Re: [PATCH RFC 04/18] rust: drm: gem: Add GEM object abstraction

Daniel Vetter
2023-04-05T11:09:00Z

Meta: I'm trying to unblock myself by limiting each reply to a narrow-ish
topic. Otherwise it's just too much. Here's the first.

On Tue, Mar 07, 2023 at 11:25:29PM +0900, Asahi Lina wrote:
> The DRM GEM subsystem is the DRM memory management subsystem used by
> most modern drivers. Add a Rust abstraction to allow Rust DRM driver
> implementations to use it.
>
> Signed-off-by: Asahi Lina <lina@asahilina.net>
> ---
> rust/bindings/bindings_helper.h | 1 +
> rust/helpers.c | 23 +++
> rust/kernel/drm/drv.rs | 4 +-
> rust/kernel/drm/gem/mod.rs | 374 ++++++++++++++++++++++++++++++++++++++++
> rust/kernel/drm/mod.rs | 1 +
> 5 files changed, 401 insertions(+), 2 deletions(-)
>
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 7d7828faf89c..7183dfe6473f 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -9,6 +9,7 @@
> #include <drm/drm_device.h>
> #include <drm/drm_drv.h>
> #include <drm/drm_file.h>
> +#include <drm/drm_gem.h>
> #include <drm/drm_ioctl.h>
> #include <linux/delay.h>
> #include <linux/device.h>
> diff --git a/rust/helpers.c b/rust/helpers.c
> index 73b2ce607f27..78ec4162b03b 100644
> --- a/rust/helpers.c
> +++ b/rust/helpers.c
> @@ -18,6 +18,7 @@
> * accidentally exposed.
> */
>
> +#include <drm/drm_gem.h>
> #include <linux/bug.h>
> #include <linux/build_bug.h>
> #include <linux/device.h>
> @@ -374,6 +375,28 @@ void rust_helper_init_completion(struct completion *c)
> }
> EXPORT_SYMBOL_GPL(rust_helper_init_completion);
>
> +#ifdef CONFIG_DRM
> +
> +void rust_helper_drm_gem_object_get(struct drm_gem_object *obj)
> +{
> + drm_gem_object_get(obj);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_drm_gem_object_get);
> +
> +void rust_helper_drm_gem_object_put(struct drm_gem_object *obj)
> +{
> + drm_gem_object_put(obj);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_drm_gem_object_put);
> +
> +__u64 rust_helper_drm_vma_node_offset_addr(struct drm_vma_offset_node *node)
> +{
> + return drm_vma_node_offset_addr(node);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_drm_vma_node_offset_addr);

Uh all the rust helper wrappers for all the kernel in a single file does
not sound good. Can we not split these up into each subsystem, and then
maybe instead of sprinkling #ifdef all over a .c file Make the compilation
of that file conditional on rust support (plus whatever other Kconfig gate
the other c files has already)?

Otherwise if rust adoption picks up there's going to be endless amounts of
cross-subsystem conflicts.

Also similarly, can we perhaps split up the bindings_helper.h file in a
per-subsystem way?

> +
> +#endif
> +
> /*
> * We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type
> * as the Rust `usize` type, so we can use it in contexts where Rust
> diff --git a/rust/kernel/drm/drv.rs b/rust/kernel/drm/drv.rs
> index 1dcb651e1417..c138352cb489 100644
> --- a/rust/kernel/drm/drv.rs
> +++ b/rust/kernel/drm/drv.rs
> @@ -126,7 +126,7 @@ pub struct AllocOps {

Similary I guess this needs to be all under rust for rust reasons. I'm
assuming that the plan is that rust patches in here get acked/reviewed by
rust people, but then merged through the drm subsystem? At least long term
I think that's the least painful way.

