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

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Message-ID: <2024071111-negotiate-spoof-da94@gregkh>
Date: Thu, 11 Jul 2024 18:34:22 +0200
From: Greg KH <greg@...ah.com>
To: Danilo Krummrich <dakr@...hat.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 06:12:08PM +0200, Danilo Krummrich wrote:
> 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

Yes, these are abuses of that and should be virtual devices as they have
nothing to do with the platform bus.

> > > 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.

Great, use the default one that the kernel gives you :)

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

Again, abuse, please do not do so.

> 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

Use the virtual device interface please, that's what it is there for.

thanks,

greg k-h