Re: [WIP RFC v2 13/35] WIP: rust: drm/kms: Add OpaqueConnector and OpaqueConnectorState

[Lyude Paul]

On Wed, 2024-11-27 at 12:51 -0300, Daniel Almeida wrote:
> Hi Lyude,
> 
> > On 30 Sep 2024, at 20:09, Lyude Paul <lyude@xxxxxxxxxx> wrote:
> > 
> > Since we allow drivers to have multiple implementations of DriverConnector
> > and DriverConnectorState (in C, the equivalent of this is having multiple
> > structs which embed drm_connector) - there are some situations we will run
> > into where it's not possible for us to know the corresponding
> > DriverConnector or DriverConnectorState for a given connector. The most
> > obvious one is iterating through all connectors on a KMS device.
> > 
> > So, take advantage of the various connector traits we added to introduce
> > OpaqueConnector<> and OpaqueConnectorState<> which both can be used as a
> > DRM connector and connector state respectively without needing to know the
> > corresponding traits.
> > 
> > Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> > 
> > ---
> > 
> > TODO:
> > * Add upcast functions for these types
> > 
> > Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> > ---
> > rust/kernel/drm/kms/connector.rs | 108 +++++++++++++++++++++++++++++++
> > 1 file changed, 108 insertions(+)
> > 
> > diff --git a/rust/kernel/drm/kms/connector.rs b/rust/kernel/drm/kms/connector.rs
> > index ec842ebc111ae..98ac7fb781d4e 100644
> > --- a/rust/kernel/drm/kms/connector.rs
> > +++ b/rust/kernel/drm/kms/connector.rs
> > @@ -359,6 +359,64 @@ unsafe fn from_raw<'a>(ptr: *mut bindings::drm_connector) -> &'a Self {
> >     T::get_modes(connector.guard(&guard), &guard)
> > }
> > 
> > +/// A [`struct drm_connector`] without a known [`DriverConnector`] implementation.
> > +///
> > +/// This is mainly for situations where our bindings can't infer the [`DriverConnector`]
> > +/// implementation for a [`struct drm_connector`] automatically. It is identical to [`Connector`],
> > +/// except that it does not provide access to the driver's private data.
> > +///
> > +/// TODO: Add upcast methods for this
> 
> You mean a way to go from OpaqueConnector to Connector?

Correct - JFYI, for the next version of this series I'm going to just move
this pattern into some macro_rules! (along with some of the other more
repetitive code).

(Haven't gone through the rest of the review yet, but figured I'd mention
this)
> 
> > +///
> > +/// # Invariants
> > +///
> > +/// - `connector` is initialized for as long as this object is exposed to users.
> > +/// - The data layout of this type is equivalent to [`struct drm_connector`].
> > +///
> > +/// [`struct drm_connector`]: srctree/include/drm/drm_connector.h
> > +#[repr(transparent)]
> > +pub struct OpaqueConnector<T: KmsDriver> {
> > +    connector: Opaque<bindings::drm_connector>,
> > +    _p: PhantomData<T>
> > +}
> > +
> > +impl<T: KmsDriver> Sealed for OpaqueConnector<T> {}
> > +
> > +impl<T: KmsDriver> AsRawConnector for OpaqueConnector<T> {
> > +    type Driver = T;
> > +    type State = OpaqueConnectorState<T>;
> > +
> > +    fn as_raw(&self) -> *mut bindings::drm_connector {
> > +        self.connector.get()
> > +    }
> > +
> > +    unsafe fn from_raw<'a>(ptr: *mut bindings::drm_connector) -> &'a Self {
> > +        // SAFETY: Our data layout is identical to `bindings::drm_connector`
> > +        unsafe { &*ptr.cast() }
> > +    }
> > +}
> > +
> > +impl<T: KmsDriver> ModeObject for OpaqueConnector<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 { &mut (*self.as_raw()).base }
> > +    }
> > +}
> > +
> > +// SAFETY: Connectors are reference counted mode objects
> > +unsafe impl<T: KmsDriver> RcModeObject for OpaqueConnector<T> {}
> > +
> > +// SAFETY: Our connector interfaces are guaranteed to be thread-safe
> > +unsafe impl<T: KmsDriver> Send for OpaqueConnector<T> {}
> > +unsafe impl<T: KmsDriver> Sync for OpaqueConnector<T> {}
> > +
> > /// A privileged [`Connector`] obtained while holding a [`ModeConfigGuard`].
> > ///
> > /// This provides access to various methods for [`Connector`] that must happen under lock, such as
> > @@ -537,6 +595,56 @@ unsafe fn from_raw_mut<'a>(ptr: *mut bindings::drm_connector_state) -> &'a mut S
> >     }
> > }
> > 
> > +/// A [`struct drm_connector_state`] without a known [`DriverConnectorState`] implementation.
> > +///
> > +/// This is mainly for situations where our bindings can't infer the [`DriverConnectorState`]
> > +/// implementation for a [`struct drm_connector_state`] automatically. It is identical to
> > +/// [`Connector`], 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_connector_state`].
> > +/// - The DRM C API and our interface guarantees that only the user has mutable access to `state`,
> > +///   up until [`drm_atomic_helper_commit_hw_done`] is called. Therefore, `connector` follows rust's
> > +///   data aliasing rules and does not need to be behind an [`Opaque`] type.
> 
> By the way, as you did in a previous commit, I wonder whether it would be better to have the invariants
> in a single place, since I’ve noticed that most of them are quite similar.
> 
> Something like “The invariants for this type are the same as the ones for Foo”
> 
> This way, if you need to update your design, these will not get out of sync that easily.
> 
> > +///
> > +/// [`struct drm_connector_state`]: srctree/include/drm/drm_connector.h
> > +/// [`drm_atomic_helper_commit_hw_done`]: srctree/include/drm/drm_atomic_helper.h
> > +#[repr(transparent)]
> > +pub struct OpaqueConnectorState<T: KmsDriver> {
> > +    state: bindings::drm_connector_state,
> > +    _p: PhantomData<T>
> > +}
> > +
> > +impl<T: KmsDriver> AsRawConnectorState for OpaqueConnectorState<T> {
> > +    type Connector = OpaqueConnector<T>;
> > +}
> > +
> > +impl<T: KmsDriver> private::AsRawConnectorState for OpaqueConnectorState<T> {
> > +    fn as_raw(&self) -> &bindings::drm_connector_state {
> > +        &self.state
> > +    }
> > +
> > +    unsafe fn as_raw_mut(&mut self) -> &mut bindings::drm_connector_state {
> > +        &mut self.state
> > +    }
> > +}
> > +
> > +impl<T: KmsDriver> FromRawConnectorState for OpaqueConnectorState<T> {
> > +    unsafe fn from_raw<'a>(ptr: *const bindings::drm_connector_state) -> &'a Self {
> > +        // SAFETY: Our data layout is identical to `bindings::drm_connector_state`
> > +        unsafe { &*ptr.cast() }
> > +    }
> > +
> > +    unsafe fn from_raw_mut<'a>(ptr: *mut bindings::drm_connector_state) -> &'a mut Self {
> > +        // SAFETY: Our data layout is identical to `bindings::drm_connector_state`
> > +        unsafe { &mut *ptr.cast() }
> > +    }
> > +}
> > +
> > unsafe extern "C" fn atomic_duplicate_state_callback<T: DriverConnectorState>(
> >     connector: *mut bindings::drm_connector
> > ) -> *mut bindings::drm_connector_state
> > -- 
> > 2.46.1
> > 
> 
> This LGTM overall.
> 
> — Daniel
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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