Meaning we need a MAINTAINERS entry for rust/kernel/drm which adds
dri-devel for review and the usual git repos somewhere earlier in the
series.
-Daniel

> }
>
> /// Trait for memory manager implementations. Implemented internally.
> -pub trait AllocImpl: Sealed {
> +pub trait AllocImpl: Sealed + drm::gem::IntoGEMObject {
> /// The C callback operations for this memory manager.
> const ALLOC_OPS: AllocOps;
> }
> @@ -263,7 +263,7 @@ impl<T: Driver> Registration<T> {
> drm,
> registered: false,
> vtable,
> - fops: Default::default(), // TODO: GEM abstraction
> + fops: drm::gem::create_fops(),
> _pin: PhantomPinned,
> _p: PhantomData,
> })
> diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
> new file mode 100644
> index 000000000000..8a7d99613718
> --- /dev/null
> +++ b/rust/kernel/drm/gem/mod.rs
> @@ -0,0 +1,374 @@
> +// SPDX-License-Identifier: GPL-2.0 OR MIT
> +
> +//! DRM GEM API
> +//!
> +//! C header: [`include/linux/drm/drm_gem.h`](../../../../include/linux/drm/drm_gem.h)
> +
> +use alloc::boxed::Box;
> +
> +use crate::{
> + bindings,
> + drm::{device, drv, file},
> + error::{to_result, Result},
> + prelude::*,
> +};
> +use core::{mem, mem::ManuallyDrop, ops::Deref, ops::DerefMut};
> +
> +/// GEM object functions, which must be implemented by drivers.
> +pub trait BaseDriverObject<T: BaseObject>: Sync + Send + Sized {
> + /// Create a new driver data object for a GEM object of a given size.
> + fn new(dev: &device::Device<T::Driver>, size: usize) -> Result<Self>;
> +
> + /// Open a new handle to an existing object, associated with a File.
> + fn open(
> + _obj: &<<T as IntoGEMObject>::Driver as drv::Driver>::Object,
> + _file: &file::File<<<T as IntoGEMObject>::Driver as drv::Driver>::File>,
> + ) -> Result {
> + Ok(())
> + }
> +
> + /// Close a handle to an existing object, associated with a File.
> + fn close(
> + _obj: &<<T as IntoGEMObject>::Driver as drv::Driver>::Object,
> + _file: &file::File<<<T as IntoGEMObject>::Driver as drv::Driver>::File>,
> + ) {
> + }
> +}
> +
> +/// Trait that represents a GEM object subtype
> +pub trait IntoGEMObject: Sized + crate::private::Sealed {
> + /// Owning driver for this type
> + type Driver: drv::Driver;
> +
> + /// Returns a pointer to the raw `drm_gem_object` structure, which must be valid as long as
> + /// this owning object is valid.
> + fn gem_obj(&self) -> *mut bindings::drm_gem_object;
> +
> + /// Returns a reference to the raw `drm_gem_object` structure, which must be valid as long as
> + /// this owning object is valid.
> + fn gem_ref(&self) -> &bindings::drm_gem_object {
> + // SAFETY: gem_obj() must be valid per the above requirement.
> + unsafe { &*self.gem_obj() }
> + }
> +
> + /// Converts a pointer to a `drm_gem_object` into a pointer to this type.
> + fn from_gem_obj(obj: *mut bindings::drm_gem_object) -> *mut Self;
> +}
> +
> +/// Trait which must be implemented by drivers using base GEM objects.
> +pub trait DriverObject: BaseDriverObject<Object<Self>> {
> + /// Parent `Driver` for this object.
> + type Driver: drv::Driver;
> +}
> +
> +unsafe extern "C" fn free_callback<T: DriverObject>(obj: *mut bindings::drm_gem_object) {
> + // SAFETY: All of our objects are Object<T>.
> + let this = crate::container_of!(obj, Object<T>, obj) as *mut Object<T>;
