spec.
>
> The device tree spec also says that in the case where #address-cells is
> not specified that a value of 2 should be assumed. So in this context, I
> find the kernel's current practice of assuming #address-cells = <0> also
> violates the spec.

Relying on defaults is not good practice and deprecated for longer
than we've had DT support on Arm. The default for the kernel on PPC is
1 which doesn't agree with dtc nor OpenFirmware. The kernel walking up
all parents to find #address-cells/#size-cells is not behavior defined
in any spec either.

6.13 now warns if parent nodes or defaults are used, but not in the
case of interrupt parsing.

> > Add a warning when this case is detected so that device tree files can
> > be fixed.
>
> I think a warning is reasonable, but perhaps we should consider making
> it an outright error. Though given the number of impacted device trees,
> perhaps that needs to be done in a couple of steps.

I'm more inclined to change the spec if needed. We only need
#address-cells if we have interrupt-map, so that's all we should
check.

If a given binding wants to be stricter, we can do that in its schema.

Rob