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

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Message-ID: <1f78898b-f703-4fd9-8f68-c0835a85de9e@sifive.com>
Date: Tue, 19 Nov 2024 10:46:59 -0600
From: Samuel Holland <samuel.holland@...ive.com>
To: Rob Herring <robh@...nel.org>
Cc: Saravana Kannan <saravanak@...gle.com>, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] of: property: fw_devlink: Do not use interrupt-parent
 directly

Hi Rob,

On 2024-11-19 9:41 AM, Rob Herring wrote:
> On Thu, Nov 14, 2024 at 11:56:49AM -0800, Samuel Holland 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
> 
> typo
> 
>> 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.
>>
>> Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
> 
> I assume this should go to stable? It needs Fixes tags.

What commit should I put in the Fixes tag? f265f06af194 ("of: property: Fix
fw_devlink handling of interrupts/interrupts-extended"), because it finished
making this code redundant? That commit didn't introduce any new bugs--this code
was always wrong--but I would be hesitant to backport this change any further,
because it might cause regressions without the "interrupts" property parsing in
place.

Regards,
Samuel

> Otherwise, the change makes sense to me.
> 
>> ---
>>
>>  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
>>