Re: [PATCH v12 2/3] rust: add dma coherent allocator abstraction.

[Abdiel Janulgue <abdiel.janulgue@gmail.com>]

On 24/02/2025 15:30, QUENTIN BOYER wrote:
> Wouldn't it be safer if the `as_slice_mut` function took a `&mut self`,
> allowing the compiler to correctly check the borrows (like `start_ptr_mut`)
> 
> Quentin

Yes that would make sense. Appreciate the feedback!

Regards,
Abdiel

> 
> On Mon Feb 24, 2025 at 12:49 PM CET, Abdiel Janulgue wrote:
>> Caution: External email. Do not open attachments or click links, unless this email comes from a known sender and you know the content is safe.
>>
>>
>> Add a simple dma coherent allocator rust abstraction. Based on
>> Andreas Hindborg's dma abstractions from the rnvme driver, which
>> was also based on earlier work by Wedson Almeida Filho.
>>
>> Nacked-by: Christoph Hellwig <hch@lst.de>
>> Signed-off-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
>> ---
>>   rust/bindings/bindings_helper.h |   1 +
>>   rust/helpers/dma.c              |  13 +
>>   rust/helpers/helpers.c          |   1 +
>>   rust/kernel/dma.rs              | 421 ++++++++++++++++++++++++++++++++
>>   rust/kernel/lib.rs              |   1 +
>>   5 files changed, 437 insertions(+)
>>   create mode 100644 rust/helpers/dma.c
>>   create mode 100644 rust/kernel/dma.rs
>>
>> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
>> index 55354e4dec14..f69b05025e52 100644
>> --- a/rust/bindings/bindings_helper.h
>> +++ b/rust/bindings/bindings_helper.h
>> @@ -11,6 +11,7 @@
>>   #include <linux/blk_types.h>
>>   #include <linux/blkdev.h>
>>   #include <linux/cred.h>
>> +#include <linux/dma-mapping.h>
>>   #include <linux/errname.h>
>>   #include <linux/ethtool.h>
>>   #include <linux/file.h>
>> diff --git a/rust/helpers/dma.c b/rust/helpers/dma.c
>> new file mode 100644
>> index 000000000000..30da079d366c
>> --- /dev/null
>> +++ b/rust/helpers/dma.c
>> @@ -0,0 +1,13 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +#include <linux/dma-mapping.h>
>> +
>> +int rust_helper_dma_set_mask_and_coherent(struct device *dev, u64 mask)
>> +{
>> +       return dma_set_mask_and_coherent(dev, mask);
>> +}
>> +
>> +int rust_helper_dma_set_mask(struct device *dev, u64 mask)
>> +{
>> +       return dma_set_mask(dev, mask);
>> +}
>> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
>> index 0640b7e115be..8f3808c8b7fe 100644
>> --- a/rust/helpers/helpers.c
>> +++ b/rust/helpers/helpers.c
>> @@ -13,6 +13,7 @@
>>   #include "build_bug.c"
>>   #include "cred.c"
>>   #include "device.c"
>> +#include "dma.c"
>>   #include "err.c"
>>   #include "fs.c"
>>   #include "io.c"
>> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
>> new file mode 100644
>> index 000000000000..b4dd5d411711
>> --- /dev/null
>> +++ b/rust/kernel/dma.rs
>> @@ -0,0 +1,421 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +//! Direct memory access (DMA).
>> +//!
>> +//! C header: [`include/linux/dma-mapping.h`](srctree/include/linux/dma-mapping.h)
>> +
>> +use crate::{
>> +    bindings, build_assert,
>> +    device::Device,
>> +    error::code::*,
>> +    error::Result,
>> +    transmute::{AsBytes, FromBytes},
>> +    types::ARef,
>> +};
>> +
>> +/// Inform the kernel about the device's DMA addressing capabilities. This will set the mask for
>> +/// both streaming and coherent APIs together.
>> +pub fn dma_set_mask_and_coherent(dev: &Device, mask: u64) -> i32 {
>> +    // SAFETY: device pointer is guaranteed as valid by invariant on `Device`.
