Re: [PATCH RFC 08/18] rust: dma_fence: Add DMA Fence abstraction

[Daniel Vetter]

On Tue, Mar 07, 2023 at 11:25:33PM +0900, Asahi Lina wrote:
> DMA fences are the internal synchronization primitive used for DMA
> operations like GPU rendering, video en/decoding, etc. Add an
> abstraction to allow Rust drivers to interact with this subsystem.
> 
> Note: This uses a raw spinlock living next to the fence, since we do
> not interact with it other than for initialization.
> TODO: Expose this to the user at some point with a safe abstraction.
> 
> Signed-off-by: Asahi Lina <lina@xxxxxxxxxxxxx>
> ---
>  rust/bindings/bindings_helper.h |   2 +
>  rust/helpers.c                  |  53 ++++
>  rust/kernel/dma_fence.rs        | 532 ++++++++++++++++++++++++++++++++++++++++

This should probably be in the dma-buf namespace like on the C side?
There's a pile of tightly coupled concepts that I expect we'll all need
sooner or later (dma-fence/buf/resv at least).

Also I guess same questions about separate files and MAINTAINER entries as
for the drm stuff.
-Daniel

>  rust/kernel/lib.rs              |   2 +
>  4 files changed, 589 insertions(+)
> 
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 9f152d373df8..705af292a5b4 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -14,6 +14,8 @@
>  #include <drm/drm_ioctl.h>
>  #include <linux/delay.h>
>  #include <linux/device.h>
> +#include <linux/dma-fence.h>
> +#include <linux/dma-fence-chain.h>
>  #include <linux/dma-mapping.h>
>  #include <linux/fs.h>
>  #include <linux/ioctl.h>
> diff --git a/rust/helpers.c b/rust/helpers.c
> index 388ff1100ea5..8e906a7a7d8a 100644
> --- a/rust/helpers.c
> +++ b/rust/helpers.c
> @@ -23,6 +23,8 @@
>  #include <linux/bug.h>
>  #include <linux/build_bug.h>
>  #include <linux/device.h>
> +#include <linux/dma-fence.h>
> +#include <linux/dma-fence-chain.h>
>  #include <linux/dma-mapping.h>
>  #include <linux/err.h>
>  #include <linux/errname.h>
> @@ -30,6 +32,7 @@
>  #include <linux/of.h>
>  #include <linux/of_device.h>
>  #include <linux/platform_device.h>
> +#include <linux/spinlock.h>
>  #include <linux/rcupdate.h>
>  #include <linux/refcount.h>
>  #include <linux/xarray.h>
> @@ -388,6 +391,56 @@ int rust_helper_sg_dma_len(const struct scatterlist *sg)
>  }
>  EXPORT_SYMBOL_GPL(rust_helper_sg_dma_len);
>  
> +void rust_helper___spin_lock_init(spinlock_t *lock, const char *name,
> +				  struct lock_class_key *key)
> +{
> +#ifdef CONFIG_DEBUG_SPINLOCK
> +# ifndef CONFIG_PREEMPT_RT
> +	__raw_spin_lock_init(spinlock_check(lock), name, key, LD_WAIT_CONFIG);
> +# else
> +	rt_mutex_base_init(&lock->lock);
> +	__rt_spin_lock_init(lock, name, key, false);
> +# endif
> +#else
> +	spin_lock_init(lock);
> +#endif
> +}
> +EXPORT_SYMBOL_GPL(rust_helper___spin_lock_init);
> +
> +#ifdef CONFIG_DMA_SHARED_BUFFER
> +
> +void rust_helper_dma_fence_get(struct dma_fence *fence)
> +{
> +	dma_fence_get(fence);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_dma_fence_get);
> +
> +void rust_helper_dma_fence_put(struct dma_fence *fence)
> +{
> +	dma_fence_put(fence);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_dma_fence_put);
> +
> +struct dma_fence_chain *rust_helper_dma_fence_chain_alloc(void)
> +{
> +	return dma_fence_chain_alloc();
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_dma_fence_chain_alloc);
> +
> +void rust_helper_dma_fence_chain_free(struct dma_fence_chain *chain)
> +{
> +	dma_fence_chain_free(chain);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_dma_fence_chain_free);
> +
> +void rust_helper_dma_fence_set_error(struct dma_fence *fence, int error)
> +{
> +	dma_fence_set_error(fence, error);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_dma_fence_set_error);
> +
> +#endif
> +
>  #ifdef CONFIG_DRM
>  
>  void rust_helper_drm_gem_object_get(struct drm_gem_object *obj)
> diff --git a/rust/kernel/dma_fence.rs b/rust/kernel/dma_fence.rs
> new file mode 100644
> index 000000000000..ca93380d9da2
> --- /dev/null
> +++ b/rust/kernel/dma_fence.rs
> @@ -0,0 +1,532 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! DMA fence abstraction.
