Re: [PATCH 1/2] rust: drm: add support for driver features

[Laura Nao]

Hi Gary,

I just submitted a v2 [1] with DRIVER_RENDER modeled as a single
constant bool flag, as using a trait for this specific feature seemed
unnecessary.

This doesn't prevent us from modeling other features such as
FEAT_MODESET differently later, possibly with a trait and a constant
type that can only be constructed when the relevant trait is
implemented.

One thing probably worth noting about your previous suggestion, in case
it's useful for future feature implementations:

> On 21 Jan 2026, at 14:31, Gary Guo <gary@xxxxxxxxxxx> wrote:
> 
> When building, you can use `TypeId` to check if it's actually implemented, and
> set bits in the feature flags automatically.
> 

Taking FEAT_MODESET as example, I believe this would translate to:

const fn compute_features() -> u32 {
    let mut features = drm::driver::FEAT_GEM;

    if TypeId::of::<T::Modeset>() != TypeId::of::<drm::driver::NoFeature>() {
        features |= drm::driver::FEAT_MODESET;
    }

    features
}

However, this results in a compiler error as PartialEq is not yet
const-stable:

error[E0658]: cannot call conditionally-const operator in constant functions
    --> rust/kernel/drm/device.rs:87:12
     |
  87 |         if TypeId::of::<T::Render>() != TypeId::of::<drm::driver::NoFeature>() {
     |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
     = note: see issue #143874 <https://github.com/rust-lang/rust/issues/143874> for more information
     = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
     = note: this compiler was built on 2026-03-25; consider upgrading it if it is out of date

  error: `PartialEq` is not yet stable as a const trait
    --> rust/kernel/drm/device.rs:87:12
     |
  87 |         if TypeId::of::<T::Render>() != TypeId::of::<drm::driver::NoFeature>() {
     |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
  help: add `#![feature(const_cmp)]` to the crate attributes to enable
    --> rust/kernel/lib.rs:68:1
     |
  68 + #![feature(const_cmp)]
     |

  error: aborting due to 2 previous errors

As TypeId comparison is currently unstable, I think a different approach
would be needed if this pattern is adopted for other features.

Best,

Laura

[1] v2: https://lore.kernel.org/all/20260505092304.108262-1-laura.nao@xxxxxxxxxxxxx/