linux-kernel - Re: [WIP RFC v2 22/35] rust: drm/kms: Add DriverPlane::atomic

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Message-Id: <E4022636-D8A6-4B03-8EF0-E9E3177E760A@collabora.com>
Date: Thu, 28 Nov 2024 10:51:22 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Lyude Paul <lyude@...hat.com>
Cc: dri-devel@...ts.freedesktop.org,
 rust-for-linux@...r.kernel.org,
 Asahi Lina <lina@...hilina.net>,
 Danilo Krummrich <dakr@...nel.org>,
 mcanal@...lia.com,
 airlied@...hat.com,
 zhiw@...dia.com,
 cjia@...dia.com,
 jhubbard@...dia.com,
 Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Wedson Almeida Filho <wedsonaf@...il.com>,
 Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <benno.lossin@...ton.me>,
 Andreas Hindborg <a.hindborg@...sung.com>,
 Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>,
 open list <linux-kernel@...r.kernel.org>
Subject: Re: [WIP RFC v2 22/35] rust: drm/kms: Add
 DriverPlane::atomic_update()



> On 30 Sep 2024, at 20:10, Lyude Paul <lyude@...hat.com> wrote:
> 
> A mandatory trait method used for implementing DRM's atomic plane update
> callback.
> 
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> ---
> rust/kernel/drm/kms/plane.rs | 39 +++++++++++++++++++++++++++++++++++-
> 1 file changed, 38 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/drm/kms/plane.rs b/rust/kernel/drm/kms/plane.rs
> index d6e11a65cc101..506ed5ced1270 100644
> --- a/rust/kernel/drm/kms/plane.rs
> +++ b/rust/kernel/drm/kms/plane.rs
> @@ -75,7 +75,7 @@ pub trait DriverPlane: Send + Sync + Sized {
>             begin_fb_access: None, // TODO: someday?
>             end_fb_access: None, // TODO: someday?
>             atomic_check: None,
> -            atomic_update: None,
> +            atomic_update: if Self::HAS_ATOMIC_UPDATE { Some(atomic_update_callback::<Self>) } else { None },
>             atomic_enable: None, // TODO
>             atomic_disable: None, // TODO
>             atomic_async_check: None, // TODO
> @@ -103,6 +103,21 @@ pub trait DriverPlane: Send + Sync + Sized {
>     ///
>     /// Drivers may use this to instantiate their [`DriverPlane`] object.
>     fn new(device: &Device<Self::Driver>, args: Self::Args) -> impl PinInit<Self, Error>;
> +
> +    /// The optional [`drm_plane_helper_funcs.atomic_update`] hook for this plane.
> +    ///
> +    /// Drivers may use this to customize the atomic update phase of their [`Plane`] objects. If not
> +    /// specified, this function is a no-op.
> +    ///
> +    /// [`drm_plane_helper_funcs.atomic_update`]: srctree/include/drm/drm_modeset_helper_vtables.h
> +    fn atomic_update(
> +        plane: &Plane<Self>,
> +        new_state: BorrowedPlaneState<'_, PlaneState<Self::State>>,
> +        old_state: &PlaneState<Self::State>,
> +        state: &AtomicStateMutator<Self::Driver>
> +    ) {
> +        build_error::build_error("This should not be reachable")
> +    }

One more comment. If this is mandatory, why not omit this default here to get the compiler to enforce
the implementation of this fn by drivers? 

> }
> 
> /// The generated C vtable for a [`DriverPlane`].
> @@ -757,3 +772,25 @@ fn deref_mut(&mut self) -> &mut Self::Target {
>     // - The cast to `drm_plane_state` is safe via `PlaneState`s type invariants.
>     unsafe { bindings::__drm_atomic_helper_plane_reset(plane, Box::into_raw(new).cast()) };
> }
> +
> +unsafe extern "C" fn atomic_update_callback<T: DriverPlane>(
> +    plane: *mut bindings::drm_plane,
> +    state: *mut bindings::drm_atomic_state,
> +) {
> +    // SAFETY:
> +    // * We're guaranteed `plane` is of type `Plane<T>` via type invariants.
> +    // * We're guaranteed by DRM that `plane` is pointing to a valid initialized state.
> +    let plane = unsafe { Plane::from_raw(plane) };
> +
> +    // SAFETY: DRM guarantees `state` points to a valid `drm_atomic_state`
> +    let state = unsafe { AtomicStateMutator::new(NonNull::new_unchecked(state)) };
> +
> +    // SAFETY: Since we are in the atomic update callback, we're guaranteed by DRM that both the old
> +    // and new atomic state are present within `state`
> +    let (old_state, new_state) = unsafe {(
> +        state.get_old_plane_state(plane).unwrap_unchecked(),
> +        state.get_new_plane_state(plane).unwrap_unchecked(),
> +    )};
> +
> +    T::atomic_update(plane, new_state, old_state, &state);
> +}
> -- 
> 2.46.1
> 
> 

— Daniel