> +//!
> +//! C header: [`include/linux/dma_fence.h`](../../include/linux/dma_fence.h)
> +
> +use crate::{
> +    bindings,
> +    error::{to_result, Result},
> +    prelude::*,
> +    sync::LockClassKey,
> +    types::Opaque,
> +};
> +use core::fmt::Write;
> +use core::ops::{Deref, DerefMut};
> +use core::ptr::addr_of_mut;
> +use core::sync::atomic::{AtomicU64, Ordering};
> +
> +/// Any kind of DMA Fence Object
> +///
> +/// # Invariants
> +/// raw() returns a valid pointer to a dma_fence and we own a reference to it.
> +pub trait RawDmaFence: crate::private::Sealed {
> +    /// Returns the raw `struct dma_fence` pointer.
> +    fn raw(&self) -> *mut bindings::dma_fence;
> +
> +    /// Returns the raw `struct dma_fence` pointer and consumes the object.
> +    ///
> +    /// The caller is responsible for dropping the reference.
> +    fn into_raw(self) -> *mut bindings::dma_fence
> +    where
> +        Self: Sized,
> +    {
> +        let ptr = self.raw();
> +        core::mem::forget(self);
> +        ptr
> +    }
> +
> +    /// Advances this fence to the chain node which will signal this sequence number.
> +    /// If no sequence number is provided, this returns `self` again.
> +    fn chain_find_seqno(self, seqno: u64) -> Result<Fence>
> +    where
> +        Self: Sized,
> +    {
> +        let mut ptr = self.into_raw();
> +
> +        // SAFETY: This will safely fail if this DmaFence is not a chain.
> +        // `ptr` is valid per the type invariant.
> +        let ret = unsafe { bindings::dma_fence_chain_find_seqno(&mut ptr, seqno) };
> +
> +        if ret != 0 {
> +            // SAFETY: This is either an owned reference or NULL, dma_fence_put can handle both.
> +            unsafe { bindings::dma_fence_put(ptr) };
> +            Err(Error::from_kernel_errno(ret))
> +        } else if ptr.is_null() {
> +            Err(EINVAL) // When can this happen?
> +        } else {
> +            // SAFETY: ptr is valid and non-NULL as checked above.
> +            Ok(unsafe { Fence::from_raw(ptr) })
> +        }
> +    }
> +
> +    /// Signal completion of this fence
> +    fn signal(&self) -> Result {
> +        to_result(unsafe { bindings::dma_fence_signal(self.raw()) })
> +    }
> +
> +    /// Set the error flag on this fence
> +    fn set_error(&self, err: Error) {
> +        unsafe { bindings::dma_fence_set_error(self.raw(), err.to_kernel_errno()) };
> +    }
> +}
> +
> +/// A generic DMA Fence Object
> +///
> +/// # Invariants
> +/// ptr is a valid pointer to a dma_fence and we own a reference to it.
> +pub struct Fence {
> +    ptr: *mut bindings::dma_fence,
> +}
> +
> +impl Fence {
> +    /// Create a new Fence object from a raw pointer to a dma_fence.
> +    ///
> +    /// # Safety
> +    /// The caller must own a reference to the dma_fence, which is transferred to the new object.
> +    pub(crate) unsafe fn from_raw(ptr: *mut bindings::dma_fence) -> Fence {
> +        Fence { ptr }
> +    }
> +
> +    /// Create a new Fence object from a raw pointer to a dma_fence.
> +    ///
> +    /// # Safety
> +    /// Takes a borrowed reference to the dma_fence, and increments the reference count.
> +    pub(crate) unsafe fn get_raw(ptr: *mut bindings::dma_fence) -> Fence {
> +        // SAFETY: Pointer is valid per the safety contract
> +        unsafe { bindings::dma_fence_get(ptr) };
> +        Fence { ptr }
> +    }
> +
> +    /// Create a new Fence object from a RawDmaFence.
