linux-kernel - Re: [PATCH] of: property: fw_devlink: Do not use interrupt-parent directly

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Message-ID: <CAGETcx9hLPxtE94cKiM==+Ep8k=d+HP4q3FCMKMbeFAjt7XW7Q@mail.gmail.com>
Date: Thu, 14 Nov 2024 12:24:34 -0800
From: Saravana Kannan <saravanak@...gle.com>
To: Samuel Holland <samuel.holland@...ive.com>, Marc Zyngier <maz@...nel.org>
Cc: Rob Herring <robh@...nel.org>, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] of: property: fw_devlink: Do not use interrupt-parent directly

On Thu, Nov 14, 2024 at 11:56 AM Samuel Holland
<samuel.holland@...ive.com> wrote:
>
> commit 7f00be96f125 ("of: property: Add device link support for
> interrupt-parent, dmas and -gpio(s)") started adding device links for
> the interrupt-parent property. Later, commit f265f06af194 ("of:
> property: Fix fw_devlink handling of interrupts/interrupts-extended")
> added full support for parsing the interrupts and interrupts-extended
> properties, which includes looking up the node of the parent domain.
> This made the handler for the interrupt-parent property redundant.
>
> In fact, creating device links based solely on interrupt-parent is
> problematic, because it can create spurious cycles. A node may have
> this property without itself being an interrupt controller or consumer.
> For example, this property is often present in the root node or a /soc
> bus node to set the default interrupt parent for child nodes. However,
> it is incorrect for the bus to depend on the interrupt controller, as
> some of the bus's childre may not be interrupt consumers at all or may
> have a different interrupt parent.
>
> Resolving these spurious dependency cycles can cause an incorrect probe
> order for interrupt controller drivers. This was observed on a RISC-V
> system with both an APLIC and IMSIC under /soc, where interrupt-parent
> in /soc points to the APLIC, and the APLIC msi-parent points to the
> IMSIC. fw_devlink found three dependency cycles and attempted to probe
> the APLIC before the IMSIC. After applying this patch, there were no
> dependency cycles and the probe order was correct.

Rob/Marc,

If the claim about the interrupt parent interpretation is correct
across the board, I'm ok with this patch.

I remember the RISC-V DT for interrupts being a mess. So, want to make
sure you agree with these claims before I Ack it.

Thanks,
Saravana

>
> Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
> ---
>
>  drivers/of/property.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/drivers/of/property.c b/drivers/of/property.c
> index 11b922fde7af..7bd8390f2fba 100644
> --- a/drivers/of/property.c
> +++ b/drivers/of/property.c
> @@ -1213,7 +1213,6 @@ DEFINE_SIMPLE_PROP(iommus, "iommus", "#iommu-cells")
>  DEFINE_SIMPLE_PROP(mboxes, "mboxes", "#mbox-cells")
>  DEFINE_SIMPLE_PROP(io_channels, "io-channels", "#io-channel-cells")
>  DEFINE_SIMPLE_PROP(io_backends, "io-backends", "#io-backend-cells")
> -DEFINE_SIMPLE_PROP(interrupt_parent, "interrupt-parent", NULL)
>  DEFINE_SIMPLE_PROP(dmas, "dmas", "#dma-cells")
>  DEFINE_SIMPLE_PROP(power_domains, "power-domains", "#power-domain-cells")
>  DEFINE_SIMPLE_PROP(hwlocks, "hwlocks", "#hwlock-cells")
> @@ -1359,7 +1358,6 @@ static const struct supplier_bindings of_supplier_bindings[] = {
>         { .parse_prop = parse_mboxes, },
>         { .parse_prop = parse_io_channels, },
>         { .parse_prop = parse_io_backends, },
> -       { .parse_prop = parse_interrupt_parent, },
>         { .parse_prop = parse_dmas, .optional = true, },
>         { .parse_prop = parse_power_domains, },
>         { .parse_prop = parse_hwlocks, },
> --
> 2.45.1
>