Message-Id: <39164069-001D-401D-A037-3C43F27373B9@collabora.com>
Date: Wed, 27 Nov 2024 13:00:30 -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 14/35] WIP: rust: drm/kms: Add OpaqueCrtc and
 OpaqueCrtcState

Hi Lyude,

> On 30 Sep 2024, at 20:09, Lyude Paul <lyude@...hat.com> wrote:
> 
> This is the same thing as OpaqueConnector and OpaqueConnectorState, but for
> CRTCs now.
> 
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> 
> ---
> 
> TODO:
> * Add upcast functions
> 
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> ---
> rust/kernel/drm/kms/crtc.rs | 131 ++++++++++++++++++++++++++++++++++++
> 1 file changed, 131 insertions(+)
> 
> diff --git a/rust/kernel/drm/kms/crtc.rs b/rust/kernel/drm/kms/crtc.rs
> index d84db49948380..1a3c9c448afcc 100644
> --- a/rust/kernel/drm/kms/crtc.rs
> +++ b/rust/kernel/drm/kms/crtc.rs
> @@ -234,6 +234,41 @@ pub fn new<'a, 'b: 'a, P, C>(
>         // SAFETY: We don't move anything
>         Ok(unsafe { &*Box::into_raw(Pin::into_inner_unchecked(this)) })
>     }
> +
> +    /// Attempt to convert an [`OpaqueCrtc`] into a fully qualified [`Crtc`].
> +    ///
> +    /// This checks if the given [`OpaqueCrtc`] uses the same [`DriverCrtc`] implementation, and
> +    /// returns the [`OpaqueCrtc`] as a [`Crtc`] object if so.
> +    pub fn try_from_opaque<'a, D>(opaque: &'a OpaqueCrtc<D>) -> Option<&'a Self>
> +    where
> +        D: KmsDriver,
> +        T: DriverCrtc<Driver = D>
> +    {
> +        // SAFETY: The vtables for a `Crtc` are initialized throughout the lifetime of the object
> +        let funcs = unsafe { (*opaque.crtc.get()).funcs };
> +
> +        // SAFETY: We only perform this transmutation if the opaque CRTC shares our vtable pointers,
> +        // so the underlying `Crtc` must share our data layout.
> +        ptr::eq(funcs, &T::OPS.funcs).then(|| unsafe { mem::transmute(opaque) })
> +    }
> +
> +    /// Convert a [`OpaqueCrtc`] into its fully qualified [`Crtc`].
> +    ///
> +    /// This is an infallible version of [`Self::try_from_opaque`]. This function is mainly useful
> +    /// for drivers where only a single [`DriverCrtc`] implementation exists.

I am confused. If a driver has a single `DriverCrtc`, why would it care for `OpaqueCrtc`?

> +    ///
> +    /// # Panics
> +    ///
> +    /// This function will panic if the underlying CRTC in the provided [`OpaqueCrtc`] does not
> +    /// belong to the same [`DriverCrtc`] implementation.
> +    pub fn from_opaque<'a, D>(opaque: &'a OpaqueCrtc<D>) -> &'a Self
> +    where
> +        D: KmsDriver,
> +        T: DriverCrtc<Driver = D>
> +    {
> +        Self::try_from_opaque(opaque)
> +            .expect("Passed OpaqueCrtc does not share this DriverCrtc implementation")
> +    }
> }
> 
> /// A trait implemented by any type that acts as a [`struct drm_crtc`] interface.
> @@ -267,6 +302,66 @@ unsafe fn from_raw<'a>(ptr: *mut bindings::drm_crtc) -> &'a Self {
>     }
> }
> 
> +/// A [`struct drm_crtc`] without a known [`DriverCrtc`] implementation.
> +///
> +/// This is mainly for situations where our bindings can't infer the [`DriverCrtc`] implementation
> +/// for a [`struct drm_crtc`] automatically. It is identical to [`Crtc`], except that it does not
> +/// provide access to the driver's private data.
> +///
> +/// It may be upcasted to a full [`Crtc`] using [`Crtc::from_opaque`] or
> +/// [`Crtc::try_from_opaque`].
> +///
> +/// # Invariants
> +///
> +/// - `crtc` is initialized for as long as this object is made available to users.
> +/// - The data layout of this structure is equivalent to [`struct drm_crtc`].

nit: Maybe worth clarifying that it’s equivalent to `bindings::drm_crtc`, not directly to
C’s `struct drm_crtc`. Although it should also be equivalent to that in practice.

