linux-kernel - Re: [WIP RFC v2 32/35] rust: drm/kms: Add Device::num

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Message-Id: <E62F0077-DC6C-4D5E-941B-8C1130AFFB4B@collabora.com>
Date: Thu, 28 Nov 2024 11:38:04 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Lyude Paul <lyude@...hat.com>
Cc: dri-devel@...ts.freedesktop.org,
 rust-for-linux@...r.kernel.org,
 Asahi Lina <lina@...hilina.net>,
 Danilo Krummrich <dakr@...nel.org>,
 mcanal@...lia.com,
 airlied@...hat.com,
 zhiw@...dia.com,
 cjia@...dia.com,
 jhubbard@...dia.com,
 Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Wedson Almeida Filho <wedsonaf@...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@...sung.com>,
 Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>,
 open list <linux-kernel@...r.kernel.org>
Subject: Re: [WIP RFC v2 32/35] rust: drm/kms: Add Device::num_crtcs()

Hi Lyude,

> On 30 Sep 2024, at 20:10, Lyude Paul <lyude@...hat.com> wrote:
> 
> A binding for checking drm_device.num_crtcs. We'll need this in a moment
> for vblank support, since setting it up requires knowing the number of
> CRTCs that a driver has initialized.
> 
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> ---
> rust/kernel/drm/kms.rs | 25 +++++++++++++++++++++----
> 1 file changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/rust/kernel/drm/kms.rs b/rust/kernel/drm/kms.rs
> index 3edd90bc0025a..d0745b44ba9b6 100644
> --- a/rust/kernel/drm/kms.rs
> +++ b/rust/kernel/drm/kms.rs
> @@ -253,10 +253,27 @@ pub fn mode_config_lock(&self) -> ModeConfigGuard<'_, T> {
> 
>     /// Return the number of registered [`Plane`](plane::Plane) objects on this [`Device`].
>     #[inline]
> -    pub fn num_plane(&self) -> i32 {
> -        // SAFETY: The only context which this could change is before registration, which must be
> -        // single-threaded anyway - so it's safe to just read this value
> -        unsafe { (*self.as_raw()).mode_config.num_total_plane }
> +    pub fn num_plane(&self) -> u32 {
> +        // SAFETY:
> +        // * This can only be modified during the single-threaded context before registration, so
> +        //   this is safe
> +        // * num_total_plane could be >= 0, but no less - so casting to u32 is fine (and better to
> +        //   prevent errors)
> +        unsafe { (*self.as_raw()).mode_config.num_total_plane as u32 }
> +    }

Shouldn’t this be introduced by the patch that introduced `num_plane()` directly?

> +
> +    /// Return the number of registered CRTCs
> +    /// TODO: while `num_crtc` is of i32, that type actually makes literally no sense here and just
> +    /// causes problems and unecessary casts. Same for num_plane(). So, fix that at some point (we
> +    /// will never get n < 0 anyway)
> +    #[inline]
> +    pub fn num_crtcs(&self) -> u32 {
> +        // SAFETY:
> +        // * This can only be modified during the single-threaded context before registration, so
> +        //   this is safe
> +        // * num_crtc could be >= 0, but no less - so casting to u32 is fine (and better to prevent
> +        //   errors)
> +        unsafe { (*self.as_raw()).mode_config.num_crtc as u32 }
>     }
> }
> 
> -- 
> 2.46.1
> 
> 

Barring the comment above, it overall LGTM.

— Daniel