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

[Greg KH]

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