[WIP RFC v2 21/35] rust: drm/kms: Introduce DriverCrtc::atomic_check()

[Lyude Paul <lyude@redhat.com>]

An optional trait method for implementing a CRTC's atomic state check.

Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 rust/kernel/drm/kms/crtc.rs | 46 +++++++++++++++++++++++++++++++++++--
 1 file changed, 44 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/drm/kms/crtc.rs b/rust/kernel/drm/kms/crtc.rs
index 7864540705f76..43c7264402b07 100644
--- a/rust/kernel/drm/kms/crtc.rs
+++ b/rust/kernel/drm/kms/crtc.rs
@@ -27,7 +27,7 @@
     marker::*,
     ptr::{NonNull, null, null_mut, addr_of_mut, self},
     ops::{Deref, DerefMut},
-    mem,
+    mem::{self, ManuallyDrop},
 };
 use macros::vtable;
 
@@ -82,7 +82,7 @@ pub trait DriverCrtc: Send + Sync + Sized {
         helper_funcs: bindings::drm_crtc_helper_funcs {
             atomic_disable: None,
             atomic_enable: None,
-            atomic_check: None,
+            atomic_check: if Self::HAS_ATOMIC_CHECK { Some(atomic_check_callback::<Self>) } else { None },
             dpms: None,
             commit: None,
             prepare: None,
@@ -117,6 +117,21 @@ pub trait DriverCrtc: Send + Sync + Sized {
     ///
     /// Drivers may use this to instantiate their [`DriverCrtc`] object.
     fn new(device: &Device<Self::Driver>, args: &Self::Args) -> impl PinInit<Self, Error>;
+
+    /// The optional [`drm_crtc_helper_funcs.atomic_check`] hook for this crtc.
+    ///
+    /// Drivers may use this to customize the atomic check phase of their [`Crtc`] objects. The
+    /// result of this function determines whether the atomic check passed or failed.
+    ///
+    /// [`drm_crtc_helper_funcs.atomic_check`]: srctree/include/drm/drm_modeset_helper_vtables.h
+    fn atomic_check(
+        crtc: &Crtc<Self>,
+        old_state: &CrtcState<Self::State>,
+        new_state: BorrowedCrtcState<'_, CrtcState<Self::State>>,
+        state: &AtomicStateComposer<Self::Driver>
+    ) -> Result {
+        build_error::build_error("This should not be reachable")
+    }
 }
 
 /// The generated C vtable for a [`DriverCrtc`].
@@ -726,3 +741,30 @@ fn as_raw(&self) -> *mut bindings::drm_crtc_state {
         )
     };
 }
+
+unsafe extern "C" fn atomic_check_callback<T: DriverCrtc>(
+    crtc: *mut bindings::drm_crtc,
+    state: *mut bindings::drm_atomic_state,
+) -> i32 {
+    // SAFETY:
+    // * We're guaranteed `crtc` is of type `Crtc<T>` via type invariants.
+    // * We're guaranteed by DRM that `crtc` is pointing to a valid initialized state.
+    let crtc = unsafe { Crtc::from_raw(crtc) };
+
+    // SAFETY: DRM guarantees `state` points to a valid `drm_atomic_state`
+    let state = unsafe {
+        ManuallyDrop::new(AtomicStateComposer::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_crtc_state(crtc).unwrap_unchecked(),
+        state.get_new_crtc_state(crtc).unwrap_unchecked(),
+    )};
+
+    from_result(|| {
+        T::atomic_check(crtc, old_state, new_state, &state)?;
+        Ok(0)
+    })
+}
-- 
2.46.1