>> +    unsafe { bindings::dma_set_mask_and_coherent(dev.as_raw(), mask) }
>> +}
>> +
>> +/// Same as `dma_set_mask_and_coherent`, but set the mask only for streaming mappings.
>> +pub fn dma_set_mask(dev: &Device, mask: u64) -> i32 {
>> +    // SAFETY: device pointer is guaranteed as valid by invariant on `Device`.
>> +    unsafe { bindings::dma_set_mask(dev.as_raw(), mask) }
>> +}
>> +
>> +/// Possible attributes associated with a DMA mapping.
>> +///
>> +/// They can be combined with the operators `|`, `&`, and `!`.
>> +///
>> +/// Values can be used from the [`attrs`] module.
>> +#[derive(Clone, Copy, PartialEq)]
>> +#[repr(transparent)]
>> +pub struct Attrs(u32);
>> +
>> +impl Attrs {
>> +    /// Get the raw representation of this attribute.
>> +    pub(crate) fn as_raw(self) -> crate::ffi::c_ulong {
>> +        self.0 as _
>> +    }
>> +
>> +    /// Check whether `flags` is contained in `self`.
>> +    pub fn contains(self, flags: Attrs) -> bool {
>> +        (self & flags) == flags
>> +    }
>> +}
>> +
>> +impl core::ops::BitOr for Attrs {
>> +    type Output = Self;
>> +    fn bitor(self, rhs: Self) -> Self::Output {
>> +        Self(self.0 | rhs.0)
>> +    }
>> +}
>> +
>> +impl core::ops::BitAnd for Attrs {
>> +    type Output = Self;
>> +    fn bitand(self, rhs: Self) -> Self::Output {
>> +        Self(self.0 & rhs.0)
>> +    }
>> +}
>> +
>> +impl core::ops::Not for Attrs {
>> +    type Output = Self;
>> +    fn not(self) -> Self::Output {
>> +        Self(!self.0)
>> +    }
>> +}
>> +
>> +/// DMA mapping attrributes.
>> +pub mod attrs {
>> +    use super::Attrs;
>> +
>> +    /// Specifies that reads and writes to the mapping may be weakly ordered, that is that reads
>> +    /// and writes may pass each other.
>> +    pub const DMA_ATTR_WEAK_ORDERING: Attrs = Attrs(bindings::DMA_ATTR_WEAK_ORDERING);
>> +
>> +    /// Specifies that writes to the mapping may be buffered to improve performance.
>> +    pub const DMA_ATTR_WRITE_COMBINE: Attrs = Attrs(bindings::DMA_ATTR_WRITE_COMBINE);
>> +
>> +    /// Lets the platform to avoid creating a kernel virtual mapping for the allocated buffer.
>> +    pub const DMA_ATTR_NO_KERNEL_MAPPING: Attrs = Attrs(bindings::DMA_ATTR_NO_KERNEL_MAPPING);
>> +
>> +    /// Allows platform code to skip synchronization of the CPU cache for the given buffer assuming
>> +    /// that it has been already transferred to 'device' domain.
>> +    pub const DMA_ATTR_SKIP_CPU_SYNC: Attrs = Attrs(bindings::DMA_ATTR_SKIP_CPU_SYNC);
>> +
>> +    /// Forces contiguous allocation of the buffer in physical memory.
>> +    pub const DMA_ATTR_FORCE_CONTIGUOUS: Attrs = Attrs(bindings::DMA_ATTR_FORCE_CONTIGUOUS);
>> +
>> +    /// This is a hint to the DMA-mapping subsystem that it's probably not worth the time to try
>> +    /// to allocate memory to in a way that gives better TLB efficiency.
>> +    pub const DMA_ATTR_ALLOC_SINGLE_PAGES: Attrs = Attrs(bindings::DMA_ATTR_ALLOC_SINGLE_PAGES);
>> +
>> +    /// This tells the DMA-mapping subsystem to suppress allocation failure reports (similarly to
>> +    /// __GFP_NOWARN).