> +
> + // SAFETY: The pointer we got has to be valid
> + unsafe { bindings::drm_gem_object_release(obj) };
> +
> + // SAFETY: All of our objects are allocated via Box<>, and we're in the
> + // free callback which guarantees this object has zero remaining references,
> + // so we can drop it
> + unsafe { Box::from_raw(this) };
> +}
> +
> +unsafe extern "C" fn open_callback<T: BaseDriverObject, U: BaseObject>(
> + raw_obj: *mut bindings::drm_gem_object,
> + raw_file: *mut bindings::drm_file,
> +) -> core::ffi::c_int {
> + // SAFETY: The pointer we got has to be valid.
> + let file = unsafe {
> + file::File::<<<U as IntoGEMObject>::Driver as drv::Driver>::File>::from_raw(raw_file)
> + };
> + let obj =
> + <<<U as IntoGEMObject>::Driver as drv::Driver>::Object as IntoGEMObject>::from_gem_obj(
> + raw_obj,
> + );
> +
> + // SAFETY: from_gem_obj() returns a valid pointer as long as the type is
> + // correct and the raw_obj we got is valid.
> + match T::open(unsafe { &*obj }, &file) {
> + Err(e) => e.to_kernel_errno(),
> + Ok(()) => 0,
> + }
> +}
> +
> +unsafe extern "C" fn close_callback<T: BaseDriverObject, U: BaseObject>(
> + raw_obj: *mut bindings::drm_gem_object,
> + raw_file: *mut bindings::drm_file,
> +) {
> + // SAFETY: The pointer we got has to be valid.
> + let file = unsafe {
> + file::File::<<<U as IntoGEMObject>::Driver as drv::Driver>::File>::from_raw(raw_file)
> + };
> + let obj =
> + <<<U as IntoGEMObject>::Driver as drv::Driver>::Object as IntoGEMObject>::from_gem_obj(
> + raw_obj,
> + );
> +
> + // SAFETY: from_gem_obj() returns a valid pointer as long as the type is
> + // correct and the raw_obj we got is valid.
> + T::close(unsafe { &*obj }, &file);
> +}
> +
> +impl<T: DriverObject> IntoGEMObject for Object<T> {
> + type Driver = T::Driver;
> +
> + fn gem_obj(&self) -> *mut bindings::drm_gem_object {
> + &self.obj as *const _ as *mut _
> + }
> +
> + fn from_gem_obj(obj: *mut bindings::drm_gem_object) -> *mut Object<T> {
> + crate::container_of!(obj, Object<T>, obj) as *mut Object<T>
> + }
> +}
> +
> +/// Base operations shared by all GEM object classes
> +pub trait BaseObject: IntoGEMObject {
> + /// Returns the size of the object in bytes.
> + fn size(&self) -> usize {
> + self.gem_ref().size
> + }
> +
> + /// Creates a new reference to the object.
> + fn reference(&self) -> ObjectRef<Self> {
> + // SAFETY: Having a reference to an Object implies holding a GEM reference
> + unsafe {
> + bindings::drm_gem_object_get(self.gem_obj());
> + }
> + ObjectRef {
> + ptr: self as *const _,
> + }
> + }
> +
> + /// Creates a new handle for the object associated with a given `File`
> + /// (or returns an existing one).
> + fn create_handle(
> + &self,
> + file: &file::File<<<Self as IntoGEMObject>::Driver as drv::Driver>::File>,
> + ) -> Result<u32> {
> + let mut handle: u32 = 0;
> + // SAFETY: The arguments are all valid per the type invariants.
> + to_result(unsafe {
> + bindings::drm_gem_handle_create(file.raw() as *mut _, self.gem_obj(), &mut handle)
> + })?;
> + Ok(handle)
> + }
> +
> + /// Looks up an object by its handle for a given `File`.
> + fn lookup_handle(
> + file: &file::File<<<Self as IntoGEMObject>::Driver as drv::Driver>::File>,
> + handle: u32,
> + ) -> Result<ObjectRef<Self>> {
