Re: [PATCH v3 15/16] rust: platform: add basic platform device / driver abstractions

[Daniel Almeida]

Hi Danilo,

> On 22 Oct 2024, at 18:31, Danilo Krummrich <dakr@xxxxxxxxxx> wrote:
> 
> Implement the basic platform bus abstractions required to write a basic
> platform driver. This includes the following data structures:
> 
> The `platform::Driver` trait represents the interface to the driver and
> provides `pci::Driver::probe` for the driver to implement.
> 
> The `platform::Device` abstraction represents a `struct platform_device`.
> 
> In order to provide the platform bus specific parts to a generic
> `driver::Registration` the `driver::RegistrationOps` trait is implemented
> by `platform::Adapter`.
> 
> Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>
> ---
> MAINTAINERS                     |   1 +
> rust/bindings/bindings_helper.h |   1 +
> rust/helpers/helpers.c          |   1 +
> rust/helpers/platform.c         |  13 ++
> rust/kernel/lib.rs              |   1 +
> rust/kernel/platform.rs         | 217 ++++++++++++++++++++++++++++++++
> 6 files changed, 234 insertions(+)
> create mode 100644 rust/helpers/platform.c
> create mode 100644 rust/kernel/platform.rs
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 87eb9a7869eb..173540375863 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -6985,6 +6985,7 @@ F: rust/kernel/device.rs
> F: rust/kernel/device_id.rs
> F: rust/kernel/devres.rs
> F: rust/kernel/driver.rs
> +F: rust/kernel/platform.rs
> 
> DRIVERS FOR OMAP ADAPTIVE VOLTAGE SCALING (AVS)
> M: Nishanth Menon <nm@xxxxxx>
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 312f03cbdce9..217c776615b9 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -18,6 +18,7 @@
> #include <linux/of_device.h>
> #include <linux/pci.h>
> #include <linux/phy.h>
> +#include <linux/platform_device.h>
> #include <linux/refcount.h>
> #include <linux/sched.h>
> #include <linux/slab.h>
> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> index 8bc6e9735589..663cdc2a45e0 100644
> --- a/rust/helpers/helpers.c
> +++ b/rust/helpers/helpers.c
> @@ -17,6 +17,7 @@
> #include "kunit.c"
> #include "mutex.c"
> #include "page.c"
> +#include "platform.c"
> #include "pci.c"
> #include "rbtree.c"
> #include "rcu.c"
> diff --git a/rust/helpers/platform.c b/rust/helpers/platform.c
> new file mode 100644
> index 000000000000..ab9b9f317301
> --- /dev/null
> +++ b/rust/helpers/platform.c
> @@ -0,0 +1,13 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/platform_device.h>
> +
> +void *rust_helper_platform_get_drvdata(const struct platform_device *pdev)
> +{
> + return platform_get_drvdata(pdev);
> +}
> +
> +void rust_helper_platform_set_drvdata(struct platform_device *pdev, void *data)
> +{
> + platform_set_drvdata(pdev, data);
> +}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 5946f59f1688..9e8dcd6d7c01 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -53,6 +53,7 @@
> pub mod net;
> pub mod of;
> pub mod page;
> +pub mod platform;
> pub mod prelude;
> pub mod print;
> pub mod rbtree;
> diff --git a/rust/kernel/platform.rs b/rust/kernel/platform.rs
> new file mode 100644
> index 000000000000..addf5356f44f
> --- /dev/null
> +++ b/rust/kernel/platform.rs
> @@ -0,0 +1,217 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Abstractions for the platform bus.
> +//!
> +//! C header: [`include/linux/platform_device.h`](srctree/include/linux/platform_device.h)
> +
> +use crate::{
> +    bindings, container_of, device,
> +    device_id::RawDeviceId,
> +    driver,
> +    error::{to_result, Result},
> +    of,
> +    prelude::*,
> +    str::CStr,
> +    types::{ARef, ForeignOwnable},
> +    ThisModule,
> +};
> +
> +/// An adapter for the registration of platform drivers.
> +pub struct Adapter<T: Driver>(T);
> +
> +impl<T: Driver + 'static> driver::RegistrationOps for Adapter<T> {
