Re: [PATCH v2 3/8] rust: drm: add driver abstractions

[Danilo Krummrich <dakr@kernel.org>]

On Mon, Sep 02, 2024 at 06:29:06PM +0200, Daniel Vetter wrote:
> On Wed, Jun 19, 2024 at 01:31:39AM +0200, Danilo Krummrich wrote:
> > Implement the DRM driver abstractions.
> > 
> > The `Driver` trait provides the interface to the actual driver to fill
> > in the driver specific data, such as the `DriverInfo`, driver features
> > and IOCTLs.
> > 
> > Co-developed-by: Asahi Lina <lina@asahilina.net>
> > Signed-off-by: Asahi Lina <lina@asahilina.net>
> > Signed-off-by: Danilo Krummrich <dakr@redhat.com>
> > ---
> >  rust/bindings/bindings_helper.h |   1 +
> >  rust/kernel/drm/drv.rs          | 141 ++++++++++++++++++++++++++++++++
> >  rust/kernel/drm/mod.rs          |   1 +
> >  3 files changed, 143 insertions(+)
> >  create mode 100644 rust/kernel/drm/drv.rs
> > 
> > diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> > index ed25b686748e..dc19cb789536 100644
> > --- a/rust/bindings/bindings_helper.h
> > +++ b/rust/bindings/bindings_helper.h
> > @@ -6,6 +6,7 @@
> >   * Sorted alphabetically.
> >   */
> >  
> > +#include <drm/drm_drv.h>
> >  #include <drm/drm_ioctl.h>
> >  #include <kunit/test.h>
> >  #include <linux/errname.h>
> > diff --git a/rust/kernel/drm/drv.rs b/rust/kernel/drm/drv.rs
> > new file mode 100644
> > index 000000000000..cd594a32f9e4
> > --- /dev/null
> > +++ b/rust/kernel/drm/drv.rs
> > @@ -0,0 +1,141 @@
> > +// SPDX-License-Identifier: GPL-2.0 OR MIT
> > +
> > +//! DRM driver core.
> > +//!
> > +//! C header: [`include/linux/drm/drm_drv.h`](srctree/include/linux/drm/drm_drv.h)
> > +
> > +use crate::{bindings, drm, private::Sealed, str::CStr, types::ForeignOwnable};
> > +use macros::vtable;
> > +
> > +/// Driver use the GEM memory manager. This should be set for all modern drivers.
> > +pub const FEAT_GEM: u32 = bindings::drm_driver_feature_DRIVER_GEM;
> > +/// Driver supports mode setting interfaces (KMS).
> > +pub const FEAT_MODESET: u32 = bindings::drm_driver_feature_DRIVER_MODESET;
> > +/// Driver supports dedicated render nodes.
> > +pub const FEAT_RENDER: u32 = bindings::drm_driver_feature_DRIVER_RENDER;
> > +/// Driver supports the full atomic modesetting userspace API.
> > +///
> > +/// Drivers which only use atomic internally, but do not support the full userspace API (e.g. not
> > +/// all properties converted to atomic, or multi-plane updates are not guaranteed to be tear-free)
> > +/// should not set this flag.
> > +pub const FEAT_ATOMIC: u32 = bindings::drm_driver_feature_DRIVER_ATOMIC;
> > +/// Driver supports DRM sync objects for explicit synchronization of command submission.
> > +pub const FEAT_SYNCOBJ: u32 = bindings::drm_driver_feature_DRIVER_SYNCOBJ;
> > +/// Driver supports the timeline flavor of DRM sync objects for explicit synchronization of command
> > +/// submission.
> > +pub const FEAT_SYNCOBJ_TIMELINE: u32 = bindings::drm_driver_feature_DRIVER_SYNCOBJ_TIMELINE;
> > +/// Driver supports compute acceleration devices. This flag is mutually exclusive with `FEAT_RENDER`
> > +/// and `FEAT_MODESET`. Devices that support both graphics and compute acceleration should be
> > +/// handled by two drivers that are connected using auxiliary bus.
> > +pub const FEAT_COMPUTE_ACCEL: u32 = bindings::drm_driver_feature_DRIVER_COMPUTE_ACCEL;
> > +/// Driver supports user defined GPU VA bindings for GEM objects.
> > +pub const FEAT_GEM_GPUVA: u32 = bindings::drm_driver_feature_DRIVER_GEM_GPUVA;
> > +/// Driver supports and requires cursor hotspot information in the cursor plane (e.g. cursor plane
> > +/// has to actually track the mouse cursor and the clients are required to set hotspot in order for
> > +/// the cursor planes to work correctly).
> > +pub const FEAT_CURSOR_HOTSPOT: u32 = bindings::drm_driver_feature_DRIVER_CURSOR_HOTSPOT;
> > +
> > +/// Information data for a DRM Driver.
> > +pub struct DriverInfo {
> > +    /// Driver major version.
> > +    pub major: i32,
> > +    /// Driver minor version.
> > +    pub minor: i32,
> > +    /// Driver patchlevel version.
> > +    pub patchlevel: i32,
> > +    /// Driver name.
> > +    pub name: &'static CStr,
> > +    /// Driver description.
> > +    pub desc: &'static CStr,
> > +    /// Driver date.
> > +    pub date: &'static CStr,
> > +}
> > +
> > +/// Internal memory management operation set, normally created by memory managers (e.g. GEM).
> > +///
> > +/// See `kernel::drm::gem` and `kernel::drm::gem::shmem`.
> > +pub struct AllocOps {
> > +    pub(crate) gem_create_object: Option<
> > +        unsafe extern "C" fn(
> > +            dev: *mut bindings::drm_device,
> > +            size: usize,
> > +        ) -> *mut bindings::drm_gem_object,
> > +    >,
> > +    pub(crate) prime_handle_to_fd: Option<
> > +        unsafe extern "C" fn(
> > +            dev: *mut bindings::drm_device,
> > +            file_priv: *mut bindings::drm_file,
> > +            handle: u32,
> > +            flags: u32,
> > +            prime_fd: *mut core::ffi::c_int,
> > +        ) -> core::ffi::c_int,
> > +    >,
> > +    pub(crate) prime_fd_to_handle: Option<
> > +        unsafe extern "C" fn(
> > +            dev: *mut bindings::drm_device,
> > +            file_priv: *mut bindings::drm_file,
> > +            prime_fd: core::ffi::c_int,
> > +            handle: *mut u32,
> > +        ) -> core::ffi::c_int,
> > +    >,
> > +    pub(crate) gem_prime_import: Option<
> > +        unsafe extern "C" fn(
> > +            dev: *mut bindings::drm_device,
> > +            dma_buf: *mut bindings::dma_buf,
> > +        ) -> *mut bindings::drm_gem_object,
> > +    >,
> > +    pub(crate) gem_prime_import_sg_table: Option<
> > +        unsafe extern "C" fn(
> > +            dev: *mut bindings::drm_device,
> > +            attach: *mut bindings::dma_buf_attachment,
> > +            sgt: *mut bindings::sg_table,
> > +        ) -> *mut bindings::drm_gem_object,
> > +    >,
> > +    pub(crate) dumb_create: Option<
> > +        unsafe extern "C" fn(
> > +            file_priv: *mut bindings::drm_file,
> > +            dev: *mut bindings::drm_device,
> > +            args: *mut bindings::drm_mode_create_dumb,
> > +        ) -> core::ffi::c_int,
> > +    >,
> > +    pub(crate) dumb_map_offset: Option<
> > +        unsafe extern "C" fn(
> > +            file_priv: *mut bindings::drm_file,
> > +            dev: *mut bindings::drm_device,
> > +            handle: u32,
> > +            offset: *mut u64,
> > +        ) -> core::ffi::c_int,
> > +    >,
> > +}
> > +
> > +/// Trait for memory manager implementations. Implemented internally.
> > +pub trait AllocImpl: Sealed {
> > +    /// The C callback operations for this memory manager.
> > +    const ALLOC_OPS: AllocOps;
> > +}
> > +
> > +/// The DRM `Driver` trait.
> > +///
> > +/// This trait must be implemented by drivers in order to create a `struct drm_device` and `struct
> > +/// drm_driver` to be registered in the DRM subsystem.
> > +#[vtable]
> > +pub trait Driver {
> > +    /// Context data associated with the DRM driver
> > +    ///
> > +    /// Determines the type of the context data passed to each of the methods of the trait.
> > +    type Data: ForeignOwnable + Sync + Send;
> > +
> > +    /// The type used to manage memory for this driver.
> > +    ///
> > +    /// Should be either `drm::gem::Object<T>` or `drm::gem::shmem::Object<T>`.
> > +    type Object: AllocImpl;
> 
> Bit similar comment to what I discussed at length with lyude, drivers
> might have a need for different implementations. But I think from the kms
> discussions we have solid solution for that, so I think we should be fine.
> 
> > +
> > +    /// Driver metadata
> > +    const INFO: DriverInfo;
> > +
> > +    /// Feature flags
> > +    const FEATURES: u32;
> 
> I think there's a type safety issue here with allowing drivers to muck
> with these directly. Example:
> 
> - If you don't set FEAT_GEM but try to use gem C functions, stuff blows up
>   because the core doesn't call drm_gem_init() in that case.

