[WIP RFC v2 32/35] rust: drm/kms: Add Device::num_crtcs()

[Daniel Almeida]

Hi Lyude,

> On 30 Sep 2024, at 20:10, Lyude Paul <lyude at redhat.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 at redhat.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