> +    pub fn from_fence(fence: &dyn RawDmaFence) -> Fence {
> +        // SAFETY: Pointer is valid per the RawDmaFence contract
> +        unsafe { Self::get_raw(fence.raw()) }
> +    }
> +}
> +
> +impl crate::private::Sealed for Fence {}
> +
> +impl RawDmaFence for Fence {
> +    fn raw(&self) -> *mut bindings::dma_fence {
> +        self.ptr
> +    }
> +}
> +
> +impl Drop for Fence {
> +    fn drop(&mut self) {
> +        // SAFETY: We own a reference to this syncobj.
> +        unsafe { bindings::dma_fence_put(self.ptr) };
> +    }
> +}
> +
> +impl Clone for Fence {
> +    fn clone(&self) -> Self {
> +        // SAFETY: `ptr` is valid per the type invariant and we own a reference to it.
> +        unsafe {
> +            bindings::dma_fence_get(self.ptr);
> +            Self::from_raw(self.ptr)
> +        }
> +    }
> +}
> +
> +unsafe impl Sync for Fence {}
> +unsafe impl Send for Fence {}
> +
> +/// Trait which must be implemented by driver-specific fence objects.
> +#[vtable]
> +pub trait FenceOps: Sized + Send + Sync {
> +    /// True if this dma_fence implementation uses 64bit seqno, false otherwise.
> +    const USE_64BIT_SEQNO: bool;
> +
> +    /// Returns the driver name. This is a callback to allow drivers to compute the name at
> +    /// runtime, without having it to store permanently for each fence, or build a cache of
> +    /// some sort.
> +    fn get_driver_name<'a>(self: &'a FenceObject<Self>) -> &'a CStr;
> +
> +    /// Return the name of the context this fence belongs to. This is a callback to allow drivers
> +    /// to compute the name at runtime, without having it to store permanently for each fence, or
> +    /// build a cache of some sort.
> +    fn get_timeline_name<'a>(self: &'a FenceObject<Self>) -> &'a CStr;
> +
> +    /// Enable software signaling of fence.
> +    fn enable_signaling(self: &FenceObject<Self>) -> bool {
> +        false
> +    }
> +
> +    /// Peek whether the fence is signaled, as a fastpath optimization for e.g. dma_fence_wait() or
> +    /// dma_fence_add_callback().
> +    fn signaled(self: &FenceObject<Self>) -> bool {
> +        false
> +    }
> +
> +    /// Callback to fill in free-form debug info specific to this fence, like the sequence number.
> +    fn fence_value_str(self: &FenceObject<Self>, _output: &mut dyn Write) {}
> +
> +    /// Fills in the current value of the timeline as a string, like the sequence number. Note that
> +    /// the specific fence passed to this function should not matter, drivers should only use it to
> +    /// look up the corresponding timeline structures.
> +    fn timeline_value_str(self: &FenceObject<Self>, _output: &mut dyn Write) {}
> +}
> +
> +unsafe extern "C" fn get_driver_name_cb<T: FenceOps>(
> +    fence: *mut bindings::dma_fence,
> +) -> *const core::ffi::c_char {
> +    // SAFETY: All of our fences are FenceObject<T>.
> +    let p = crate::container_of!(fence, FenceObject<T>, fence) as *mut FenceObject<T>;
> +
> +    // SAFETY: The caller is responsible for passing a valid dma_fence subtype
> +    T::get_driver_name(unsafe { &mut *p }).as_char_ptr()
> +}
> +
> +unsafe extern "C" fn get_timeline_name_cb<T: FenceOps>(
> +    fence: *mut bindings::dma_fence,
> +) -> *const core::ffi::c_char {
> +    // SAFETY: All of our fences are FenceObject<T>.
> +    let p = crate::container_of!(fence, FenceObject<T>, fence) as *mut FenceObject<T>;
> +
> +    // SAFETY: The caller is responsible for passing a valid dma_fence subtype
> +    T::get_timeline_name(unsafe { &mut *p }).as_char_ptr()
> +}
> +
> +unsafe extern "C" fn enable_signaling_cb<T: FenceOps>(fence: *mut bindings::dma_fence) -> bool {
> +    // SAFETY: All of our fences are FenceObject<T>.
