linux-kernel - Re: [PATCH v2 3/3] mfd: syscon: Allow syscon nodes without a "syscon" compatible

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Message-ID: <Z2HTAJmBeIUlWysh@google.com>
Date: Tue, 17 Dec 2024 11:37:36 -0800
From: William McVicker <willmcvicker@...gle.com>
To: "Rob Herring (Arm)" <robh@...nel.org>
Cc: Lee Jones <lee@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	Pankaj Dubey <pankaj.dubey@...sung.com>,
	Heiko Stuebner <heiko@...ech.de>, Liviu Dudau <liviu.dudau@....com>,
	Sudeep Holla <sudeep.holla@....com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Peter Griffin <peter.griffin@...aro.org>,
	John Madieu <john.madieu.xa@...renesas.com>,
	Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 3/3] mfd: syscon: Allow syscon nodes without a
 "syscon" compatible

On 12/17/2024, Rob Herring (Arm) wrote:
> of_syscon_register_regmap() was added for nodes which need a custom
> regmap setup. It's not really correct for those nodes to claim they are
> compatible with "syscon" as the default handling likely doesn't work in
> those cases. If device_node_get_regmap() happens to be called first,
> then of_syscon_register() will be called and an incorrect regmap will be
> created (barring some other error). That may lead to unknown results in
> the worst case. In the best case, of_syscon_register_regmap() will fail
> with -EEXIST. This problem remains unless these cases drop "syscon" (an
> ABI issue) or we exclude them using their specific compatible. ATM,
> there is only one user: "google,gs101-pmu"
> 
> There are also cases of adding "syscon" compatible to existing nodes
> after the fact in order to register the syscon. That presents a
> potential DT ABI problem. Instead, if there's a kernel change needing a
> syscon for a node, then it should be possible to allow the kernel to
> register a syscon without a DT change. That's only possible by using
> of_syscon_register_regmap() currently, but in the future we may want to
> support a match list for cases which don't need a custom regmap.
> 
> With this change, the lookup functions will succeed for any node
> registered by of_syscon_register_regmap() regardless of whether the node
> compatible contains "syscon".
> 
> Signed-off-by: Rob Herring (Arm) <robh@...nel.org>

I verified this works on my Pixel 6. Thanks!

Tested-by: Will McVicker <willmcvicker@...gle.com>

Thanks,
Will