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

[Lyude Paul]

On Thu, 2024-11-28 at 10:37 -0300, Daniel Almeida wrote:
> Hi Lyude,
> 
> > On 30 Sep 2024, at 20:10, Lyude Paul <lyude@xxxxxxxxxx> 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?

Yep! As well, the state can be further mutated within the atomic check
> 
> 
> > 
> > Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> > ---
> > 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?

Yes - tl;dr basically every atomic hook needs this pattern, since KMS doesn't
have/need a concept of composers or mutators but most of these hooks are able
to mutate the 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
> > 
> 
> — Daniel
> 

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

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