> +    type RegType = bindings::platform_driver;
> +
> +    fn register(
> +        pdrv: &mut Self::RegType,
> +        name: &'static CStr,
> +        module: &'static ThisModule,
> +    ) -> Result {
> +        pdrv.driver.name = name.as_char_ptr();
> +        pdrv.probe = Some(Self::probe_callback);
> +
> +        // Both members of this union are identical in data layout and semantics.
> +        pdrv.__bindgen_anon_1.remove = Some(Self::remove_callback);
> +        pdrv.driver.of_match_table = T::ID_TABLE.as_ptr();
> +
> +        // SAFETY: `pdrv` is guaranteed to be a valid `RegType`.
> +        to_result(unsafe { bindings::__platform_driver_register(pdrv, module.0) })
> +    }
> +
> +    fn unregister(pdrv: &mut Self::RegType) {
> +        // SAFETY: `pdrv` is guaranteed to be a valid `RegType`.
> +        unsafe { bindings::platform_driver_unregister(pdrv) };
> +    }
> +}
> +
> +impl<T: Driver + 'static> Adapter<T> {
> +    fn id_info(pdev: &Device) -> Option<&'static T::IdInfo> {
> +        let table = T::ID_TABLE;
> +        let id = T::of_match_device(pdev)?;
> +
> +        Some(table.info(id.index()))
> +    }
> +
> +    extern "C" fn probe_callback(pdev: *mut bindings::platform_device) -> core::ffi::c_int {
> +        // SAFETY: The platform bus only ever calls the probe callback with a valid `pdev`.
> +        let dev = unsafe { device::Device::from_raw(&mut (*pdev).dev) };
> +        // SAFETY: `dev` is guaranteed to be embedded in a valid `struct platform_device` by the
> +        // call above.
> +        let mut pdev = unsafe { Device::from_dev(dev) };
> +
> +        let info = Self::id_info(&pdev);
> +        match T::probe(&mut pdev, info) {
> +            Ok(data) => {
> +                // Let the `struct platform_device` own a reference of the driver's private data.
> +                // SAFETY: By the type invariant `pdev.as_raw` returns a valid pointer to a
> +                // `struct platform_device`.
> +                unsafe { bindings::platform_set_drvdata(pdev.as_raw(), data.into_foreign() as _) };
> +            }
> +            Err(err) => return Error::to_errno(err),
> +        }
> +
> +        0
> +    }
> +
> +    extern "C" fn remove_callback(pdev: *mut bindings::platform_device) {
> +        // SAFETY: `pdev` is a valid pointer to a `struct platform_device`.
> +        let ptr = unsafe { bindings::platform_get_drvdata(pdev) };
> +
> +        // SAFETY: `remove_callback` is only ever called after a successful call to
> +        // `probe_callback`, hence it's guaranteed that `ptr` points to a valid and initialized
> +        // `KBox<T>` pointer created through `KBox::into_foreign`.
> +        let _ = unsafe { KBox::<T>::from_foreign(ptr) };
> +    }
> +}
> +
> +/// Declares a kernel module that exposes a single platform driver.
> +///
> +/// # Examples
> +///
> +/// ```ignore
> +/// kernel::module_platform_driver! {
> +///     type: MyDriver,
> +///     name: "Module name",
> +///     author: "Author name",
> +///     description: "Description",
> +///     license: "GPL v2",
> +/// }
> +/// ```
> +#[macro_export]
> +macro_rules! module_platform_driver {
> +    ($($f:tt)*) => {
> +        $crate::module_driver!(<T>, $crate::platform::Adapter<T>, { $($f)* });
> +    };
> +}
> +
> +/// IdTable type for platform drivers.
> +pub type IdTable<T> = &'static dyn kernel::device_id::IdTable<of::DeviceId, T>;
> +
> +/// The platform driver trait.
> +///
> +/// # Example
> +///
> +///```
> +/// # use kernel::{bindings, c_str, of, platform};
> +///
> +/// struct MyDriver;
> +///
> +/// kernel::of_device_table!(
> +///     OF_TABLE,
> +///     MODULE_OF_TABLE,
> +///     <MyDriver as platform::Driver>::IdInfo,
> +///     [
> +///         (of::DeviceId::new(c_str!("redhat,my-device")), ())
> +///     ]
> +/// );