>> +    pub const DMA_ATTR_NO_WARN: Attrs = Attrs(bindings::DMA_ATTR_NO_WARN);
>> +
>> +    /// Used to indicate that the buffer is fully accessible at an elevated privilege level (and
>> +    /// ideally inaccessible or at least read-only at lesser-privileged levels).
>> +    pub const DMA_ATTR_PRIVILEGED: Attrs = Attrs(bindings::DMA_ATTR_PRIVILEGED);
>> +}
>> +
>> +/// An abstraction of the `dma_alloc_coherent` API.
>> +///
>> +/// This is an abstraction around the `dma_alloc_coherent` API which is used to allocate and map
>> +/// large consistent DMA regions.
>> +///
>> +/// A [`CoherentAllocation`] instance contains a pointer to the allocated region (in the
>> +/// processor's virtual address space) and the device address which can be given to the device
>> +/// as the DMA address base of the region. The region is released once [`CoherentAllocation`]
>> +/// is dropped.
>> +///
>> +/// # Invariants
>> +///
>> +/// For the lifetime of an instance of [`CoherentAllocation`], the cpu address is a valid pointer
>> +/// to an allocated region of consistent memory and we hold a reference to the device.
>> +pub struct CoherentAllocation<T: AsBytes + FromBytes> {
>> +    dev: ARef<Device>,
>> +    dma_handle: bindings::dma_addr_t,
>> +    count: usize,
>> +    cpu_addr: *mut T,
>> +    dma_attrs: Attrs,
>> +}
>> +
>> +impl<T: AsBytes + FromBytes> CoherentAllocation<T> {
>> +    /// Allocates a region of `size_of::<T> * count` of consistent memory.
>> +    ///
>> +    /// # Examples
>> +    ///
>> +    /// ```
>> +    /// use kernel::device::Device;
>> +    /// use kernel::dma::{attrs::*, CoherentAllocation};
>> +    ///
>> +    /// # fn test(dev: &Device) -> Result {
>> +    /// let c: CoherentAllocation<u64> = CoherentAllocation::alloc_attrs(dev.into(), 4, GFP_KERNEL,
>> +    ///                                                                  DMA_ATTR_NO_WARN)?;
>> +    /// # Ok::<(), Error>(()) }
>> +    /// ```
>> +    pub fn alloc_attrs(
>> +        dev: ARef<Device>,
>> +        count: usize,
>> +        gfp_flags: kernel::alloc::Flags,
>> +        dma_attrs: Attrs,
>> +    ) -> Result<CoherentAllocation<T>> {
>> +        build_assert!(
>> +            core::mem::size_of::<T>() > 0,
>> +            "It doesn't make sense for the allocated type to be a ZST"
>> +        );
>> +
>> +        let size = count
>> +            .checked_mul(core::mem::size_of::<T>())
>> +            .ok_or(EOVERFLOW)?;
>> +        let mut dma_handle = 0;
>> +        // SAFETY: device pointer is guaranteed as valid by invariant on `Device`.
>> +        // We ensure that we catch the failure on this function and throw an ENOMEM
>> +        let ret = unsafe {
>> +            bindings::dma_alloc_attrs(
>> +                dev.as_raw(),
>> +                size,
>> +                &mut dma_handle,
>> +                gfp_flags.as_raw(),
>> +                dma_attrs.as_raw(),
>> +            )
>> +        };
>> +        if ret.is_null() {
>> +            return Err(ENOMEM);
>> +        }
>> +        // INVARIANT: We just successfully allocated a coherent region which is accessible for
>> +        // `count` elements, hence the cpu address is valid. We also hold a refcounted reference
>> +        // to the device.
>> +        Ok(Self {
>> +            dev,
>> +            dma_handle,
>> +            count,
>> +            cpu_addr: ret as *mut T,
>> +            dma_attrs,
>> +        })
>> +    }
>> +
>> +    /// Performs the same functionality as `alloc_attrs`, except the `dma_attrs` is 0 by default.
