Re: [PATCH 2/2] rust/kernel: Add platform::ModuleDevice

[Lyude Paul <lyude@redhat.com>]

So digging further at this I can already tell that the example I found still
wasn't entirely what we want - while not mentioned in any documentation I can
find (correct me if I am wrong), work was done to make it possible to allow
classes to be specified in read-only memory - so I'll make sure that the
bindings we have for this in rust only allow this as well.

I'll also go ahead and add the missing mention of this in class_create(), and
look into writing the totally missing documentation for class_register() which
appears to be the most likely to be the correct way of doing this.

On Thu, 2025-01-23 at 19:33 -0500, Lyude Paul wrote:
> On Thu, 2025-01-23 at 07:23 +0100, Greg Kroah-Hartman wrote:
> > On Wed, Jan 22, 2025 at 06:49:22PM -0500, Lyude Paul wrote:
> > > A number of kernel modules work with virtual devices, where being virtual
> > > implies that there's no physical device to actually be plugged into the
> > > system. Because of that, such modules need to be able to manually
> > > instantiate a kernel device themselves - which can then be probed in the
> > > same manner as any other kernel device.
> > > 
> > > This adds support for such a usecase by introducing another platform device
> > > type, ModuleDevice. This type is interchangeable with normal platform
> > > devices, with the one exception being that it controls the lifetime of the
> > > registration of the device.
> > 
> > Sorry, but a "virtual" device is NOT a platform device at all.  Platform
> > devices are things that are not on a real bus and are described by
> > firmware somehow.
> > 
> > The kernel has "virtual" devices today just fine, look at
> > /sys/devices/virtual/ so why not just use that api instead of making up
> > something new?
> 
> Honestly I never even knew this was a thing! Taking a closer look though I can
> see why - unless I'm missing something it feels like /sys/devices/virtual is
> pretty much the only indication this exists (and lots of examples of platform
> devices being used for this purpose: vkms, vgem, virmidi, etc.). The actual
> platform bus documentation doesn't even really suggest it exists, it mostly
> just discourages legacy style device probing and doesn't really provide an
> alternative / warning that "platform devices should be real".
> 
> That being said though I'm more then happy to add something for virtual
> devices, looking at drivers/gpu/drm/display/drm_dp_aux_dev.c this doesn't seem
> too difficult to write bindings for. I'm also happy to amend the platform
> device documentation to mention this, and maybe add a document describing the
> process for creating virtual devices that we can link back to in the platform
> documentation as the recommended alternative to abusing platform devices. This
> misunderstanding seems to happen often enough in the kernel I'd expect that to
> be the best spot to mention it.
> 
> > 
> > And modules are code, not data.  Let's not start to even attempt to tie
> > lifetimes of device structures to module code, that way lies madness and
> > rolls back the work we did decades ago to split the two apart :)
> 
> To make sure I'm understanding you properly, by "are code not data" you're
> suggesting that resources (devices, driver registrations, etc.) should have
> their lifetime entirely managed by the kernel and not as part of the module
> data structure correct?
> 
> If so - I do agree! I actually tried to reduce tying resources to the module
> as much as possible, previously everything (device, sysfs resources for the
> device, etc.) were all explicit resources managed by the module. Obviously if
> we can do better then that (and it sounds like we can) I'm happy to. I do want
> to make sure I'm not misunderstanding something here though, because looking
> at the way this works in drm_dp_aux_dev.c the process seems to be like this:
> 
> (excuse me if some terminology is wrong, it has been ages since I looked at
> this portion of the kernel)
> 
>  * Create a device class (in this case, drm_dp_aux_dev), this gives us a home
>    in /sys/devices/virtual using class_create(). The device class appears to
>    be tied to the lifetime of the module declared in
>    drivers/gpu/drm/display/drm_display_helper_mod.c
>  * Create a char dev using register_chrdev() with major = 0, which gives us a
>    dynamically allocated major device number to use
>  * Actual device creation is handled by DRM devices calling
>    drm_dp_aux_dev_alloc(), using the DRM device as the parent device
> 
> Resulting in the device class and chrdev being tied to the lifetime of the
> drm_display_helper module, and the aux devices being tied to the lifetime of
> their respective DRM parent devices.
> 
> Since we don't have another device to rely on as a parent though, we'd need to
> use the kernel parent device that you mentioned. That's fine, but unless
> there's some way to annotate the module so the kernel knows we need a device
> created then that still implies creating a struct that destroys the device on
> drop. In other words, the lifetime of the device is still tied to the module
> except with extra steps.
> 
> Don't get me wrong, I totally agree using virtual devices seems better then
> using platform devices here! It's just I'm not sure where you're seeing the
> lifetime distinction here with virtual devices vs. platform devices. Any kind