> +///
> +/// [`struct drm_crtc`]: srctree/include/drm/drm_crtc.h
> +#[repr(transparent)]
> +pub struct OpaqueCrtc<T: KmsDriver> {
> +    crtc: Opaque<bindings::drm_crtc>,
> +    _p: PhantomData<T>
> +}
> +
> +impl<T: KmsDriver> Sealed for OpaqueCrtc<T> {}
> +
> +impl<T: KmsDriver> AsRawCrtc for OpaqueCrtc<T> {
> +    type State = OpaqueCrtcState<T>;
> +
> +    fn as_raw(&self) -> *mut bindings::drm_crtc {
> +        self.crtc.get()
> +    }
> +
> +    unsafe fn from_raw<'a>(ptr: *mut bindings::drm_crtc) -> &'a Self {
> +        // SAFETY: Our data layout starts with `bindings::drm_crtc`
> +        unsafe { &*ptr.cast() }
> +    }
> +}
> +
> +impl<T: KmsDriver> ModeObject for OpaqueCrtc<T> {
> +    type Driver = T;
> +
> +    fn drm_dev(&self) -> &Device<Self::Driver> {
> +        // SAFETY: The parent device for a DRM connector will never outlive the connector, and this
> +        // pointer is invariant through the lifetime of the connector
> +        unsafe { Device::borrow((*self.as_raw()).dev) }
> +    }
> +
> +    fn raw_mode_obj(&self) -> *mut bindings::drm_mode_object {
> +        // SAFETY: We don't expose DRM connectors to users before `base` is initialized
> +        unsafe { addr_of_mut!((*self.as_raw()).base) }
> +    }
> +}
> +
> +// SAFETY: CRTCs are non-refcounted modesetting objects
> +unsafe impl<T: KmsDriver> StaticModeObject for OpaqueCrtc<T> {}
> +
> +// SAFETY: Our CRTC interface is guaranteed to be thread-safe
> +unsafe impl<T: KmsDriver> Send for OpaqueCrtc<T> {}
> +
> +// SAFETY: Our CRTC interface is guaranteed to be thread-safe
> +unsafe impl<T: KmsDriver> Sync for OpaqueCrtc<T> {}
> +
> unsafe impl Zeroable for bindings::drm_crtc_state { }
> 
> impl<T: DriverCrtcState> Sealed for CrtcState<T> {}
> @@ -400,6 +495,42 @@ unsafe fn from_raw<'a>(ptr: *const bindings::drm_crtc_state) -> &'a Self {
>     }
> }
> 
> +/// A [`struct drm_crtc_state`] without a known [`DriverCrtcState`] implementation.
> +///
> +/// This is mainly for situations where our bindings can't infer the [`DriverCrtcState`]
> +/// implementation for a [`struct drm_crtc_state`] automatically. It is identical to [`Crtc`],
> +/// except that it does not provide access to the driver's private data.
> +///
> +/// TODO: Add upcast functions
> +///
> +/// # Invariants
> +///
> +/// - `state` is initialized for as long as this object is exposed to users.
> +/// - The data layout of this type is identical to [`struct drm_crtc_state`].
> +///
> +/// [`struct drm_crtc_state`]: srctree/include/drm/drm_crtc.h
> +#[repr(transparent)]
> +pub struct OpaqueCrtcState<T: KmsDriver> {
> +    state: Opaque<bindings::drm_crtc_state>,
> +    _p: PhantomData<T>
> +}
> +
> +impl<T: KmsDriver> AsRawCrtcState for OpaqueCrtcState<T> {
> +    type Crtc = OpaqueCrtc<T>;
> +}
> +
> +impl<T: KmsDriver> private::AsRawCrtcState for OpaqueCrtcState<T> {
> +    fn as_raw(&self) -> *mut bindings::drm_crtc_state {
> +        self.state.get()
> +    }
> +}
> +
> +impl<T: KmsDriver> FromRawCrtcState for OpaqueCrtcState<T> {
> +    unsafe fn from_raw<'a>(ptr: *const bindings::drm_crtc_state) -> &'a Self {
> +        // SAFETY: Our data layout is identical to `bindings::drm_crtc_state`
> +        unsafe { &*(ptr.cast()) }
> +    }
> +}
> unsafe extern "C" fn crtc_destroy_callback<T: DriverCrtc>(
>     crtc: *mut bindings::drm_crtc
> ) {
> -- 
> 2.46.1
> 
> 

— Daniel

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