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Wed, 03 Jun 2026 09:41:25 -0700 (PDT) From: Daniel Almeida X-Google-Original-From: Daniel Almeida Content-Type: text/plain; charset=utf-8 Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3826.700.81\)) Subject: Re: [PATCH 3/4] rust: Add dma_fence abstractions In-Reply-To: <20260530143541.229628-5-phasta@kernel.org> Date: Wed, 3 Jun 2026 13:41:02 -0300 Cc: Miguel Ojeda , Boqun Feng , Gary Guo , =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= , Benno Lossin , Andreas Hindborg , Alice Ryhl , Trevor Gross , Danilo Krummrich , Sumit Semwal , =?utf-8?Q?Christian_K=C3=B6nig?= , "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Greg Kroah-Hartman , Igor Korotin , Lorenzo Stoakes , Alexandre Courbot , FUJITA Tomonori , Krishna Ketan Rai , Shankari Anand , manos@pitsidianak.is, Boris Brezillon , linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, rcu@vger.kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: <4F8E8E04-5AB5-4E6B-9194-5FC467E2313F@collabora.com> References: <20260530143541.229628-2-phasta@kernel.org> <20260530143541.229628-5-phasta@kernel.org> To: Philipp Stanner X-Mailer: Apple Mail (2.3826.700.81) Hi Philipp, As I said on the cover letter, this works on Tyr (with our own = JobQueue), but there are a few things to point out. > On 30 May 2026, at 11:35, Philipp Stanner wrote: >=20 > C's dma_fence's are synchronisation primitives that will be needed by = all > Rust GPU drivers. >=20 > The dma_fence framework sets a number of rules, notably: > - fences must only be signalled once > - all fences must be signalled at some point > - fence error codes must only be set before signalling > - every pointer to a fence must be backed by a reference >=20 > All those rules are being addressed by these abstractions. >=20 > To cleanly decouple fence issuers and consumers, two types are = provided: > - DriverFence: the only fence type that can be signalled and that > carries driver-specific data. > - Fence: the fence type to be shared with other drivers and / or > userspace. The only type callbacks can be registered on. > Cannot be signalled. >=20 > Hereby, a Fence lives in the same chunk of memory as a DriverFence. = Both > share the refcount of the underlying C dma_fence. Since this > implementation does not provide a custom = dma_fence_backend_ops.release() > function, the memory is freed by the dma_fence backend once the = refcount > drops to 0. >=20 > To create a DriverFence, the user must first allocate a > DriverFenceAllocation, so that the creation of the DriverFence later = on > can always succeed. Otherwise, deadlocks could occur if fences need to > be created in a GPU job submission path. >=20 > Synchronization is ensured by the dma_fence backend. >=20 > All DriverFence's created through this abstraction must be signalled = by > the creator with an error code. In case a DriverFence drops without > being signalled beforehand, it is signalled with -ECANCELLED as its > error and a warning is printed. This allows the Rust abstraction to = very > cleanly decouple fence issuer and consumer by relying on the = decoupling > mechanisms in the C backend, which ensures through RCU and the > 'signalled' fence-flag that dma_fence_backend_ops functions cannot > access the potentially unloaded driver code anymore. >=20 > Signalling fences on drop thus grants many advantages. Not signalling > fences on drop would risk deadlock and does not grant real advantages: > By definition only the drivers can ensure that a fence always = represents > the hardware's state correctly. >=20 > This implementation models a DmaFenceCtx (fence context) object on = which > fences are to be created, thereby ensuring correct sequence numbering > according to the timeline. >=20 > dma_fence supports a variety of callbacks. The mandatory callbacks > (get_timeline_name() and get_driver_name()) are implemented in this > patch. For convenience, they store those name parameters in the fence > context, saving the driver from implementing these two callbacks. >=20 > Support for other callbacks (like for hardware signalling) is prepared > for through the fact that both DriverFence and Fence live in the same > allocation, allowing for usage of container_of from the callback to > access the driver-specific data. >=20 > Synchronization for backend_ops callbacks is ensured through RCU which > prevents UAF-bugs should a DriverFence drop while a Fence callback > is currently operating on the associated driver data. >=20 > Add abstractions for dma_fence in Rust. >=20 > Signed-off-by: Philipp Stanner > --- > rust/bindings/bindings_helper.h | 1 + > rust/helpers/dma_fence.c | 48 ++ > rust/helpers/helpers.c | 1 + > rust/kernel/dma_buf/dma_fence.rs | 821 +++++++++++++++++++++++++++++++ > rust/kernel/dma_buf/mod.rs | 13 + > rust/kernel/lib.rs | 1 + > 6 files changed, 885 insertions(+) > create mode 100644 rust/helpers/dma_fence.c > create mode 100644 rust/kernel/dma_buf/dma_fence.rs > create mode 100644 rust/kernel/dma_buf/mod.rs >=20 > diff --git a/rust/bindings/bindings_helper.h = b/rust/bindings/bindings_helper.h > index 2011645c7cfb..69daeb790f77 100644 > --- a/rust/bindings/bindings_helper.h > +++ b/rust/bindings/bindings_helper.h > @@ -52,6 +52,7 @@ > #include > #include > #include > +#include > #include > #include > #include > diff --git a/rust/helpers/dma_fence.c b/rust/helpers/dma_fence.c > new file mode 100644 > index 000000000000..6244a5a61038 > --- /dev/null > +++ b/rust/helpers/dma_fence.c > @@ -0,0 +1,48 @@ > +// SPDX-License-Identifier: GPL-2.0 > + > +#include > + > +__rust_helper void rust_helper_dma_fence_get(struct dma_fence *f) > +{ > + dma_fence_get(f); > +} > + > +__rust_helper void rust_helper_dma_fence_put(struct dma_fence *f) > +{ > + dma_fence_put(f); > +} > + > +__rust_helper bool rust_helper_dma_fence_begin_signalling(void) > +{ > + return dma_fence_begin_signalling(); > +} > + > +__rust_helper void rust_helper_dma_fence_end_signalling(bool cookie) > +{ > + dma_fence_end_signalling(cookie); > +} > + > +__rust_helper bool rust_helper_dma_fence_is_signaled(struct dma_fence = *f) > +{ > + return dma_fence_is_signaled(f); > +} > + > +__rust_helper bool rust_helper_dma_fence_is_signaled_locked(struct = dma_fence *f) > +{ > + return dma_fence_is_signaled_locked(f); > +} > + > +__rust_helper void rust_helper_dma_fence_lock_irqsave(struct = dma_fence *f, unsigned long *flags) > +{ > + dma_fence_lock_irqsave(f, *flags); > +} > + > +__rust_helper void rust_helper_dma_fence_unlock_irqrestore(struct = dma_fence *f, unsigned long *flags) > +{ > + dma_fence_unlock_irqrestore(f, *flags); > +} > + > +__rust_helper void rust_helper_dma_fence_set_error(struct dma_fence = *f, int error) > +{ > + dma_fence_set_error(f, error); > +} > diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c > index 625921e27dfb..d9114d0b3c8f 100644 > --- a/rust/helpers/helpers.c > +++ b/rust/helpers/helpers.c > @@ -57,6 +57,7 @@ > #include "cred.c" > #include "device.c" > #include "dma.c" > +#include "dma_fence.c" > #include "dma-resv.c" > #include "drm.c" > #include "err.c" > diff --git a/rust/kernel/dma_buf/dma_fence.rs = b/rust/kernel/dma_buf/dma_fence.rs > new file mode 100644 > index 000000000000..7dc1f5c16b02 > --- /dev/null > +++ b/rust/kernel/dma_buf/dma_fence.rs > @@ -0,0 +1,821 @@ > +// SPDX-License-Identifier: GPL-2.0 > +// > +// Copyright (C) 2025, 2026 Red Hat Inc.: > +// - Philipp Stanner > + > +//! DriverFence support. > +//! > +//! Reference: = > +//! > +//! C header: = [`include/linux/dma-fence.h`](srctree/include/linux/dma-fence.h) > + > +use crate::{ > + alloc::AllocError, > + bindings, > + container_of, > + error::to_result, > + prelude::*, > + sync::rcu::RcuBox, > + types::ForeignOwnable, > + types::Opaque, > + warn_on, // > +}; > + > +use pin_init::pin_init_from_closure; > + > +use core::{ > + marker::PhantomData, // > + ops::Deref, > + ptr, > + ptr::{ > + drop_in_place, > + NonNull, // > + }, > + sync::atomic::{ > + AtomicU64, > + Ordering, // > + }, > +}; > + > +use bindings::ECANCELED; > + > +use kernel::str::CString; > +use kernel::sync::{ > + aref::{ > + ARef, > + AlwaysRefCounted, // > + }, > + Arc, > + ArcBorrow, // > +}; > + > +/// VTable for dma_fence backend_ops callbacks. > +// > +// Mandatory dma_fence backend_ops are implemented implicitly through > +// [`FenceCtx`]. Additional ones shall get implemented on this trait, = which then > +// shall be demanded for the fence context data. > +pub trait FenceCtxOps {} > + > +/// A dma-fence context. A fence context takes care of associating = related fences with each other, > +/// providing each with raising sequence numbers and a common = identifier. > +#[pin_data(PinnedDrop)] > +pub struct FenceCtx { IMHO, I think we should avoid acronyms. This can be called =E2=80=9CFenceContext=E2=80=9D just fine. > + /// The fence context number. > + nr: u64, > + /// The sequence number for the next fence created. > + seqno: AtomicU64, > + // The name parameters live in RcuBox because they can be = accessed by the > + // dma_fence backend_ops. Those accesses are guarded by the = rcu_read_lock(), > + // so dropping them must be delayed by a grace period. > + /// The name of the driver this FenceCtx's fences belong to. > + driver_name: CString, > + /// The name of the timeline this FenceCtx's fences belong to. > + timeline_name: CString, > + #[pin] > + data: C, > + fence_type: PhantomData, > +} > + > +#[allow(unused_unsafe)] > +impl = FenceCtx { > + // This can later be extended as a vtable in case other parties = need support > + // for the more "exotic" callbacks. > + const OPS: bindings::dma_fence_ops =3D bindings::dma_fence_ops { > + get_driver_name: Some(Self::get_driver_name), > + get_timeline_name: Some(Self::get_timeline_name), > + enable_signaling: None, > + signaled: None, > + wait: None, > + release: None, > + set_deadline: None, > + }; > + > + /// Create a new `FenceCtx`. [`FenceCtx`] (or [`FenceContext`], if you agree with the change.=20 > + pub fn new( > + driver_name: CString, > + timeline_name: CString, > + data: impl PinInit, > + ) -> Result> { > + let ctx =3D pin_init!