> of resource you allocate in rust code is going to have a structure that
> represents its lifetime, unless something else creates the resource for us -
> e.g. a PCI device being created by the kernel as the result of a bus being
> probed. Is there something like that we could take advantage of here, some
> sort of module annotation maybe?
> 
> Some more comments below:
> 
> > > 
> > > Signed-off-by: Lyude Paul <lyude@redhat.com>
> > > Co-authored-by: Maíra Canal <mairacanal@riseup.net>
> > > ---
> > >  rust/kernel/platform.rs | 96 ++++++++++++++++++++++++++++++++++++++++-
> > >  1 file changed, 94 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/rust/kernel/platform.rs b/rust/kernel/platform.rs
> > > index 75dc7824eccf4..b5d38bb182e93 100644
> > > --- a/rust/kernel/platform.rs
> > > +++ b/rust/kernel/platform.rs
> > > @@ -13,8 +13,11 @@
> > >      types::{ARef, ForeignOwnable, Opaque},
> > >      ThisModule,
> > >  };
> > > -
> > > -use core::ptr::{NonNull, addr_of_mut};
> > > +use core::{
> > > +    mem::ManuallyDrop,
> > > +    ops::*,
> > > +    ptr::{addr_of_mut, NonNull},
> > > +};
> > >  
> > >  /// An adapter for the registration of platform drivers.
> > >  pub struct Adapter<T: Driver>(T);
> > > @@ -213,3 +216,92 @@ fn as_ref(&self) -> &device::Device {
> > >          &self.0
> > >      }
> > >  }
> > > +
> > > +/// A platform device ID specifier.
> > > +///
> > > +/// This type is used for selecting the kind of device ID to use when constructing a new
> > > +/// [`ModuleDevice`].
> > > +#[derive(Copy, Clone)]
> > > +pub enum ModuleDeviceId {
> > > +    /// Do not use a device ID with a device.
> > > +    None,
> > > +    /// Automatically allocate a device ID for a device.
> > > +    Auto,
> > > +    /// Explicitly specify a device ID for a device.
> > > +    Explicit(i32),
> > > +}
> > > +
> > > +impl ModuleDeviceId {
> > > +    fn as_raw(self) -> Result<i32> {
> > > +        match self {
> > > +            ModuleDeviceId::Explicit(id) => {
> > > +                if matches!(
> > > +                    id,
> > > +                    bindings::PLATFORM_DEVID_NONE | bindings::PLATFORM_DEVID_AUTO
> > > +                ) {
> > > +                    Err(EINVAL)
> > > +                } else {
> > > +                    Ok(id)
> > > +                }
> > > +            }
> > > +            ModuleDeviceId::None => Ok(bindings::PLATFORM_DEVID_NONE),
> > > +            ModuleDeviceId::Auto => Ok(bindings::PLATFORM_DEVID_AUTO),
> > > +        }
> > > +    }
> > > +}
> > > +
> > > +/// A platform device that was created by a module.
> > 
> > Again, no, sorry.
> > 
> > If you want a virtual device, make a virtual device.  Ideally using the
> > auxbus api, but if you REALLY want a "raw" struct device, wonderful,
> > create that and register it with the driver core which will throw it
> > under /sys/devices/virtual/ which is where it belongs.
> > 
> > But I don't think you want that, what you want is all the power of a
> > real bus, but none of the hassle, which is why people abuse the platform
> > device code so much.  And why I complain about it so much.
> 
> Keeping in mind this is a virtual device and there is no conceivable bus, what
> exactly are you asking for here? Just that we register a virtual bus with the
> kernel and then simulate an event to say our virtual device has been plugged
> into it? That would be fine with me if you have examples.
> 
> > 
> > So, sorry, I am not going to allow the abuse of the platform device code
> > to carry over into rust drivers if I can possibly help it.  Make a real
> > bus.  Or a raw device.  Or use the busses you have today (again,
> > auxbus), but do NOT abuse platform devices for things they are not.
> 
> Just a heads up: you're not going to get an argument on this now or ever ;).
> I'm not interested in abusing APIs or the easiest solution, being a kernel
> maintainer myself I'm pretty intimately familiar with how quickly technical
> debt from that stacks up and makes things difficult for everyone - so I'm
> always more interested in a correct solution then a lazy one.
> 
> But the correct solution in a codebase as large as the kernel with as many
> incorrect historical examples as the kernel has is not always obvious, and
> just like I'm doing now I am relying on feedback from people like yourself to
> know if I'm going in the wrong direction. I wouldn't be happy with any kind of
> interface in rust that uses core kernel APIs wrong, it goes against the whole
> purpose of us codifying all of this in rust in the first place.
> 
> So, I'm saying this just to emphasize I'm more then happy to work with you and
> everyone here to make sure we get this right the first time, and I'm even
> happy to amend documentation/write examples/etc. to make your job easier so
> you don't have to point this out to people all the time. The whole benefit of
> RFL should be making correct solutions more obvious to any kernel developer so
> we can get a future where you don't have to spend as much time preventing API
> abuse. But until then, don't be surprised if every now and then a suggestion
> of "this alternative would be better" is needed :). This is uncharted
> territory for me but I'm always happy to listen to reason.
> 
> > 
> > thanks,
> > 
> > greg k-h
> > 
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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