> +    let p = crate::container_of!(fence, FenceObject<T>, fence) as *mut FenceObject<T>;
> +
> +    // SAFETY: The caller is responsible for passing a valid dma_fence subtype
> +    T::enable_signaling(unsafe { &mut *p })
> +}
> +
> +unsafe extern "C" fn signaled_cb<T: FenceOps>(fence: *mut bindings::dma_fence) -> bool {
> +    // SAFETY: All of our fences are FenceObject<T>.
> +    let p = crate::container_of!(fence, FenceObject<T>, fence) as *mut FenceObject<T>;
> +
> +    // SAFETY: The caller is responsible for passing a valid dma_fence subtype
> +    T::signaled(unsafe { &mut *p })
> +}
> +
> +unsafe extern "C" fn release_cb<T: FenceOps>(fence: *mut bindings::dma_fence) {
> +    // SAFETY: All of our fences are FenceObject<T>.
> +    let p = crate::container_of!(fence, FenceObject<T>, fence) as *mut FenceObject<T>;
> +
> +    // SAFETY: p is never used after this
> +    unsafe {
> +        core::ptr::drop_in_place(&mut (*p).inner);
> +    }
> +
> +    // SAFETY: All of our fences are allocated using kmalloc, so this is safe.
> +    unsafe { bindings::dma_fence_free(fence) };
> +}
> +
> +unsafe extern "C" fn fence_value_str_cb<T: FenceOps>(
> +    fence: *mut bindings::dma_fence,
> +    string: *mut core::ffi::c_char,
> +    size: core::ffi::c_int,
> +) {
> +    let size: usize = size.try_into().unwrap_or(0);
> +
> +    if size == 0 {
> +        return;
> +    }
> +
> +    // SAFETY: All of our fences are FenceObject<T>.
> +    let p = crate::container_of!(fence, FenceObject<T>, fence) as *mut FenceObject<T>;
> +
> +    // SAFETY: The caller is responsible for the validity of string/size
> +    let mut f = unsafe { crate::str::Formatter::from_buffer(string as *mut _, size) };
> +
> +    // SAFETY: The caller is responsible for passing a valid dma_fence subtype
> +    T::fence_value_str(unsafe { &mut *p }, &mut f);
> +    let _ = f.write_str("\0");
> +
> +    // SAFETY: `size` is at least 1 per the check above
> +    unsafe { *string.add(size - 1) = 0 };
> +}
> +
> +unsafe extern "C" fn timeline_value_str_cb<T: FenceOps>(
> +    fence: *mut bindings::dma_fence,
> +    string: *mut core::ffi::c_char,
> +    size: core::ffi::c_int,
> +) {
> +    let size: usize = size.try_into().unwrap_or(0);
> +
> +    if size == 0 {
> +        return;
> +    }
> +
> +    // SAFETY: All of our fences are FenceObject<T>.
> +    let p = crate::container_of!(fence, FenceObject<T>, fence) as *mut FenceObject<T>;
> +
> +    // SAFETY: The caller is responsible for the validity of string/size
> +    let mut f = unsafe { crate::str::Formatter::from_buffer(string as *mut _, size) };
> +
> +    // SAFETY: The caller is responsible for passing a valid dma_fence subtype
> +    T::timeline_value_str(unsafe { &mut *p }, &mut f);
> +    let _ = f.write_str("\0");
> +
> +    // SAFETY: `size` is at least 1 per the check above
> +    unsafe { *string.add(size - 1) = 0 };
> +}
> +
> +// Allow FenceObject<Self> to be used as a self argument, for ergonomics
> +impl<T: FenceOps> core::ops::Receiver for FenceObject<T> {}
> +
> +/// A driver-specific DMA Fence Object
> +///
> +/// # Invariants
> +/// ptr is a valid pointer to a dma_fence and we own a reference to it.