> +///
> +/// impl platform::Driver for MyDriver {
> +///     type IdInfo = ();
> +///     const ID_TABLE: platform::IdTable<Self::IdInfo> = &OF_TABLE;
> +///
> +///     fn probe(
> +///         _pdev: &mut platform::Device,
> +///         _id_info: Option<&Self::IdInfo>,
> +///     ) -> Result<Pin<KBox<Self>>> {
> +///         Err(ENODEV)
> +///     }
> +/// }
> +///```
> +/// Drivers must implement this trait in order to get a platform driver registered. Please refer to
> +/// the `Adapter` documentation for an example.
> +pub trait Driver {
> +    /// The type holding information about each device id supported by the driver.
> +    ///
> +    /// TODO: Use associated_type_defaults once stabilized:
> +    ///
> +    /// type IdInfo: 'static = ();
> +    type IdInfo: 'static;
> +
> +    /// The table of device ids supported by the driver.
> +    const ID_TABLE: IdTable<Self::IdInfo>;
> +
> +    /// Platform driver probe.
> +    ///
> +    /// Called when a new platform device is added or discovered.
> +    /// Implementers should attempt to initialize the device here.
> +    fn probe(dev: &mut Device, id_info: Option<&Self::IdInfo>) -> Result<Pin<KBox<Self>>>;
> +
> +    /// Find the [`of::DeviceId`] within [`Driver::ID_TABLE`] matching the given [`Device`], if any.
> +    fn of_match_device(pdev: &Device) -> Option<&of::DeviceId> {
> +        let table = Self::ID_TABLE;
> +
> +        // SAFETY:
> +        // - `table` has static lifetime, hence it's valid for read,
> +        // - `dev` is guaranteed to be valid while it's alive, and so is
> +        //   `pdev.as_dev().as_raw()`.
> +        let raw_id = unsafe { bindings::of_match_device(table.as_ptr(), pdev.as_dev().as_raw()) };
> +
> +        if raw_id.is_null() {
> +            None
> +        } else {
> +            // SAFETY: `DeviceId` is a `#[repr(transparent)` wrapper of `struct of_device_id` and
> +            // does not add additional invariants, so it's safe to transmute.
> +            Some(unsafe { &*raw_id.cast::<of::DeviceId>() })
> +        }
> +    }
> +}
> +
> +/// The platform device representation.
> +///
> +/// A platform device is based on an always reference counted `device:Device` instance. Cloning a
> +/// platform device, hence, also increments the base device' reference count.
> +///
> +/// # Invariants
> +///
> +/// `Device` holds a valid reference of `ARef<device::Device>` whose underlying `struct device` is a
> +/// member of a `struct platform_device`.
> +#[derive(Clone)]
> +pub struct Device(ARef<device::Device>);
> +
> +impl Device {
> +    /// Convert a raw kernel device into a `Device`
> +    ///
> +    /// # Safety
> +    ///
> +    /// `dev` must be an `Aref<device::Device>` whose underlying `bindings::device` is a member of a
> +    /// `bindings::platform_device`.
> +    unsafe fn from_dev(dev: ARef<device::Device>) -> Self {
> +        Self(dev)
> +    }
> +
> +    fn as_dev(&self) -> &device::Device {

This has to be pub, since a platform::Device is at least as useful as a device::Device.

IOW: if an API takes &device::Device, there is no reason why someone with a &platform::Device
shouldn’t be able to call it.

In particular, having this as private makes it impossible for a platform driver to use Abdiel’s DMA allocator at [0].

> +        &self.0
> +    }
> +
> +    fn as_raw(&self) -> *mut bindings::platform_device {
> +        // SAFETY: By the type invariant `self.0.as_raw` is a pointer to the `struct device`
> +        // embedded in `struct platform_device`.
> +        unsafe { container_of!(self.0.as_raw(), bindings::platform_device, dev) }.cast_mut()
> +    }
> +}
> +
> +impl AsRef<device::Device> for Device {
> +    fn as_ref(&self) -> &device::Device {
> +        &self.0
> +    }
> +}
> -- 
> 2.46.2
> 

— Daniel

[0]: https://lkml.org/lkml/2024/12/3/1281