Re: [PATCH v5 2/4] rust/drm: Don't setup private driver data until registration

[Daniel Almeida <daniel.almeida@collabora.com>]

Hi Lyude,

> On 30 Jan 2026, at 21:13, Lyude Paul <lyude@redhat.com> wrote:
> 
> Now that we have a DeviceContext that we can use to represent whether a
> Device is known to have been registered, we can make it so that drivers can
> create their Devices but wait until the registration phase to assign their
> private data to the Device. This is desirable as some drivers need to make
> use of the DRM device early on before finalizing their private driver data.
> 
> As such, this change makes it so that the driver's private data can
> currently only be accessed through Device<T, Registered> types and not
> Device<T, Uninit>.
> 
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> ---
> V4:
> * Remove accidental double-aliasing &mut in Device::release()
> ---
> drivers/gpu/drm/nova/driver.rs |  4 +--
> drivers/gpu/drm/tyr/driver.rs  |  4 +--
> rust/kernel/drm/device.rs      | 64 +++++++++++++++++++++-------------
> rust/kernel/drm/driver.rs      | 19 ++++++++--
> 4 files changed, 59 insertions(+), 32 deletions(-)
> 
> diff --git a/drivers/gpu/drm/nova/driver.rs b/drivers/gpu/drm/nova/driver.rs
> index 99d6841b69cbc..8cea5f68c3b04 100644
> --- a/drivers/gpu/drm/nova/driver.rs
> +++ b/drivers/gpu/drm/nova/driver.rs
> @@ -56,8 +56,8 @@ impl auxiliary::Driver for NovaDriver {
>     fn probe(adev: &auxiliary::Device<Core>, _info: &Self::IdInfo) -> impl PinInit<Self, Error> {
>         let data = try_pin_init!(NovaData { adev: adev.into() });
> 
> -        let drm = drm::UnregisteredDevice::<Self>::new(adev.as_ref(), data)?;
> -        let drm = drm::Registration::new_foreign_owned(drm, adev.as_ref(), 0)?;
> +        let drm = drm::UnregisteredDevice::<Self>::new(adev.as_ref())?;
> +        let drm = drm::Registration::new_foreign_owned(drm, adev.as_ref(), data, 0)?;
> 
>         Ok(Self { drm: drm.into() })
>     }
> diff --git a/drivers/gpu/drm/tyr/driver.rs b/drivers/gpu/drm/tyr/driver.rs
> index 0d479b5d7bd62..b574ee1c24587 100644
> --- a/drivers/gpu/drm/tyr/driver.rs
> +++ b/drivers/gpu/drm/tyr/driver.rs
> @@ -133,8 +133,8 @@ fn probe(
>                 gpu_info,
>         });
> 
> -        let tdev = drm::UnregisteredDevice::<Self>::new(pdev.as_ref(), data)?;
> -        let tdev = drm::driver::Registration::new_foreign_owned(tdev, pdev.as_ref(), 0)?;
> +        let tdev = drm::UnregisteredDevice::<Self>::new(pdev.as_ref())?;
> +        let tdev = drm::driver::Registration::new_foreign_owned(tdev, pdev.as_ref(), data, 0)?;
> 
>         let driver = TyrDriver {
>             _device: tdev.into(),
> diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
> index 8f1780b8f0196..a1d9b0e92f3fd 100644
> --- a/rust/kernel/drm/device.rs
> +++ b/rust/kernel/drm/device.rs
> @@ -26,13 +26,15 @@
> };
> use core::{
>     alloc::Layout,
> +    cell::UnsafeCell,
>     marker::PhantomData,
> -    mem,
> -    ops::Deref,
> -    ptr::{
> +    mem::{
>         self,
> -        NonNull, //
> +        MaybeUninit, //
>     },
> +    ops::Deref,
> +    ptr::NonNull,
> +    sync::atomic::*,
> };
> 
> #[cfg(CONFIG_DRM_LEGACY)]
> @@ -141,7 +143,7 @@ impl DeviceContext for Uninit {}
> ///
> /// The device in `self.0` is guaranteed to be a newly created [`Device`] that has not yet been
> /// registered with userspace until this type is dropped.
> -pub struct UnregisteredDevice<T: drm::Driver>(ARef<Device<T, Uninit>>, NotThreadSafe);
> +pub struct UnregisteredDevice<T: drm::Driver>(pub(crate) ARef<Device<T, Uninit>>, NotThreadSafe);
> 
> impl<T: drm::Driver> Deref for UnregisteredDevice<T> {
>     type Target = Device<T, Uninit>;
> @@ -188,7 +190,7 @@ impl<T: drm::Driver> UnregisteredDevice<T> {
>     /// Create a new `UnregisteredDevice` for a `drm::Driver`.
>     ///
>     /// This can be used to create a [`Registration`](kernel::drm::Registration).
> -    pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<Self> {
> +    pub fn new(dev: &device::Device) -> Result<Self> {
>         // `__drm_dev_alloc` uses `kmalloc()` to allocate memory, hence ensure a `kmalloc()`
>         // compatible `Layout`.
>         let layout = Kmalloc::aligned_layout(Layout::new::<Self>());
> @@ -207,22 +209,6 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<S
>         .cast();
>         let raw_drm = NonNull::new(from_err_ptr(raw_drm)?).ok_or(ENOMEM)?;
> 
> -        // SAFETY: `raw_drm` is a valid pointer to `Self`.
> -        let raw_data = unsafe { ptr::addr_of_mut!((*raw_drm.as_ptr()).data) };
> -
> -        // SAFETY:
> -        // - `raw_data` is a valid pointer to uninitialized memory.
> -        // - `raw_data` will not move until it is dropped.
> -        unsafe { data.__pinned_init(raw_data) }.inspect_err(|_| {
> -            // SAFETY: `raw_drm` is a valid pointer to `Self`, given that `__drm_dev_alloc` was
> -            // successful.
> -            let drm_dev = unsafe { Device::into_drm_device(raw_drm) };
> -
> -            // SAFETY: `__drm_dev_alloc()` was successful, hence `drm_dev` must be valid and the
> -            // refcount must be non-zero.
> -            unsafe { bindings::drm_dev_put(drm_dev) };
> -        })?;
> -
>         // SAFETY: The reference count is one, and now we take ownership of that reference as a
>         // `drm::Device`.
>         // INVARIANT: We just created the device above, but have yet to call `drm_dev_register`.
> @@ -253,7 +239,15 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<S
> #[repr(C)]
> pub struct Device<T: drm::Driver, C: DeviceContext = Registered> {
>     dev: Opaque<bindings::drm_device>,
> -    data: T::Data,
> +
> +    /// Keeps track of whether we've initialized the device data yet.
> +    pub(crate) data_is_init: AtomicBool,

What about this?

	/**
	 * @registered:
	 *
	 * Internally used by drm_dev_register() and drm_connector_register().
	 */
	bool registered;

Can’t we use this in lieu of this flag you’ve added?

> +
> +    /// The Driver's private data.
> +    ///
> +    /// This must only be written to from [`drm::Registration::new`].
> +    pub(crate) data: UnsafeCell<MaybeUninit<T::Data>>,
> +
>     _ctx: PhantomData<C>,
> }
> 

— Daniel