On 2026-09-01 at 06:08 +1000, Danilo Krummrich <[email protected]> wrote... > On Fri Aug 28, 2026 at 5:35 AM CEST, Alistair Popple wrote: > > +/// API handle for the auxiliary bus child drivers to interact with > > nova-core. > > +pub struct NovaCoreApi<'bound> { > > + #[expect(unused)] > > + pub(crate) gpu: Pin<&'bound Gpu<'bound>>, > > +} > > + > > +impl NovaCoreApi<'_> { > > + /// Obtain a [`NovaCoreApi`] handle from an auxiliary device registered > > + /// by nova-core. > > + pub fn of(adev: &auxiliary::Device<Bound>) -> > > Result<Pin<&NovaCoreApi<'_>>> { > > + adev.registration_data::<CovariantForLt!(NovaCoreApi<'_>)>() > > + } > > +} > > CovariantForLt does not hold anymore on latest drm-rust-next, as Cmdq has a > Mutex. So, this needs ForLt now and therefore the approach that I shared in > [1] > a while ago. I applied the changes in [2] to fix it up. > > I think the closure access through api.with(|api| ...) is fine in most cases, > but there are a few options if we run into cases where we consider it a bit > inconvinient. > > I think it should be possible to support projections into T: 'static and > covariant fields. The reason I differentiate them is because T: 'static is > very > convinient, but non-'static covariant types need an annoying turbofish. > > For T: 'static types it would turn out like this > > let spec = reg_data.api.project(|api| api.spec()); > > such that everything that needs spec does not need to be in the closure > anymore. > > For non-'static covariant fields we could have > > let foo = reg_data.api.project_lt::<CovariantForLt!(Foo<'_>)>(|api| > api.foo()); > > but as mentioned it unfortunately needs the turbofish. Of course we could > invent > a macro around it to get rid of the turbofish, but we'd still need to > explicitly > mention the type Foo<'_>, so it doesn't buy us a lot. > > This is the implementation I came up with in nova-core > > /// Projects a `'static` sub-field out of the registration data. > /// > /// `T` is fully inferred from the closure. For projected types with a > lifetime parameter, > /// use [`Self::project_lt`]. > pub fn project<T: 'static>( > &self, > f: impl for<'b> FnOnce(Pin<&'b NovaCoreApi<'b>>) -> &'b T, > ) -> &'a T { > self.adev > .registration_data_field::<ForLt!(NovaCoreApi<'_>), T>(f) > .expect("TypeId was validated in NovaCoreApiHandle::of()") > } > > /// Projects a covariant sub-field out of the registration data. > /// > /// Supports projected types with a lifetime parameter via a > /// [`CovariantForLt`](trait@CovariantForLt) encoding. Unlike > [`Self::project`], `G` cannot > /// be inferred and must be specified explicitly. > pub fn project_lt<G: CovariantForLt + 'static>( > &self, > f: impl for<'b> FnOnce(Pin<&'b NovaCoreApi<'b>>) -> &'b G::Of<'b>, > ) -> &'a G::Of<'a> { > self.adev > .registration_data_project::<ForLt!(NovaCoreApi<'_>), G>(f) > .expect("TypeId was validated in NovaCoreApiHandle::of()") > } > > and this is what we'd need in the auxiliary bus > > /// Projects a covariant sub-field out of potentially invariant > registration data. > /// > /// `F` is the [`ForLt`](trait@ForLt) encoding of the registration data > type. `G` is the > /// [`CovariantForLt`](trait@CovariantForLt) encoding of the projected > sub-field type. > /// > /// For projected types that are `'static`, prefer > [`Self::registration_data_field`] which > /// does not require a [`CovariantForLt`](trait@CovariantForLt) > encoding and fully infers `T`. > /// > /// Returns [`EINVAL`] if `F` does not match the type used by the > parent driver when calling > /// [`Registration::new()`]. Returns [`ENOENT`] if no registration data > has been set. > #[inline] > pub fn registration_data_project<F, G>( > &self, > project: impl for<'a> FnOnce(Pin<&'a