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

[Rob Herring]

On Tue, Jul 16, 2024 at 9:22 AM Greg KH <greg@xxxxxxxxx> 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