> +#[repr(C)]
> +pub struct FenceObject<T: FenceOps> {
> +    fence: bindings::dma_fence,
> +    lock: Opaque<bindings::spinlock>,
> +    inner: T,
> +}
> +
> +impl<T: FenceOps> FenceObject<T> {
> +    const SIZE: usize = core::mem::size_of::<Self>();
> +
> +    const VTABLE: bindings::dma_fence_ops = bindings::dma_fence_ops {
> +        use_64bit_seqno: T::USE_64BIT_SEQNO,
> +        get_driver_name: Some(get_driver_name_cb::<T>),
> +        get_timeline_name: Some(get_timeline_name_cb::<T>),
> +        enable_signaling: if T::HAS_ENABLE_SIGNALING {
> +            Some(enable_signaling_cb::<T>)
> +        } else {
> +            None
> +        },
> +        signaled: if T::HAS_SIGNALED {
> +            Some(signaled_cb::<T>)
> +        } else {
> +            None
> +        },
> +        wait: None, // Deprecated
> +        release: Some(release_cb::<T>),
> +        fence_value_str: if T::HAS_FENCE_VALUE_STR {
> +            Some(fence_value_str_cb::<T>)
> +        } else {
> +            None
> +        },
> +        timeline_value_str: if T::HAS_TIMELINE_VALUE_STR {
> +            Some(timeline_value_str_cb::<T>)
> +        } else {
> +            None
> +        },
> +    };
> +}
> +
> +impl<T: FenceOps> Deref for FenceObject<T> {
> +    type Target = T;
> +
> +    fn deref(&self) -> &T {
> +        &self.inner
> +    }
> +}
> +
> +impl<T: FenceOps> DerefMut for FenceObject<T> {
> +    fn deref_mut(&mut self) -> &mut T {
> +        &mut self.inner
> +    }
> +}
> +
> +impl<T: FenceOps> crate::private::Sealed for FenceObject<T> {}
> +impl<T: FenceOps> RawDmaFence for FenceObject<T> {
> +    fn raw(&self) -> *mut bindings::dma_fence {
> +        &self.fence as *const _ as *mut _
> +    }
> +}
> +
> +/// A unique reference to a driver-specific fence object
> +pub struct UniqueFence<T: FenceOps>(*mut FenceObject<T>);
> +
> +impl<T: FenceOps> Deref for UniqueFence<T> {
> +    type Target = FenceObject<T>;
> +
> +    fn deref(&self) -> &FenceObject<T> {
> +        unsafe { &*self.0 }
> +    }
> +}
> +
> +impl<T: FenceOps> DerefMut for UniqueFence<T> {
> +    fn deref_mut(&mut self) -> &mut FenceObject<T> {
> +        unsafe { &mut *self.0 }
> +    }
> +}
> +
> +impl<T: FenceOps> crate::private::Sealed for UniqueFence<T> {}
> +impl<T: FenceOps> RawDmaFence for UniqueFence<T> {
> +    fn raw(&self) -> *mut bindings::dma_fence {
> +        unsafe { addr_of_mut!((*self.0).fence) }
> +    }
> +}
> +
> +impl<T: FenceOps> From<UniqueFence<T>> for UserFence<T> {
> +    fn from(value: UniqueFence<T>) -> Self {
> +        let ptr = value.0;
> +        core::mem::forget(value);
> +
> +        UserFence(ptr)
> +    }
> +}
> +
> +impl<T: FenceOps> Drop for UniqueFence<T> {
> +    fn drop(&mut self) {
> +        // SAFETY: We own a reference to this fence.
> +        unsafe { bindings::dma_fence_put(self.raw()) };
> +    }
> +}
> +
> +unsafe impl<T: FenceOps> Sync for UniqueFence<T> {}
> +unsafe impl<T: FenceOps> Send for UniqueFence<T> {}
> +
> +/// A shared reference to a driver-specific fence object
> +pub struct UserFence<T: FenceOps>(*mut FenceObject<T>);
> +
> +impl<T: FenceOps> Deref for UserFence<T> {
> +    type Target = FenceObject<T>;
> +
> +    fn deref(&self) -> &FenceObject<T> {
> +        unsafe { &*self.0 }
> +    }
> +}
> +
> +impl<T: FenceOps> Clone for UserFence<T> {
> +    fn clone(&self) -> Self {
> +        // SAFETY: `ptr` is valid per the type invariant and we own a reference to it.
> +        unsafe {
> +            bindings::dma_fence_get(self.raw());
> +            Self(self.0)
> +        }
> +    }
> +}
> +
> +impl<T: FenceOps> crate::private::Sealed for UserFence<T> {}
> +impl<T: FenceOps> RawDmaFence for UserFence<T> {
> +    fn raw(&self) -> *mut bindings::dma_fence {
> +        unsafe { addr_of_mut!((*self.0).fence) }
> +    }
> +}
> +
> +impl<T: FenceOps> Drop for UserFence<T> {
> +    fn drop(&mut self) {
> +        // SAFETY: We own a reference to this fence.
> +        unsafe { bindings::dma_fence_put(self.raw()) };
> +    }
> +}
> +
> +unsafe impl<T: FenceOps> Sync for UserFence<T> {}
> +unsafe impl<T: FenceOps> Send for UserFence<T> {}
> +
> +/// An array of fence contexts, out of which fences can be created.
