linux-kernel - Re: [PATCH v3] of/irq: Support #msi-cells=<0> in of_msi_get

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Message-ID: <172409282414.2143677.12162426898759329021.robh@kernel.org>
Date: Mon, 19 Aug 2024 12:40:28 -0600
From: "Rob Herring (Arm)" <robh@...nel.org>
To: Andrew Jones <ajones@...tanamicro.com>
Cc: loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org,
	palmer@...belt.com, Anup Patel <apatel@...tanamicro.com>,
	linux-mips@...r.kernel.org, maz@...nel.org,
	linux-riscv@...ts.infradead.org, aou@...s.berkeley.edu,
	mark.rutland@....com, linux-arm-kernel@...ts.infradead.org,
	saravanak@...gle.com, paul.walmsley@...ive.com,
	devicetree@...r.kernel.org
Subject: Re: [PATCH v3] of/irq: Support #msi-cells=<0> in of_msi_get_domain


On Sat, 17 Aug 2024 09:41:08 +0200, Andrew Jones wrote:
> An 'msi-parent' property with a single entry and no accompanying
> '#msi-cells' property is considered the legacy definition as opposed
> to its definition after being expanded with commit 126b16e2ad98
> ("Docs: dt: add generic MSI bindings"). However, the legacy
> definition is completely compatible with the current definition and,
> since of_phandle_iterator_next() tolerates missing and present-but-
> zero *cells properties since commit e42ee61017f5 ("of: Let
> of_for_each_phandle fallback to non-negative cell_count"), there's no
> need anymore to special case the legacy definition in
> of_msi_get_domain().
> 
> Indeed, special casing has turned out to be harmful, because, as of
> commit 7c025238b47a ("dt-bindings: irqchip: Describe the IMX MU block
> as a MSI controller"), MSI controller DT bindings have started
> specifying '#msi-cells' as a required property (even when the value
> must be zero) as an effort to make the bindings more explicit. But,
> since the special casing of 'msi-parent' only uses the existence of
> '#msi-cells' for its heuristic, and not whether or not it's also
> nonzero, the legacy path is not taken. Furthermore, the path to
> support the new, broader definition isn't taken either since that
> path has been restricted to the platform-msi bus.
> 
> But, neither the definition of 'msi-parent' nor the definition of
> '#msi-cells' is platform-msi-specific (the platform-msi bus was just
> the first bus that needed '#msi-cells'), so remove both the special
> casing and the restriction. The code removal also requires changing
> to of_parse_phandle_with_optional_args() in order to ensure the
> legacy (but compatible) use of 'msi-parent' remains supported. This
> not only simplifies the code but also resolves an issue with PCI
> devices finding their MSI controllers on riscv, as the riscv,imsics
> binding requires '#msi-cells=<0>'.
> 
> Signed-off-by: Andrew Jones <ajones@...tanamicro.com>
> ---
> v3:
>  - switch to of_for_each_phandle() to further cleanup/simplify the
>    code [Rob]
> v2:
>  - switch to of_parse_phandle_with_optional_args() to ensure the
>    absence of #msi-cells means count=0
> 
>  drivers/of/irq.c | 35 ++++++++---------------------------
>  1 file changed, 8 insertions(+), 27 deletions(-)
> 

Applied, thanks!