F::Of<'a>>) -> &'a G::Of<'a>, > ) -> Result<&G::Of<'_>> > where > F: ForLt + 'static, > G: CovariantForLt + 'static, > { > let ptr = self.registration_data_with::<F, *const ()>(|data| { > core::ptr::from_ref::<G::Of<'_>>(project(data)).cast::<()>() > })?; > > // SAFETY: > // - The HRTB bound on `project` ensures the returned pointer is > derived from the > // registration data (nothing else lives for universally > quantified `'a`). > // - `G: CovariantForLt` guarantees that shortening the lifetime of > `G::Of` is sound. > // - The registration data is heap-allocated and outlives the > device's bound state. > Ok(unsafe { &*ptr.cast::<G::Of<'_>>() }) > } > > /// Projects a `'static` sub-field out of potentially invariant > registration data. > /// > /// Simplified variant of [`Self::registration_data_project`] for > projected types that are > /// `'static`. Since `&'a T` is trivially covariant when `T: 'static`, > no > /// [`CovariantForLt`](trait@CovariantForLt) encoding is needed and `T` > is fully inferred > /// from the closure. > /// > /// Returns [`EINVAL`] if `F` does not match the type used by the > parent driver when calling > /// [`Registration::new()`]. Returns [`ENOENT`] if no registration data > has been set. > #[inline] > pub fn registration_data_field<F: ForLt + 'static, T: 'static>( > &self, > project: impl for<'a> FnOnce(Pin<&'a F::Of<'a>>) -> &'a T, > ) -> Result<&T> { > let ptr = self.registration_data_with::<F, *const T>(|data| { > core::ptr::from_ref(project(data)) > })?; > > // SAFETY: > // - The HRTB bound on `project` ensures the returned pointer is > derived from the > // registration data (nothing else lives for universally > quantified `'a`). > // - `T: 'static` means `&T` is trivially covariant; lifetime > shortening is sound. > // - The registration data is heap-allocated and outlives the > device's bound state. > Ok(unsafe { &*ptr }) > } > > with the documentation being written by an LLM and unchecked. > > I think at least the T: 'static projection can provide an ergonomic advantage > and might be useful to add. Please let me know what you think.
Not being a Rust variance ninja it took me a while to figure this out :) But now that I have I agree the projections for 'static at least are quite nice and I think Gary's suggestion helps a bit as well for non-static. So assuming you're ok with me adding this code with your Co-developed-by or other appropriate tag then I will incorporate this idea into v6 so everything keeps working if/when Covariance no longer holds for Gpu. Thanks! - Alistair > > Thanks, > Danilo > > [1] https://lore.kernel.org/all/[email protected]/ > [2] CovariantForLt to ForLt changes for nova-core > > diff --git a/drivers/gpu/drm/nova/driver.rs b/drivers/gpu/drm/nova/driver.rs > index 7fc0baef5f04..028020213ac5 100644 > --- a/drivers/gpu/drm/nova/driver.rs > +++ b/drivers/gpu/drm/nova/driver.rs > @@ -1,7 +1,5 @@ > // SPDX-License-Identifier: GPL-2.0 > > -use core::pin::Pin; > - > use kernel::{ > auxiliary, > device::{ > @@ -20,7 +18,10 @@ > use crate::file::File; > use crate::gem::NovaObject; > > -use nova_core::api::NovaCoreApi; > +use nova_core::api::{ > + NovaCoreApi, > + NovaCoreApiHandle, // > +}; > > pub(crate) struct NovaDriver; > > @@ -32,7 +33,7 @@ pub(crate) struct Nova<'bound> { > > /// DRM registration data, accessible from ioctl handlers via the > registration guard. > pub(crate) struct DrmRegData<'bound> { > - pub(crate) api: Pin<&'bound NovaCoreApi<'bound>>, > + pub(crate) api: NovaCoreApiHandle<'bound>, > } > > /// Convienence type alias for the DRM device type for this driver > @@ -69,7 +70,7 @@ fn probe<'bound>( > ) -> impl PinInit<Self::Data<'bound>, Error> + 'bound { > let drm = drm::UnregisteredDevice::<Self>::new(adev, Ok(()))?; > let