(Self { > + // SAFETY: `dma_fence_context_alloc()` merely works on a = global atomic. Parameter `1` > + // is the number of contexts we want to allocate. > + nr: unsafe { bindings::dma_fence_context_alloc(1) }, > + seqno: AtomicU64::new(0), > + driver_name, > + timeline_name, > + data <- data, > + fence_type: PhantomData, > + }); > + > + Arc::pin_init(ctx, GFP_KERNEL) > + } > + > + fn get_next_fence_seqno(&self) -> u64 { > + self.seqno.fetch_add(1, Ordering::Relaxed) > + } I=E2=80=99d personally avoid using =E2=80=9Cget=E2=80=9D prefixes in = general in Rust.=20 Also be aware, in the future this will need to be pub(crate), i.e.: as soon as you have syncobjs. > + > + /// Allocate the memory for a [`DriverFence`] and already store = `data` inside. > + /// > + /// This is needed because many times, creation of a = [`DriverFence`] must not > + /// fail, and allocating might deadlock in some situations. > + /// > + /// The `data` you pass here must not perform any operations that = are illegal > + /// in atomic context in its [`Drop`] implementation. > + pub fn new_fence_allocation( > + self: ArcBorrow<'_, Self>, > + data: F, > + ) -> Result> { > + let fctx =3D Arc::::from(self); > + > + DriverFenceAllocation::new(fctx, data) > + } > + > + /// Create a new fence, consuming `data`. > + /// > + /// The fence will increment the refcount of the fence context = associated with this > + /// [`FenceCtx`]. > + pub fn new_fence(&self, memory: DriverFenceAllocation) -> = DriverFence { > + let seqno: u64 =3D self.get_next_fence_seqno(); > + > + // We feed the C dma_fence backend a NULL for the spinlock so = that it > + // uses per-fence locks automatically. > + let null_ptr: *mut bindings::spinlock =3D ptr::null_mut(); > + let fence_ptr =3D memory.as_raw(); > + // SAFETY: `fence_ptr` has been created directly above. It = will live > + // at least as long as `Self`. The same applies to = `&Self::OPS`. > + unsafe { bindings::dma_fence_init(fence_ptr, &Self::OPS, = null_ptr, self.nr, seqno) }; > + > + // A `DriverFenceAllocation`'s purpose is to carry allocated = memory, so that > + // `DriverFence`s can always be created without allocating. = In this > + // method, ownership over that memory is transferred to the = new > + // `DriverFence` and managed through refcounting. The C = dma_fence > + // backend will ultimately free the memory once the refcount = reaches 0. > + let ptr =3D KBox::into_raw(memory.data); > + // SAFETY: `ptr` was just created validly directly above. > + let ptr =3D unsafe { NonNull::new_unchecked(ptr) }; > + > + DriverFence { data: ptr } > + } > + > + extern "C" fn get_driver_name(ptr: *mut bindings::dma_fence) -> = *const c_char { > + // SAFETY: The C backend only invokes this callback with = `ptr` pointing > + // to a valid, unsignaled `bindings::dma_fence`. All fences = created > + // in this module always reside within `Fence` which always = resides in > + // a `DriverFenceData`, thus satisfying the function's safety = requirements. > + let fctx =3D unsafe { Self::from_raw_fence(ptr) }; > + > + fctx.driver_name.as_char_ptr() > + } > + > + extern "C" fn get_timeline_name(ptr: *mut bindings::dma_fence) -> = *const c_char { > + // SAFETY: The C backend only invokes this callback with = `ptr` pointing > + // to a valid, unsignaled `bindings::dma_fence`. All fences = created > + // in this module always reside within `Fence` which always = resides in > + // a `DriverFenceData`, thus satisfying the function's safety = requirements. > + let fctx =3D unsafe { Self::from_raw_fence(ptr) }; > + > + fctx.timeline_name.as_char_ptr() > + } > + > + /// Create a [`FenceCtx`] from an associated = [`bindings::dma_fence`]. > + /// > + /// # Safety > + /// > + /// `ptr` must be a valid pointer to a dma_fence which resides = within a [`Fence`], > + /// which in turn resides in a [`DriverFenceData`]. > + unsafe fn from_raw_fence<'a>(ptr: *mut bindings::dma_fence) -> = &'a Self { > + let opaque_fence =3D Opaque::cast_from(ptr); > + > + // SAFETY: Safe due to the function's overall safety = requirements. > + let fence_ptr =3D unsafe { container_of!(opaque_fence, Fence, = inner) }; > + > + // DriverFenceData is repr(C) and a Fence is its first = member. > + let fence_data_ptr =3D fence_ptr as *mut DriverFenceData; > + > + // SAFETY: Safe because of the safety comment directly above. > + let fence_data =3D unsafe { &*fence_data_ptr }; > + > + &fence_data.fctx > + } > +} > + > +// FenceCtx's drop() ensures that the driver cannot unload while = there are still > +// dma_fence callbacks running. This also prevents UAF problems with = fctx.driver_name > +// and fctx.timeline_name. > +// > +// DriverFence data gets dropped through call_rcu() in = DriverFence::drop. > +// This `rcu_barrier()` also serves to wait for their completion. > +#[pinned_drop] > +impl PinnedDrop for FenceCtx { > + fn drop(self: Pin<&mut Self>) { > + // SAFETY: `rcu_barrier()` is always safe to be called. > + unsafe { bindings::rcu_barrier() }; > + } > +} > + > +/// Error type for fence callback registration. > +/// > +/// Generic over `T` so that `AlreadySignaled` can return the = callback to the > +/// caller, allowing it to reclaim any resources owned