> + // SAFETY: The arguments are all valid per the type invariants.
> + let ptr = unsafe { bindings::drm_gem_object_lookup(file.raw() as *mut _, handle) };
> +
> + if ptr.is_null() {
> + Err(ENOENT)
> + } else {
> + Ok(ObjectRef {
> + ptr: ptr as *const _,
> + })
> + }
> + }
> +
> + /// Creates an mmap offset to map the object from userspace.
> + fn create_mmap_offset(&self) -> Result<u64> {
> + // SAFETY: The arguments are valid per the type invariant.
> + to_result(unsafe {
> + // TODO: is this threadsafe?
> + bindings::drm_gem_create_mmap_offset(self.gem_obj())
> + })?;
> + Ok(unsafe {
> + bindings::drm_vma_node_offset_addr(&self.gem_ref().vma_node as *const _ as *mut _)
> + })
> + }
> +}
> +
> +impl<T: IntoGEMObject> BaseObject for T {}
> +
> +/// A base GEM object.
> +#[repr(C)]
> +pub struct Object<T: DriverObject> {
> + obj: bindings::drm_gem_object,
> + // The DRM core ensures the Device exists as long as its objects exist, so we don't need to
> + // manage the reference count here.
> + dev: ManuallyDrop<device::Device<T::Driver>>,
> + inner: T,
> +}
> +
> +impl<T: DriverObject> Object<T> {
> + /// The size of this object's structure.
> + pub const SIZE: usize = mem::size_of::<Self>();
> +
> + const OBJECT_FUNCS: bindings::drm_gem_object_funcs = bindings::drm_gem_object_funcs {
> + free: Some(free_callback::<T>),
> + open: Some(open_callback::<T, Object<T>>),
> + close: Some(close_callback::<T, Object<T>>),
> + print_info: None,
> + export: None,
> + pin: None,
> + unpin: None,
> + get_sg_table: None,
> + vmap: None,
> + vunmap: None,
> + mmap: None,
> + vm_ops: core::ptr::null_mut(),
> + };
> +
> + /// Create a new GEM object.
> + pub fn new(dev: &device::Device<T::Driver>, size: usize) -> Result<UniqueObjectRef<Self>> {
> + let mut obj: Box<Self> = Box::try_new(Self {
> + // SAFETY: This struct is expected to be zero-initialized
> + obj: unsafe { mem::zeroed() },
> + // SAFETY: The drm subsystem guarantees that the drm_device will live as long as
> + // the GEM object lives, so we can conjure a reference out of thin air.
> + dev: ManuallyDrop::new(unsafe { device::Device::from_raw(dev.ptr) }),
> + inner: T::new(dev, size)?,
> + })?;
> +
> + obj.obj.funcs = &Self::OBJECT_FUNCS;
> + to_result(unsafe {
> + bindings::drm_gem_object_init(dev.raw() as *mut _, &mut obj.obj, size)
> + })?;
> +
> + let obj_ref = UniqueObjectRef {
> + ptr: Box::leak(obj),
> + };
> +
> + Ok(obj_ref)
> + }
> +
> + /// Returns the `Device` that owns this GEM object.
> + pub fn dev(&self) -> &device::Device<T::Driver> {
> + &self.dev
> + }
> +}
> +
> +impl<T: DriverObject> crate::private::Sealed for Object<T> {}
> +
> +impl<T: DriverObject> Deref for Object<T> {
> + type Target = T;
> +
> + fn deref(&self) -> &Self::Target {
> + &self.inner
> + }
> +}
> +
> +impl<T: DriverObject> DerefMut for Object<T> {
> + fn deref_mut(&mut self) -> &mut Self::Target {
> + &mut self.inner
> + }
> +}
> +
> +impl<T: DriverObject> drv::AllocImpl for Object<T> {
> + const ALLOC_OPS: drv::AllocOps = drv::AllocOps {
> + gem_create_object: None,
> + prime_handle_to_fd: Some(bindings::drm_gem_prime_handle_to_fd),
> + prime_fd_to_handle: Some(bindings::drm_gem_prime_fd_to_handle),
> + gem_prime_import: None,
> + gem_prime_import_sg_table: None,
> + gem_prime_mmap: Some(bindings::drm_gem_prime_mmap),
