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

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Message-ID: <CAL_Jsq+OHqoo9Lxpw5GE0315dmjQPvRo60=PsJXCx=heOfmBNw@mail.gmail.com>
Date: Tue, 16 Jul 2024 09:53:15 -0600
From: Rob Herring <robh@...nel.org>
To: Greg KH <greg@...ah.com>
Cc: Danilo Krummrich <dakr@...nel.org>, Danilo Krummrich <dakr@...hat.com>, 
	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>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 8/8] cpufreq: Add Rust based cpufreq-dt driver

On Tue, Jul 16, 2024 at 9:22 AM Greg KH <greg@...ah.com> wrote:
>
> On Tue, Jul 16, 2024 at 05:15:25PM +0200, Danilo Krummrich wrote:
> > On Thu, Jul 11, 2024 at 06:34:22PM +0200, Greg KH wrote:
> > > 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.
> >
> > For those drivers, wouldn't it be better if proper devices would be derived from
> > the CPU OF nodes directly? This seems to be a common problem for cpuidle and
> > cpufreq drivers.
>
> Yes they should.

Well, which one do we bind? The cpufreq driver or cpuidle driver? Or
there's the thermal f/w throttling as well. It's messy. Also, the CPUs
already have a struct device associated with them for the topology
stuff, but no driver IIRC.

Another complication is it is not the CPU that determines what
cpufreq/cpuidle drivers to use, but a platform decision. That decision
may evolve as well which means it can't be driven from the DT.

> > But it's quite a while ago I dealt with such drivers, maybe there are reasons
> > not to do so.
>
> I think people just got lazy :)

Virtual device was probably the right thing given there isn't directly
any device we are controlling/programming. This driver is just built
on top of other subsystems (clock and regulator).

Rob