reg_data = DrmRegData { > - api: NovaCoreApi::of(adev)?, > + api: NovaCoreApi::handle(adev)?, > }; > // SAFETY: `reg` is stored in `Nova` and dropped when the driver is > unbound; it is > // never forgotten. > diff --git a/drivers/gpu/drm/nova/file.rs b/drivers/gpu/drm/nova/file.rs > index e1223a29f8b1..798b14f33e20 100644 > --- a/drivers/gpu/drm/nova/file.rs > +++ b/drivers/gpu/drm/nova/file.rs > @@ -32,13 +32,15 @@ > > impl GpuInfo { > fn new(reg_data: &DrmRegData<'_>) -> Self { > - Self(uapi::drm_nova_gpu_info { > - architecture: reg_data.api.architecture(), > - implementation: reg_data.api.implementation(), > - vram_size: reg_data.api.vram_size(), > - gpu_name: reg_data.api.gpu_name(), > - gpu_short_name: reg_data.api.gpu_short_name(), > - gpu_gid: reg_data.api.gpu_gid(), > + reg_data.api.with(|api| { > + Self(uapi::drm_nova_gpu_info { > + architecture: api.architecture(), > + implementation: api.implementation(), > + vram_size: api.vram_size(), > + gpu_name: api.gpu_name(), > + gpu_short_name: api.gpu_short_name(), > + gpu_gid: api.gpu_gid(), > + }) > }) > } > } > @@ -74,7 +76,7 @@ pub(crate) fn get_param( > _file: &drm::File<File>, > ) -> Result<u32> { > let value = match getparam.param as u32 { > - uapi::NOVA_GETPARAM_VRAM_BAR_SIZE => reg_data.api.bar1_size()?, > + uapi::NOVA_GETPARAM_VRAM_BAR_SIZE => reg_data.api.with(|api| > api.bar1_size())?, > _ => return Err(EINVAL), > }; > > diff --git a/drivers/gpu/nova-core/api.rs b/drivers/gpu/nova-core/api.rs > index ad4b62db1e5f..c9ae48d278af 100644 > --- a/drivers/gpu/nova-core/api.rs > +++ b/drivers/gpu/nova-core/api.rs > @@ -10,7 +10,7 @@ > device::Bound, > pci, > prelude::*, > - types::CovariantForLt, // > + types::ForLt, // > }; > > use crate::gpu::{ > @@ -39,10 +39,9 @@ impl NovaCoreApi<'_> { > *self.gpu.gsp_static_info.gpu_gid() > } > > - /// Obtain a [`NovaCoreApi`] handle from an auxiliary device registered > - /// by nova-core. > - pub fn of(adev: &auxiliary::Device<Bound>) -> > Result<Pin<&NovaCoreApi<'_>>> { > - adev.registration_data::<CovariantForLt!(NovaCoreApi<'_>)>() > + /// Obtain a [`NovaCoreApiHandle`] from an auxiliary device registered > by nova-core. > + pub fn handle(adev: &auxiliary::Device<Bound>) -> > Result<NovaCoreApiHandle<'_>> { > + NovaCoreApiHandle::of(adev) > } > > /// Returns the architecture identifier of this GPU. > @@ -66,3 +65,22 @@ pub fn vram_size(&self) -> u64 { > self.gpu.gsp_static_info.vram_size() > } > } > + > +/// Closure-based API handle for invariant registration data types. > +pub struct NovaCoreApiHandle<'a> { > + adev: &'a auxiliary::Device<Bound>, > +} > + > +impl<'a> NovaCoreApiHandle<'a> { > + fn of(adev: &'a auxiliary::Device<Bound>) -> Result<Self> { > + adev.registration_data_with::<ForLt!(NovaCoreApi<'_>), ()>(|_| ())?; > + Ok(Self { adev }) > + } > + > + /// Access the [`NovaCoreApi`] through a closure. > + pub fn with<R>(&self, f: impl for<'b> FnOnce(Pin<&'b NovaCoreApi<'b>>) > -> R) -> R { > + self.adev > + .registration_data_with::<ForLt!(NovaCoreApi<'_>), R>(f) > + .expect("TypeId was validated in NovaCoreApiHandle::of()") > + } > +} > diff --git a/drivers/gpu/nova-core/driver.rs b/drivers/gpu/nova-core/driver.rs > index 4d3c18d6a733..025ba2869d6c 100644 > --- a/drivers/gpu/nova-core/driver.rs > +++ b/drivers/gpu/nova-core/driver.rs > @@ -15,7 +15,7 @@ > Atomic, > Relaxed, // > }, > - types::CovariantForLt, > + types::ForLt, > }; > > use crate::{ > @@ -29,7 +29,7 @@ > #[pin_data] > pub(crate) struct NovaCore<'bound> { > #[allow(clippy::type_complexity)] > - _reg: auxiliary::Registration<'bound, CovariantForLt!(NovaCoreApi<'_>)>, > + _reg: auxiliary::Registration<'bound, ForLt!(NovaCoreApi<'_>)>, > #[pin] > pub(crate) gpu: Gpu<'bound>, > bar: pci::Bar<'bound, BAR0_SIZE>,