>> +    pub fn alloc_coherent(
>> +        dev: ARef<Device>,
>> +        count: usize,
>> +        gfp_flags: kernel::alloc::Flags,
>> +    ) -> Result<CoherentAllocation<T>> {
>> +        CoherentAllocation::alloc_attrs(dev, count, gfp_flags, Attrs(0))
>> +    }
>> +
>> +    /// Create a duplicate of the `CoherentAllocation` object but prevent it from being dropped.
>> +    pub fn skip_drop(self) -> CoherentAllocation<T> {
>> +        let me = core::mem::ManuallyDrop::new(self);
>> +        Self {
>> +            // SAFETY: The refcount of `dev` will not be decremented because this doesn't actually
>> +            // duplicafe `ARef` and the use of `ManuallyDrop` forgets the originals.
>> +            dev: unsafe { core::ptr::read(&me.dev) },
>> +            dma_handle: me.dma_handle,
>> +            count: me.count,
>> +            cpu_addr: me.cpu_addr,
>> +            dma_attrs: me.dma_attrs,
>> +        }
>> +    }
>> +
>> +    /// Returns the base address to the allocated region in the CPU's virtual address space.
>> +    pub fn start_ptr(&self) -> *const T {
>> +        self.cpu_addr
>> +    }
>> +
>> +    /// Returns the base address to the allocated region in the CPU's virtual address space as
>> +    /// a mutable pointer.
>> +    pub fn start_ptr_mut(&mut self) -> *mut T {
>> +        self.cpu_addr
>> +    }
>> +
>> +    /// Returns a DMA handle which may given to the device as the DMA address base of
>> +    /// the region.
>> +    pub fn dma_handle(&self) -> bindings::dma_addr_t {
>> +        self.dma_handle
>> +    }
>> +
>> +    /// Returns the data from the region starting from `offset` as a slice.
>> +    /// `offset` and `count` are in units of `T`, not the number of bytes.
>> +    ///
>> +    /// Due to the safety requirements of slice, the caller should consider that the region could
>> +    /// be modified by the device at anytime (see the safety block below). For ringbuffer type of
>> +    /// r/w access or use-cases where the pointer to the live data is needed, `start_ptr()` or
>> +    /// `start_ptr_mut()` could be used instead.
>> +    ///
>> +    /// # Safety
>> +    ///
>> +    /// Callers must ensure that no hardware operations that involve the buffer are currently
>> +    /// taking place while the returned slice is live.
>> +    pub unsafe fn as_slice(&self, offset: usize, count: usize) -> Result<&[T]> {
>> +        let end = offset.checked_add(count).ok_or(EOVERFLOW)?;
>> +        if end >= self.count {
>> +            return Err(EINVAL);
>> +        }
>> +        // SAFETY:
>> +        // - The pointer is valid due to type invariant on `CoherentAllocation`,
>> +        // we've just checked that the range and index is within bounds. The immutability of the
>> +        // of data is also guaranteed by the safety requirements of the function.
>> +        // - `offset` can't overflow since it is smaller than `self.count` and we've checked
>> +        // that `self.count` won't overflow early in the constructor.
>> +        Ok(unsafe { core::slice::from_raw_parts(self.cpu_addr.add(offset), count) })
>> +    }
>> +
>> +    /// Performs the same functionality as `as_slice`, except that a mutable slice is returned.
>> +    /// See that method for documentation and safety requirements.
>> +    ///
>> +    /// # Safety
>> +    ///
>> +    /// It is the callers responsibility to avoid separate read and write accesses to the region
>> +    /// while the returned slice is live.
>> +    pub unsafe fn as_slice_mut(&self, offset: usize, count: usize) -> Result<&mut [T]> {
>> +        let end = offset.checked_add(count).ok_or(EOVERFLOW)?;
>> +        if end >= self.count {
>> +            return Err(EINVAL);
>> +        }
>> +        // SAFETY:
>> +        // - The pointer is valid due to type invariant on `CoherentAllocation`,
>> +        // we've just checked that the range and index is within bounds. The immutability of the
>> +        // of data is also guaranteed by the safety requirements of the function.