by the = callback (e.g., > +/// a fence handle that needs to be signaled). > +#[derive(Debug)] > +pub enum CallbackError { > + /// The fence was already signaled. The callback is returned so = the caller > + /// can extract owned resources without losing them. > + AlreadySignaled(T), > + /// Some other error occurred during registration. > + Other(Error), > +} > + > +impl From> for Error { > + fn from(err: CallbackError) -> Self { > + match err { > + CallbackError::AlreadySignaled(_) =3D> ENOENT, > + CallbackError::Other(e) =3D> e, > + } > + } > +} > + > +impl From for CallbackError { > + fn from(e: AllocError) -> Self { > + CallbackError::Other(Error::from(e)) > + } > +} > + > +/// Trait for callbacks that can be registered on fences. > +/// > +/// When the fence signals, the callback will be invoked. > +/// > +/// # Example > +/// > +/// ```rust > +/// use kernel::dma_buf::FenceCb; > +/// > +/// struct MyCallback { > +/// // Your callback state here > +/// } > +/// > +/// impl FenceCb for MyCallback { > +/// fn called(&mut self) { > +/// pr_info!("Fence signaled!"); > +/// // Handle fence completion > +/// } > +/// } > +/// ``` > +pub trait FenceCb: Send + 'static { Same here, this can be called =E2=80=9CFenceCallback=E2=80=9D just fine. > + /// Called when the fence is signaled. > + /// > + /// This is called from the fence signaling path, which may be in = interrupt > + /// context or with locks held, which is why `self` is only = borrowed, so that > + /// it cannot drop. Implementations must not sleep or perform > + /// long-running operations. > + /// > + /// An implementation likely wants to inform itself (e.g., = through a work item) > + /// within this callback that the associated = [`FenceCbRegistration`] can now be > + /// dropped. > + fn called(&mut self); This is a central point. We ideally would want this to consume self, = because we may want to move things out of the callback. =20 Consider a fence design where signal() consumes self. Now consider this: ``` impl FenceCb for MyCallback { fn called(&mut self) { // Can't move the fence out, so we have to put an Option just to = be able // to move. if let Some(f) =3D self.some_fence.take() { f.signal(); } } ``` This used to be the case when our version of the job queue used the = "proxy fence" design: ``` // Callback on the hw fence impl FenceCb for MyCallback { fn called(&mut self) { if let Some(f) =3D self.submit_fence.take() { f.signal(); } } ``` Although this is not the case anymore, since we phased out this design = given Christian's recent work. Still, we should ideally not require Option = here in general just to make resource transfer possible. > +} > + > +/// A callback registration on a fence. > +/// > +/// When this object is dropped, the callback is automatically = removed if it > +/// hasn't been called yet. > +/// > +/// # Invariants > +/// > +/// If `callback` is `Some`, then `cb` is registered with the fence = and the > +/// callback hasn't been invoked yet. If `None`, the callback has = been invoked > +/// or the fence was already signaled when we tried to register. > +#[pin_data(PinnedDrop)] > +pub struct FenceCbRegistration { > + #[pin] > + cb: Opaque, > + callback: T, > + fence: ARef, > +} > + > +impl FenceCbRegistration { > + /// Register a callback on a fence. > + /// > + /// On success the callback is pinned in place and will fire when = the fence > + /// signals. On `AlreadySignaled` the callback is returned to the = caller so > + /// that owned resources can be reclaimed. > + pub fn new<'a>(fence: &'a Fence, callback: T) -> impl = PinInit> + 'a > + where > + T: 'a, > + { > + // Uses `pin_init_from_closure` instead of `try_pin_init!` so = that on > + // `-ENOENT` (already signaled) the callback can be read back = from the > + // partially-initialized slot and returned through the error. > + // > + // SAFETY: `pin_init_from_closure` requires: > + // - On `Ok(())`: the slot is fully initialized and valid for = `Drop`. > + // - On `Err(_)`: the slot is clean, i.e.: no = partially-initialized fields > + // remain, and the slot can be deallocated without = dropping. > + // > + // We uphold this as follows: > + // - On success: all three fields are initialized. Ok(()) is = returned. > + // - On ENOENT (already signaled): `callback` and `fence` are = read back > + // from the slot via `ptr::read`, leaving the slot clean. = `cb` was > + // initialized by `dma_fence_add_callback` (it calls > + // `INIT_LIST_HEAD(&cb->node)` even on error), but `cb` is > + // `Opaque` which has no `Drop`, so not = dropping it is > + // fine. The callback is returned through = `AlreadySignaled(T)`. > + // - On other errors: same cleanup as ENOENT, error returned = as > + // `Other(e)`. > + unsafe { > + pin_init_from_closure(move |slot: *mut Self| { > + let slot_callback =3D &raw mut (*slot).callback; > + let slot_fence =3D &raw mut (*slot).fence; > + let slot_cb =3D &raw mut (*slot).cb; > + > + // Write callback and fence first =E2=80=94 must be = visible before > + // dma_fence_add_callback makes the registration = live. > + core::ptr::write(slot_callback, callback); > + core::ptr::write(slot_fence, ARef::from(fence)); > + > + let ret =3D = to_result(bindings::dma_fence_add_callback( > + fence.inner.get(), > + Opaque::cast_into(slot_cb), > + Some(Self::dma_fence_callback), > + )); > + > + match ret { > + Ok(()) =3D> Ok(()), > + Err(e) =3D> { > + // Read back what we wrote to leave the slot = clean. > + let cb_back =3D = core::ptr::read(slot_callback); > + let _fence_back =3D = core::ptr::read(slot_fence); > + > + if e.to_errno() =3D=3D ENOENT.to_errno() { > + = Err(CallbackError::AlreadySignaled(cb_back)) > + } else { > + Err(CallbackError::Other(e)) > + } > + } > + } > + }) > + } > + } > + > + /// Raw dma fence callback that is called by the C code. > + /// > + /// # Safety > + /// > + /// This is only called by the dma_fence subsystem with valid = pointers. > + unsafe extern "C" fn dma_fence_callback( > + _fence: *mut bindings::dma_fence, > + cb: *mut bindings::dma_fence_cb, > + ) { > + let ptr =3D Opaque::cast_from(cb).cast_mut(); > + > + // SAFETY: All `cb` we can receive here have been created in = such a way > + // that they are embedded into a `FenceCbRegistration`. The = backend > + // ensures synchronisation so whoever holds the registration = object > + // cannot drop it while this code is running. See = `FenceCbRegistration::drop`. > + unsafe { > + let reg: *mut Self =3D container_of!(ptr, Self, cb); > + > + (*reg).callback.called(); > + } > + } > + > + /// Returns a reference to the fence this callback is registered = on. > + pub fn fence(self: Pin<&Self>) -> &Fence { > + &self.get_ref().fence > + } > +} > + > +#[pinned_drop] > +impl PinnedDrop for FenceCbRegistration { > + fn drop(self: Pin<&mut Self>) { > + // Always call dma_fence_remove_callback, even if `callback` = has already > + // been taken by `dma_fence_callback`. This is necessary for > + // synchronization: `dma_fence_remove_callback` acquires = `fence->lock`, > + // which ensures that any in-flight `dma_fence_signal` (which = calls our > + // callback while holding the same lock) has completed before = we free > + // the struct. > + // > + // Without this, Drop can race with a concurrent signal: > + // CPU0 (signal, lock held): take() -> signaled(fence_ref) = (in progress) > + // CPU1 (drop): sees is_some()=3D=3Dfalse -> skips lock -> = frees struct > + // CPU0: accesses fence_ref -> use-after-free > + // > + // When the callback has already fired, the signal path = detached the > + // list node via INIT_LIST_HEAD, so dma_fence_remove_callback = just sees > + // an empty node and returns false =E2=80=94 the lock = acquisition is the only > + // thing that matters. > + // > + // SAFETY: The fence pointer is valid and the cb was = initialized by > + // dma_fence_add_callback during construction. > + unsafe { > + bindings::dma_fence_remove_callback(self.fence.as_raw(), = self.cb.get()); > + } > + } > +} > + > +// SAFETY: FenceCbRegistration can be sent between threads > +unsafe impl Send for FenceCbRegistration {} > + > +// SAFETY: &FenceCbRegistration can be shared between threads if &T = can. > +unsafe impl Sync for FenceCbRegistration where T: Sync = {} > + > +/// The receiving counterpart of a [`DriverFence`], designed to = register callbacks > +/// on, check the signalled state etc. A [`Fence`] cannot be = signalled. > +/// A [`Fence`] is always refcounted. > +pub struct Fence { > + /// The actual dma_fence passed to C. > + inner: Opaque, > +} > + > +// SAFETY: Fences are literally designed to be shared between = threads. > +unsafe impl Send for Fence {} > +// SAFETY: Fences are literally designed to be shared between = threads. > +unsafe impl Sync for Fence {} > + > +impl Fence { > + /// Check whether the fence was signalled at the moment of the = function call. > + pub fn is_signaled(&self) -> bool { > + // SAFETY: self is by definition still valid. The backend = ensures proper > + // locking. > + unsafe { bindings::dma_fence_is_signaled(self.as_raw()) } > + } > + > + fn as_raw(&self) -> *mut bindings::dma_fence { > + self.inner.get() > + } Same here, this will need to be pub(crate) as soon as you have syncobjs, = JFYI. > + > + /// Create a [`Fence`] from a raw C [`bindings::dma_fence`]. > + /// > + /// # Safety > + /// > + /// `ptr` must point to an initialized fence that is embedded = into a [`Fence`]. > + pub unsafe fn from_raw<'a>(ptr: *mut bindings::dma_fence) -> &'a = Self { > + // SAFETY: Safe as per the function's overall safety = requirements. > + unsafe { &*ptr.cast() } > + } > +} > + > +// SAFETY: These implement the C backends refcounting methods which = are proven to work correctly. > +unsafe impl AlwaysRefCounted for Fence { > + fn inc_ref(&self) { > + // SAFETY: `self.as_raw()` is a pointer to a valid `struct = dma_fence`. > + unsafe { bindings::dma_fence_get(self.as_raw()) } > + } > + > + /// # Safety > + /// > + /// `ptr`must be a valid pointer to a [`DriverFence`]. > + unsafe fn dec_ref(ptr: NonNull) { > + // SAFETY: `ptr` is never a NULL pointer; and when = `dec_ref()` is called > + // the fence is by definition still valid. > + let fence =3D unsafe { (*ptr.as_ptr()).inner.get() }; > + > + // SAFETY: Valid because `fence` was created validly above. > + unsafe { bindings::dma_fence_put(fence) } > + } > +} > + > +#[repr(C)] // Necessary to guarantee that `inner` always comes first = so that we can cast. > +#[pin_data] > +struct DriverFenceData { > + #[pin] > + /// The inner fence. > + inner: Fence, > + /// Pointer to access the FenceCtx. Useful for obtaining name = parameters. > + // The FenceCtx lives as long as at least all its fences, hence = this is safe. > + fctx: Arc>, > + /// The API user's data. As required by = [`DriverFenceAllowedData`], this either > + /// does not need drop, or must live in a [`rcu::RcuBox`]. It is = essential > + /// that the data only performs operations legal in atomic = context in its > + /// [`Drop`] implementation. > + #[pin] > + data: F, > +} > + > +/// A trait to enforce that all data in a [`DriverFence`] either does = not need > +/// drop, or lives in a [`RcuBox`]. > +pub trait DriverFenceAllowedData: private::Sealed {} > + > +mod private { > + pub trait Sealed {} > +} > + > +impl DriverFenceAllowedData for F {} > +impl DriverFenceAllowedData for RcuBox {} > + > +impl private::Sealed for F {} > +impl private::Sealed for RcuBox {} > + > +/// A synchronization primitive mainly for GPU drivers. > +/// > +/// Fences are always reference counted. The typical use case is that = one side registers > +/// callbacks on the fence which will perform a certain action (such = as queueing work) once the > +/// other side signals the fence. > +/// > +/// # Examples > +/// > +/// ``` > +/// use kernel::dma_buf::{DriverFence, FenceCtx, FenceCb, = FenceCbRegistration}; > +/// use kernel::str::CString; > +/// use kernel::sync::{ > +/// aref::ARef, > +/// rcu::RcuBox, // > +/// }; > +/// use core::ops::Deref; > +/// use core::fmt::Display; > +/// > +/// struct CallbackData { } > +/// > +/// impl FenceCb for CallbackData { > +/// fn called(&mut self) { > +/// pr_info!("DmaFence callback executed.\n"); > +/// } > +/// } > +/// > +/// let driver_name =3D CString::try_from_fmt(fmt!("dummy_driver"))?; > +/// let timeline_name =3D = CString::try_from_fmt(fmt!("dummy_timeline"))?; > +/// > +/// let fctx =3D FenceCtx::new(driver_name, timeline_name, ())?; > +/// > +/// let fence_data =3D CString::try_from_fmt(fmt!("dummy_data"))?; > +/// // DriverFence::data must either not need drop, or live in an = RcuBox. > +/// let fence_data =3D RcuBox::new(fence_data, GFP_KERNEL)?; > +/// > +/// let fence_alloc =3D = fctx.as_arc_borrow().new_fence_allocation(fence_data)?; > +/// let mut fence =3D fctx.new_fence(fence_alloc); > +/// > +/// let cb_data =3D CallbackData { }; > +/// let waiting_fence =3D ARef::from(fence.as_fence()); > +/// let cb_reg =3D FenceCbRegistration::new(&waiting_fence, cb_data); > +/// let cb_reg =3D KBox::pin_init(cb_reg, GFP_KERNEL)?; > +/// > +/// // DriverFence implements Deref. > +/// // FIXME: unit test claims that CString does not implement = Display. Why? > +/// // pr_info!("Fence's inner data is: {}", fence.deref().deref()); > +/// > +/// // TODO begin_signalling > +/// fence.signal(Ok(())); > +/// assert_eq!(waiting_fence.is_signaled(), true); > +/// > +/// Ok::<(), Error>(()) > +/// ``` > +pub struct DriverFence { C is () on Tyr, yet we have to drag this extra generic everywhere in the = API. I feel this is a bit of an ergonomics regression given our version on = tyr-dev, and it will get worse if we add a third generic later for the typestate. As I said before, my concern is that this extra generic does not stay = local to the fence layer. It leaks into APIs that do not actually care about it. = For example, queue code and types like PreparedJob, SubmitResult, and = similar types usually care about jobs plus fence behavior. They do not care about this = C type enough to have it show up in the API. With the extra generic on the fence, that context type starts showing up everywhere: pub trait QueueOps { type Job; type FencePayload; type FenceCtxData; fn submit( &self, job: &Self::Job, fence: DriverFence, ) -> Result>; } pub enum SubmitResult { Submitted, NoResources(DriverFence), } pub struct PreparedJob { job: Arc, fence: DriverFence, } // etc > + /// The actual content of the fence. Lives in a raw pointer so = that its > + /// memory can be managed independently. Valid until both the = [`DriverFence`] > + /// and all associated [`Fence`]s have disappeared. > + data: NonNull>, We used to have a ManuallyDrop here, I wonder why you decided to move = away from that? I'm asking because now, the lifetime is not explicit on the types, = and now you have to manually implement them correctly. Which you do, but = it's easier to get wrong. > +} > + > +/// A pre-prepared DMA fence, carrying the user's data and the memory = it and the Not sure what pre-prepared means here. I guess the useful fact is that dma_fence_init has not been called, hence =E2=80=9CUninitFence=E2=80=9D = being a better name IMHO. But...I guess this is a matter of opinion, so no big deal either way. > +/// fence reside in. Only useful for creating a [`DriverFence`]. = Splitting > +/// allocation and full