> + dumb_create: None,
> + dumb_map_offset: None,
> + dumb_destroy: None,
> + };
> +}
> +
> +/// A reference-counted shared reference to a base GEM object.
> +pub struct ObjectRef<T: IntoGEMObject> {
> + // Invariant: the pointer is valid and initialized, and this ObjectRef owns a reference to it.
> + ptr: *const T,
> +}
> +
> +/// SAFETY: GEM object references are safe to share between threads.
> +unsafe impl<T: IntoGEMObject> Send for ObjectRef<T> {}
> +unsafe impl<T: IntoGEMObject> Sync for ObjectRef<T> {}
> +
> +impl<T: IntoGEMObject> Clone for ObjectRef<T> {
> + fn clone(&self) -> Self {
> + self.reference()
> + }
> +}
> +
> +impl<T: IntoGEMObject> Drop for ObjectRef<T> {
> + fn drop(&mut self) {
> + // SAFETY: Having an ObjectRef implies holding a GEM reference.
> + // The free callback will take care of deallocation.
> + unsafe {
> + bindings::drm_gem_object_put((*self.ptr).gem_obj());
> + }
> + }
> +}
> +
> +impl<T: IntoGEMObject> Deref for ObjectRef<T> {
> + type Target = T;
> +
> + fn deref(&self) -> &Self::Target {
> + // SAFETY: The pointer is valid per the invariant
> + unsafe { &*self.ptr }
> + }
> +}
> +
> +/// A unique reference to a base GEM object.
> +pub struct UniqueObjectRef<T: IntoGEMObject> {
> + // Invariant: the pointer is valid and initialized, and this ObjectRef owns the only reference
> + // to it.
> + ptr: *mut T,
> +}
> +
> +impl<T: IntoGEMObject> UniqueObjectRef<T> {
> + /// Downgrade this reference to a shared reference.
> + pub fn into_ref(self) -> ObjectRef<T> {
> + let ptr = self.ptr as *const _;
> + core::mem::forget(self);
> +
> + ObjectRef { ptr }
> + }
> +}
> +
> +impl<T: IntoGEMObject> Drop for UniqueObjectRef<T> {
> + fn drop(&mut self) {
> + // SAFETY: Having a UniqueObjectRef implies holding a GEM
> + // reference. The free callback will take care of deallocation.
> + unsafe {
> + bindings::drm_gem_object_put((*self.ptr).gem_obj());
> + }
> + }
> +}
> +
> +impl<T: IntoGEMObject> Deref for UniqueObjectRef<T> {
> + type Target = T;
> +
> + fn deref(&self) -> &Self::Target {
> + // SAFETY: The pointer is valid per the invariant
> + unsafe { &*self.ptr }
> + }
> +}
> +
> +impl<T: IntoGEMObject> DerefMut for UniqueObjectRef<T> {
> + fn deref_mut(&mut self) -> &mut Self::Target {
> + // SAFETY: The pointer is valid per the invariant
> + unsafe { &mut *self.ptr }
> + }
> +}
> +
> +pub(super) fn create_fops() -> bindings::file_operations {
> + bindings::file_operations {
> + owner: core::ptr::null_mut(),
> + open: Some(bindings::drm_open),
> + release: Some(bindings::drm_release),
> + unlocked_ioctl: Some(bindings::drm_ioctl),
> + #[cfg(CONFIG_COMPAT)]
> + compat_ioctl: Some(bindings::drm_compat_ioctl),
> + #[cfg(not(CONFIG_COMPAT))]
> + compat_ioctl: None,
> + poll: Some(bindings::drm_poll),
> + read: Some(bindings::drm_read),
> + llseek: Some(bindings::noop_llseek),
> + mmap: Some(bindings::drm_gem_mmap),
> + ..Default::default()
> + }
> +}
> diff --git a/rust/kernel/drm/mod.rs b/rust/kernel/drm/mod.rs
> index a767942d0b52..c44760a1332f 100644
> --- a/rust/kernel/drm/mod.rs
> +++ b/rust/kernel/drm/mod.rs
> @@ -5,4 +5,5 @@
> pub mod device;
> pub mod drv;
> pub mod file;
> +pub mod gem;
> pub mod ioctl;
>
> --
> 2.35.1
>

--
Daniel Vetter
Software Engineer, Intel Corporation