> +pub struct FenceContexts {
> +    start: u64,
> +    count: u32,
> +    seqnos: Vec<AtomicU64>,
> +    lock_name: &'static CStr,
> +    lock_key: &'static LockClassKey,
> +}
> +
> +impl FenceContexts {
> +    /// Create a new set of fence contexts.
> +    pub fn new(
> +        count: u32,
> +        name: &'static CStr,
> +        key: &'static LockClassKey,
> +    ) -> Result<FenceContexts> {
> +        let mut seqnos: Vec<AtomicU64> = Vec::new();
> +
> +        seqnos.try_reserve(count as usize)?;
> +
> +        for _ in 0..count {
> +            seqnos.try_push(Default::default())?;
> +        }
> +
> +        let start = unsafe { bindings::dma_fence_context_alloc(count as core::ffi::c_uint) };
> +
> +        Ok(FenceContexts {
> +            start,
> +            count,
> +            seqnos,
> +            lock_name: name,
> +            lock_key: key,
> +        })
> +    }
> +
> +    /// Create a new fence in a given context index.
> +    pub fn new_fence<T: FenceOps>(&self, context: u32, inner: T) -> Result<UniqueFence<T>> {
> +        if context > self.count {
> +            return Err(EINVAL);
> +        }
> +
> +        let p = unsafe {
> +            bindings::krealloc(
> +                core::ptr::null_mut(),
> +                FenceObject::<T>::SIZE,
> +                bindings::GFP_KERNEL | bindings::__GFP_ZERO,
> +            ) as *mut FenceObject<T>
> +        };
> +
> +        if p.is_null() {
> +            return Err(ENOMEM);
> +        }
> +
> +        let seqno = self.seqnos[context as usize].fetch_add(1, Ordering::Relaxed);
> +
> +        // SAFETY: The pointer is valid, so pointers to members are too.
> +        // After this, all fields are initialized.
> +        unsafe {
> +            addr_of_mut!((*p).inner).write(inner);
> +            bindings::__spin_lock_init(
> +                addr_of_mut!((*p).lock) as *mut _,
> +                self.lock_name.as_char_ptr(),
> +                self.lock_key.get(),
> +            );
> +            bindings::dma_fence_init(
> +                addr_of_mut!((*p).fence),
> +                &FenceObject::<T>::VTABLE,
> +                addr_of_mut!((*p).lock) as *mut _,
> +                self.start + context as u64,
> +                seqno,
> +            );
> +        };
> +
> +        Ok(UniqueFence(p))
> +    }
> +}
> +
> +/// A DMA Fence Chain Object
> +///
> +/// # Invariants
> +/// ptr is a valid pointer to a dma_fence_chain which we own.
> +pub struct FenceChain {
> +    ptr: *mut bindings::dma_fence_chain,
> +}
> +
> +impl FenceChain {
> +    /// Create a new DmaFenceChain object.
> +    pub fn new() -> Result<Self> {
> +        // SAFETY: This function is safe to call and takes no arguments.
> +        let ptr = unsafe { bindings::dma_fence_chain_alloc() };
> +
> +        if ptr.is_null() {
> +            Err(ENOMEM)
> +        } else {
> +            Ok(FenceChain { ptr })
> +        }
> +    }
> +
> +    /// Convert the DmaFenceChain into the underlying raw pointer.
> +    ///
> +    /// This assumes the caller will take ownership of the object.
> +    pub(crate) fn into_raw(self) -> *mut bindings::dma_fence_chain {
> +        let ptr = self.ptr;
> +        core::mem::forget(self);
> +        ptr
> +    }
> +}
> +
> +impl Drop for FenceChain {
> +    fn drop(&mut self) {
> +        // SAFETY: We own this dma_fence_chain.
> +        unsafe { bindings::dma_fence_chain_free(self.ptr) };
> +    }
> +}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index cb23d24c6718..31866069e0bc 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -36,6 +36,8 @@ mod allocator;
>  mod build_assert;
>  pub mod delay;
>  pub mod device;
> +#[cfg(CONFIG_DMA_SHARED_BUFFER)]
> +pub mod dma_fence;
>  pub mod driver;
>  #[cfg(CONFIG_RUST_DRM)]
>  pub mod drm;
> 
> -- 
> 2.35.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch