Re: [PATCH v2 1/1] rust: drm: add RENDER_CAPABILITY flag for render node support

[Alice Ryhl <aliceryhl@google.com>]

On Tue, May 05, 2026 at 11:23:04AM +0200, Laura Nao wrote:
> Add RENDER_CAPABILITY bool constant to the Driver trait to control
> render node support. When enabled, the driver exposes /dev/dri/renderDXX
> render nodes to userspace. The flag defaults to false, drivers can opt
> in by setting it to true in their Driver implementation.
> 
> This is then enabled in the Tyr driver, while it's left disabled for
> Nova for the time being.
> 
> Co-developed-by: Daniel Almeida <daniel.almeida@collabora.com>
> Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
> Signed-off-by: Laura Nao <laura.nao@collabora.com>

Overall looks good to me!

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

>  drivers/gpu/drm/tyr/driver.rs |  1 +
>  rust/kernel/drm/device.rs     | 12 +++++++++++-
>  rust/kernel/drm/driver.rs     |  5 +++++
>  3 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/tyr/driver.rs b/drivers/gpu/drm/tyr/driver.rs
> index e20a5978eed6..b7ae37ce3a1b 100644
> --- a/drivers/gpu/drm/tyr/driver.rs
> +++ b/drivers/gpu/drm/tyr/driver.rs
> @@ -186,6 +186,7 @@ impl drm::Driver for TyrDrmDriver {
>      type Object = drm::gem::shmem::Object<BoData>;
>  
>      const INFO: drm::DriverInfo = INFO;
> +    const RENDER_CAPABILITY: bool = true;
>  
>      kernel::declare_drm_ioctls! {
>          (PANTHOR_DEV_QUERY, drm_panthor_dev_query, ioctl::RENDER_ALLOW, TyrDrmFileData::dev_query),
> diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
> index adbafe8db54d..e121303d88f0 100644
> --- a/rust/kernel/drm/device.rs
> +++ b/rust/kernel/drm/device.rs
> @@ -80,6 +80,16 @@ pub struct Device<T: drm::Driver> {
>  }
>  
>  impl<T: drm::Driver> Device<T> {
> +    const fn compute_features() -> u32 {
> +        let mut features = drm::driver::FEAT_GEM;
> +
> +        if T::RENDER_CAPABILITY {
> +            features |= drm::driver::FEAT_RENDER;
> +        }
> +
> +        features
> +    }
> +
>      const VTABLE: bindings::drm_driver = drm_legacy_fields! {
>          load: None,
>          open: Some(drm::File::<T::File>::open_callback),
> @@ -105,7 +115,7 @@ impl<T: drm::Driver> Device<T> {
>          name: crate::str::as_char_ptr_in_const_context(T::INFO.name).cast_mut(),
>          desc: crate::str::as_char_ptr_in_const_context(T::INFO.desc).cast_mut(),
>  
> -        driver_features: drm::driver::FEAT_GEM,
> +        driver_features: Self::compute_features(),
>          ioctls: T::IOCTLS.as_ptr(),
>          num_ioctls: T::IOCTLS.len() as i32,
>          fops: &Self::GEM_FOPS,
> diff --git a/rust/kernel/drm/driver.rs b/rust/kernel/drm/driver.rs
> index 5233bdebc9fc..92cbc26ce11f 100644
> --- a/rust/kernel/drm/driver.rs
> +++ b/rust/kernel/drm/driver.rs
> @@ -16,6 +16,8 @@
>  
>  /// Driver use the GEM memory manager. This should be set for all modern drivers.
>  pub(crate) const FEAT_GEM: u32 = bindings::drm_driver_feature_DRIVER_GEM;
> +/// Driver supports render nodes, i.e.: /dev/dri/renderDXX devices.
> +pub(crate) const FEAT_RENDER: u32 = bindings::drm_driver_feature_DRIVER_RENDER;
>  
>  /// Information data for a DRM Driver.
>  pub struct DriverInfo {
> @@ -115,6 +117,9 @@ pub trait Driver {
>  
>      /// IOCTL list. See `kernel::drm::ioctl::declare_drm_ioctls!{}`.
>      const IOCTLS: &'static [drm::ioctl::DrmIoctlDescriptor];
> +
> +    /// Sets the `DRIVER_RENDER` feature for this driver.
> +    const RENDER_CAPABILITY: bool = false;

I think it would be nice for this documentation to elaborate more on
what this feature actually does. After all, it clearly took us a while
to understand it, so probably others are confused too.

Something along these lines:

/// Sets the `DRIVER_RENDER` feature for this driver.
/// 
/// When enabled, the driver exposes `/dev/dri/renderDXX` render nodes to
/// userspace. The render node is an alternate low-priviledge way to access
/// the driver, which is enforced on a per-ioctl level. Userspace processes
/// that open the render node can only invoke ioctls explicitly listed as
/// usable from the render node, whereas userspace processes using the
/// master node can invoke any ioctl.
const RENDER_CAPABILITY: bool = false;

Also, I'd probably call this const `FEAT_RENDER` for consistency / to
make it show up in grep.

Alice