linux-kernel - Re: [RFC v3 27/33] rust: drm/kms: Add DriverCrtc::atomic_begin() and atomic

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Message-ID: <20250314-aspiring-prehistoric-caracara-ba6435@houat>
Date: Fri, 14 Mar 2025 13:25:17 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Lyude Paul <lyude@...hat.com>
Cc: dri-devel@...ts.freedesktop.org, rust-for-linux@...r.kernel.org, 
	Danilo Krummrich <dakr@...nel.org>, mcanal@...lia.com, Alice Ryhl <aliceryhl@...gle.com>, 
	Simona Vetter <sima@...ll.ch>, Daniel Almeida <daniel.almeida@...labora.com>, 
	Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...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@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v3 27/33] rust: drm/kms: Add DriverCrtc::atomic_begin() and
 atomic_flush()

On Wed, Mar 05, 2025 at 05:59:43PM -0500, Lyude Paul wrote:
> Optional trait methods for implementing the atomic_begin and atomic_flush
> callbacks for a CRTC.
> 
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> ---
>  rust/kernel/drm/kms/crtc.rs | 90 ++++++++++++++++++++++++++++++++++++-
>  1 file changed, 88 insertions(+), 2 deletions(-)
> 
> diff --git a/rust/kernel/drm/kms/crtc.rs b/rust/kernel/drm/kms/crtc.rs
> index aaa208b35c3c1..131d10505ba07 100644
> --- a/rust/kernel/drm/kms/crtc.rs
> +++ b/rust/kernel/drm/kms/crtc.rs
> @@ -90,8 +90,16 @@ pub trait DriverCrtc: Send + Sync + Sized {
>              mode_set: None,
>              mode_valid: None,
>              mode_fixup: None,
> -            atomic_begin: None,
> -            atomic_flush: None,
> +            atomic_begin: if Self::HAS_ATOMIC_BEGIN {
> +                Some(atomic_begin_callback::<Self>)
> +            } else {
> +                None
> +            },
> +            atomic_flush: if Self::HAS_ATOMIC_FLUSH {
> +                Some(atomic_flush_callback::<Self>)
> +            } else {
> +                None
> +            },
>              mode_set_nofb: None,
>              mode_set_base: None,
>              mode_set_base_atomic: None,
> @@ -132,6 +140,36 @@ fn atomic_check(
>      ) -> Result {
>          build_error::build_error("This should not be reachable")
>      }
> +
> +    /// The optional [`drm_crtc_helper_funcs.atomic_begin`] hook.
> +    ///
> +    /// This hook will be called before a set of [`Plane`] updates are performed for the given
> +    /// [`Crtc`].
> +    ///
> +    /// [`drm_crtc_helper_funcs.atomic_begin`]: srctree/include/drm/drm_modeset_helper_vtables.h
> +    fn atomic_begin(
> +        _crtc: &Crtc<Self>,
> +        _old_state: &CrtcState<Self::State>,
> +        _new_state: CrtcStateMutator<'_, CrtcState<Self::State>>,
> +        _state: &AtomicStateMutator<Self::Driver>,
> +    ) {
> +        build_error::build_error("This should not be reachable")
> +    }

Another dumb question I guess. If it's optional, and the default
implementation of a trait errors out at build time, how can it not break
if we don't implement the method?

Maxime

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