Hi Lyude,
> On 30 Sep 2024, at 20:10, Lyude Paul <ly...@redhat.com> wrote:
>
> An optional trait method for implementing a CRTC's atomic state check.
A more thorough explanation like you had in your last patch would be nice here.
By `atomic state check` you mean after the state has been duplicated, and
mutated, right?
So it’s the stage where we check whether the hardware can accept the new
parameters?
>
> Signed-off-by: Lyude Paul <ly...@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")
> + }
> }
>
I am confused. If this is optional, why do we have a default implementation
with a build_error ?
> /// 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)))
> + };
> +
Some comments on why ManuallyDrop is required here would also be useful. Is it
related to the
use of ManuallyDrop in the preceding patch?
> + // 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
>
— Daniel