initialization is necessary because fences = cannot be > +/// allocated dynamically in some circumstances (deadlock). > +pub struct DriverFenceAllocation { > + /// The memory for the actual content of the fence. > + /// Handed over to a [`DriverFence`], or deallocated once the > + /// [`DriverFenceAllocation`] drops. > + data: KBox>, > +} > + > +impl = DriverFenceAllocation { > + /// Create a new allocation slot that can later be used to create = a fully > + /// initialized [`DriverFence`] without the need to allocate. > + pub fn new(fctx: Arc>, data: F) -> Result { > + let fence_data =3D DriverFenceData { > + // `inner` remains uninitialized until a [`DriverFence`] = takes over. > + inner: Fence { > + inner: Opaque::uninit(), > + }, > + fctx, > + data, > + }; > + > + // In order to support the C dma_fence callbacks, it is = necessary for > + // a `Fence` and a `DriverFence` to live in the same = allocation, > + // because the C backend passes a dma_fence, from which the = driver most > + // likely wants to be able to access its `data` in = `DriverFence`. > + // > + // Hence, we need the manage the memory manually. It will be = freed by the > + // C backend automatically once the refcount within `Fence` = drops to 0. > + let data =3D KBox::new(fence_data, GFP_KERNEL | __GFP_ZERO)?; > + > + Ok(Self { data }) > + } > + > + fn as_raw(&self) -> *mut bindings::dma_fence { > + self.data.inner.inner.get() > + } > +} > + > +impl DriverFence { > + fn as_raw(&self) -> *mut bindings::dma_fence { > + // SAFETY: Valid because `self` is valid. > + let fence_data =3D unsafe { &mut *self.data.as_ptr() }; > + > + fence_data.inner.inner.get() > + } > + > + /// Create a [`DriverFence`] from a raw pointer to a = [`bindings::dma_fence`]. > + /// > + /// # Safety > + /// > + /// `ptr` must be a valid pointer to a `dma_fence` that was = obtained through > + /// a [`DriverFence`] with matching generic data for both fence = and associated > + /// [`FenceCtx`]. > + unsafe fn from_raw(ptr: *mut bindings::dma_fence) -> Self { > + let opaque_fence =3D Opaque::cast_from(ptr); > + > + // SAFETY: Safe due to the function's overall safety = requirements. > + let fence_ptr =3D unsafe { container_of!(opaque_fence, Fence, = inner) }; > + > + // DriverFenceData is repr(C) and a Fence is its first = member. > + let fence_data_ptr =3D fence_ptr as *mut DriverFenceData; > + > + // SAFETY: `fence_data_ptr` was created validly above. > + let data =3D unsafe { NonNull::new_unchecked(fence_data_ptr) = }; > + > + Self { data } > + } > + > + /// Return the underlying [`Fence`]. > + pub fn as_fence(&self) -> &Fence { AsRef ? > + // SAFETY: `self` is by definition still valid, and it cannot = drop until > + // this new reference is gone. > + unsafe { Fence::from_raw(self.as_raw()) } > + } > + > + /// Signal the fence. This will invoke all registered callbacks. > + pub fn signal(self, res: Result) { > + let fence =3D self.as_raw(); > + let mut fence_flags: usize =3D 0; > + let flag_ptr =3D &raw mut fence_flags; > + > + // SAFETY: Once a `DriverFence` is initialized, the inner = `fence` is > + // valid and initialized. It is valid until the refcount = drops > + // to 0, which can earliest happen once the `DriverFence` has = been dropped. > + unsafe { > + bindings::dma_fence_lock_irqsave(fence, flag_ptr); > + if !bindings::dma_fence_is_signaled_locked(fence) { > + if let Err(err) =3D res { > + bindings::dma_fence_set_error(fence, = err.to_errno()); > + } > + bindings::dma_fence_signal_locked(fence); > + } > + bindings::dma_fence_unlock_irqrestore(fence, flag_ptr); > + } > + } > +} > + > +// SAFETY: Fences are literally designed to be shared between = threads. > +unsafe impl Send for DriverFence {} > + > +impl Deref for DriverFence { > + type Target =3D F; > + > + fn deref(&self) -> &Self::Target { > + // SAFETY: Thanks to refcounting, `data` is always valid as = long as `self` is. > + let data =3D unsafe { &*self.data.as_ptr() }; > + > + &data.data > + } > +} > + > +/// A borrowed [`DriverFence`]. All you can do with it is access your = user data > +/// and obtain a [`Fence`]. > +pub struct DriverFenceBorrow { > + /// The actual content of the fence. Lives in a raw pointer so = that its > + /// memory can be managed independently. Valid until both the = [`DriverFence`] > + /// and all associated [`Fence`]s have disappeared. > + data: NonNull>, Same here, why no ManuallyDrop? Also, why no BorrowMut? I know we = don=E2=80=99t _need_ one, but still... > +} > + > +impl Deref for DriverFenceBorrow { > + type Target =3D F; > + > + fn deref(&self) -> &Self::Target { > + // SAFETY: Thanks to refcounting, `data` is always valid as = long as `self` is. > + let data =3D unsafe { &*self.data.as_ptr() }; > + > + &data.data > + } > +} > + > +impl DriverFenceBorrow { > + fn as_raw(&self) -> *mut bindings::dma_fence { > + // SAFETY: Valid because `self` is valid. > + let fence_data =3D unsafe { &mut *self.data.as_ptr() }; > + > + fence_data.inner.inner.get() > + } Well this is Borrow, not BorrowMut. I think we should return a const = pointer here. > + > + /// Return the underlying [`Fence`]. > + pub fn as_fence(&self) -> &Fence { > + // SAFETY: `self` is by definition still valid, and it cannot = drop until > + // this new reference is gone. > + unsafe { Fence::from_raw(self.as_raw()) } > + } AsRef? > + > + /// Get a [`DriverFenceBorrow`] from a raw pointer. > + /// > + /// # Safety > + /// > + /// `ptr` must point to a raw dma_fence within a [`Fence`] within = a [`DriverFenceData`]. > + unsafe fn from_raw(ptr: *mut bindings::dma_fence) -> Self { > + let opaque_fence =3D Opaque::cast_from(ptr); > + > + // SAFETY: Safe due to the function's overall safety = requirements. > + let fence_ptr =3D unsafe { container_of!