Indeed. Ideally, we'd want the feature flags to be automatically set, when a
corresponding implementation is provided by the driver.

For now, as you say, GEM can just be mandatory I think.

> 
> - For modesetting it's more fun because there mandatory init functions are
>   meant to be called by the driver, in the right sequence, interleaved
>   with other driver setup code for all the right modeset objects. If you
>   get it wrong you go boom.

My proposal was to just have another associated `Kms` type for `Driver` that
provides the corresponding callbacks for initialization. KMS initialization
functions could then be exposed only through those callbacks, such that they
can't be called after registration.

> 
> For the modeset side of things I've dumped a pile of comments on lyude's
> patches already: Essentially during registration I think we need a special
> drmKmsDriverInit object or phantom type or so, so that you can proof
> you're registering kms objects at the right time, with the rust
> abstraction calling all the other functions around that in the right
> order.
> 
> For gem I think we should flat out not allow non-gem drivers in rust. At
> least until someone comes up with a need for a non-gem driver.
> 
> For some of the values like hotspot cursor support we might need to change
> the rust abstraction to compute these at runtime driver init, and then set
> them somewhere in the runtime data structure instead of having them
> statically sepcified in drm_driver->features.
> 
> In general these feature flag are midlayer design and that tends to be
> bad, rust is just the messenger here.
> 
> Cheers, Sima
> 
> 
> > +
> > +    /// IOCTL list. See `kernel::drm::ioctl::declare_drm_ioctls!{}`.
> > +    const IOCTLS: &'static [drm::ioctl::DrmIoctlDescriptor];
> > +}
> > diff --git a/rust/kernel/drm/mod.rs b/rust/kernel/drm/mod.rs
> > index 9ec6d7cbcaf3..d987c56b3cec 100644
> > --- a/rust/kernel/drm/mod.rs
> > +++ b/rust/kernel/drm/mod.rs
> > @@ -2,4 +2,5 @@
> >  
> >  //! DRM subsystem abstractions.
> >  
> > +pub mod drv;
> >  pub mod ioctl;
> > -- 
> > 2.45.1
> > 
> 
> -- 
> Simona Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
>