linux-kernel - Re: [PATCH V4 8/8] cpufreq: Add Rust based cpufreq-dt driver

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Message-ID: <ZpAEWAzETnrVI-cs@pollux>
Date: Thu, 11 Jul 2024 18:12:08 +0200
From: Danilo Krummrich <dakr@...hat.com>
To: Greg KH <greg@...ah.com>
Cc: Viresh Kumar <viresh.kumar@...aro.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>, linux-pm@...r.kernel.org,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Stephen Boyd <sboyd@...nel.org>, Nishanth Menon <nm@...com>,
	rust-for-linux@...r.kernel.org,
	Manos Pitsidianakis <manos.pitsidianakis@...aro.org>,
	Erik Schilling <erik.schilling@...aro.org>,
	Alex Bennée <alex.bennee@...aro.org>,
	Joakim Bech <joakim.bech@...aro.org>, Rob Herring <robh@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 8/8] cpufreq: Add Rust based cpufreq-dt driver

On Thu, Jul 11, 2024 at 04:37:50PM +0200, Greg KH wrote:
> On Thu, Jul 11, 2024 at 03:21:31PM +0200, Danilo Krummrich wrote:
> > (2) You require drivers to always implement a "dummy" struct platform_device,
> > there is platform_device_register_simple() for that purpose.
> 
> No, NEVER do that.  platform devices are only for real platform devices,
> do not abuse that interface any more than it already is.

I thought we're talking about cases like [1] or [2], but please correct me if
those are considered abusing the platform bus as well.

(Those drivers read the CPU OF nodes, instead of OF nodes that represent a
separate device.)

[1] https://elixir.bootlin.com/linux/latest/source/drivers/cpuidle/cpuidle-riscv-sbi.c#L586
[2] https://elixir.bootlin.com/linux/latest/source/drivers/cpuidle/cpuidle-psci.c#L441

> 
> > I think (2) is the preferred option.
> 
> No, not at all, sorry.
> 
> Use a real device, you have one somewhere that relates to this hardware,
> otherwise you aren't controlling anything and then you can use a virtual
> device.

Of course we should stick to a real device if there is one, I didn't meant to
say anything else.

But since it came up now, some virtual drivers still require a parent device.

For instance, in DRM we have vGEM [3] and vKMS [4], that use
platform_device_register_simple() for this purpose.

What should they use instead? I'm happy to fix things up if required.

[3] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/vgem/vgem_drv.c
[4] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/vkms/vkms_drv.c

> 
> Again, do NOT abuse the platform subsystem.  It's one reason I am loath
> to even want to allow rust bindings to it.

How is that related to Rust?

> 
> greg k-h
>