(opaque_fence, Fence, = inner) }; > + > + // DriverFenceData is repr(C) and a Fence is its first = member. > + let fence_data_ptr =3D fence_ptr as *mut DriverFenceData; > + > + // SAFETY: `fence_data_ptr` was created validly above. > + let data =3D unsafe { NonNull::new_unchecked(fence_data_ptr) = }; > + > + Self { data } > + } > +} > + > +// SAFETY: The Rust dma_fence abstractions are already designed = around the inner > +// C `dma_fence`, which can serve safely as the identification point = when being > +// owned by C. Moreover, safety is ensured by not dropping = `DriverFence` and by > +// only allowing operations without side effects on the Borrowed = type. > +unsafe impl = ForeignOwnable > + for DriverFence > +{ > + // `DriverFence` is merely a wrapper around a raw pointer. Thus, = we can just > + // use it directly. > + type Borrowed<'a> =3D DriverFenceBorrow; > + type BorrowedMut<'a> =3D DriverFenceBorrow; > + > + const FOREIGN_ALIGN: usize =3D = core::mem::align_of::(); > + > + fn into_foreign(self) -> *mut c_void { > + let fence =3D self; > + > + let ptr =3D fence.as_raw(); > + > + // DriverFence must not drop. > + core::mem::forget(fence); > + > + ptr.cast() > + } > + > + unsafe fn from_foreign(ptr: *mut c_void) -> Self { > + // SAFETY: Safe because the trait implementation only invokes = this with > + // a valid `ptr`, associated to a `DriverFence` with matching = generic data. > + unsafe { Self::from_raw(ptr.cast()) } > + } > + > + unsafe fn borrow<'a>(ptr: *mut c_void) -> Self::Borrowed<'a> { > + // SAFETY: The trait implementation ensures that `ptr` always = resides > + // within a [`Fence`] within a [`DriverFenceData`]. > + unsafe { DriverFenceBorrow::from_raw(ptr.cast()) } > + } > + > + unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> = Self::BorrowedMut<'a> { > + // SAFETY: The trait implementation ensures that `ptr` always = resides > + // within a [`Fence`] within a [`DriverFenceData`]. > + unsafe { DriverFenceBorrow::from_raw(ptr.cast()) } > + } > +} > + > +impl Drop for DriverFence { > + fn drop(&mut self) { > + let fence =3D self.as_raw(); > + let mut fence_flags: usize =3D 0; > + let flag_ptr =3D &raw mut fence_flags; > + > + // SAFETY: Once a `DriverFence` is initialized, the inner = `fence` is > + // valid and initialized. It is valid until the refcount = drops > + // to 0, which can earliest happen once the `DriverFence` has = been dropped. > + unsafe { > + bindings::dma_fence_lock_irqsave(fence, flag_ptr); > + #[allow(unused_unsafe)] > + if = warn_on!(!bindings::dma_fence_is_signaled_locked(fence)) { > + bindings::dma_fence_set_error(fence, ECANCELED as = i32); > + bindings::dma_fence_signal_locked(fence); > + } > + bindings::dma_fence_unlock_irqrestore(fence, flag_ptr); > + } > + > + // SAFETY: `self.data` is owned by the DriverFence, but could = be accessed > + // through some dma_fence callbacks right now. Access is = being revoked > + // above by signalling the fence. The DriverFenceAllowedData = trait > + // ensures that the data either does not need drop, or if it = does it > + // lives in a RcuBox which will delay dropping by one grace = period, hence > + // ensuring that all readers have disappeared. > + unsafe { drop_in_place(self.data.as_ptr()) }; > + > + // SAFETY: Once a `DriverFence` is initialized, the inner = `fence` is > + // valid and initialized. It is valid until the refcount = drops > + // to 0, which can earliest happen once the `DriverFence` has = been dropped. > + unsafe { > + bindings::dma_fence_put(fence); > + } > + > + // The actual memory the data associated with a `DriverFence` = lives in > + // gets freed by the C dma_fence backend once the fence's = refcount reaches 0. > + } > +} > diff --git a/rust/kernel/dma_buf/mod.rs b/rust/kernel/dma_buf/mod.rs > new file mode 100644 > index 000000000000..d9da3dc57fce > --- /dev/null > +++ b/rust/kernel/dma_buf/mod.rs > @@ -0,0 +1,13 @@ > +// SPDX-License-Identifier: GPL-2.0 OR MIT > + > +//! DMA-buf subsystem abstractions. > + > +pub mod dma_fence; > + > +pub use self::dma_fence::{ > + DriverFence, > + Fence, > + FenceCb, > + FenceCbRegistration, > + FenceCtx, // > +}; > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs > index b72b2fbe046d..a05ccaa7598c 100644 > --- a/rust/kernel/lib.rs > +++ b/rust/kernel/lib.rs > @@ -63,6 +63,7 @@ > pub mod device_id; > pub mod devres; > pub mod dma; > +pub mod dma_buf; > pub mod driver; > #[cfg(CONFIG_DRM =3D "y")] > pub mod drm; > --=20 > 2.54.0 >=20