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

[Lyude Paul <lyude@redhat.com>]

On Thu, 2024-11-28 at 11:38 -0300, Daniel Almeida wrote:
> Hi Lyude,
> 
> > On 30 Sep 2024, at 20:10, Lyude Paul <lyude@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@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?

Yes it should! Thanks for pointing this out

> 
> > +
> > +    /// 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
> 

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

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