>> +        // - `offset` can't overflow since it is smaller than `self.count` and we've checked
>> +        // that `self.count` won't overflow early in the constructor.
>> +        Ok(unsafe { core::slice::from_raw_parts_mut(self.cpu_addr.add(offset), count) })
>> +    }
>> +
>> +    /// Writes data to the region starting from `offset`. `offset` is in units of `T`, not the
>> +    /// number of bytes.
>> +    ///
>> +    /// # Examples
>> +    ///
>> +    /// ```
>> +    /// # fn test(alloc: &mut kernel::dma::CoherentAllocation<u8>) -> Result {
>> +    /// let somedata: [u8; 4] = [0xf; 4];
>> +    /// let buf: &[u8] = &somedata;
>> +    /// alloc.write(buf, 0)?;
>> +    /// # Ok::<(), Error>(()) }
>> +    /// ```
>> +    pub fn write(&self, src: &[T], offset: usize) -> Result {
>> +        let end = offset.checked_add(src.len()).ok_or(EOVERFLOW)?;
>> +        if end >= self.count {
>> +            return Err(EINVAL);
>> +        }
>> +        // SAFETY:
>> +        // - The pointer is valid due to type invariant on `CoherentAllocation`
>> +        // and we've just checked that the range and index is within bounds.
>> +        // - `offset` can't overflow since it is smaller than `self.count` and we've checked
>> +        // that `self.count` won't overflow early in the constructor.
>> +        unsafe {
>> +            core::ptr::copy_nonoverlapping(src.as_ptr(), self.cpu_addr.add(offset), src.len())
>> +        };
>> +        Ok(())
>> +    }
>> +
>> +    /// Retrieve a single entry from the region with bounds checking. `offset` is in units of `T`,
>> +    /// not the number of bytes.
>> +    pub fn item_from_index(&self, offset: usize) -> Result<*mut T> {
>> +        if offset >= self.count {
>> +            return Err(EINVAL);
>> +        }
>> +        // SAFETY:
>> +        // - The pointer is valid due to type invariant on `CoherentAllocation`
>> +        // and we've just checked that the range and index is within bounds.
>> +        // - `offset` can't overflow since it is smaller than `self.count` and we've checked
>> +        // that `self.count` won't overflow early in the constructor.
>> +        Ok(unsafe { &mut *self.cpu_addr.add(offset) })
>> +    }
>> +
>> +    /// Reads the value of `field` and ensures that its type is `FromBytes`
>> +    ///
>> +    /// # Safety:
>> +    ///
>> +    /// This must be called from the `dma_read` macro which ensures that the `field` pointer is
>> +    /// validated beforehand.
>> +    ///
>> +    /// Public but hidden since it should only be used from `dma_read` macro.
>> +    #[doc(hidden)]
>> +    pub unsafe fn field_read<F: FromBytes>(&self, field: *const F) -> F {
>> +        // SAFETY: By the safety requirements field is valid
>> +        unsafe { field.read() }
>> +    }
>> +
>> +    /// Writes a value to `field` and ensures that its type is `AsBytes`
>> +    ///
>> +    /// # Safety:
>> +    ///
>> +    /// This must be called from the `dma_write` macro which ensures that the `field` pointer is
>> +    /// validated beforehand.
>> +    ///
>> +    /// Public but hidden since it should only be used from `dma_write` macro.
>> +    #[doc(hidden)]
>> +    pub unsafe fn field_write<F: AsBytes>(&self, field: *mut F, val: F) {
>> +        // SAFETY: By the safety requirements field is valid
>> +        unsafe { field.write(val) }
>> +    }
>> +}
>> +
>> +/// Reads a field of an item from an allocated region of structs.
>> +/// # Examples
>> +///
>> +/// ```
>> +/// struct MyStruct { field: u32, }
>> +/// // SAFETY: All bit patterns are acceptable values for MyStruct.
>> +/// unsafe impl kernel::transmute::FromBytes for MyStruct{};
>> +/// // SAFETY: Instances of MyStruct have no uninitialized portions.
>> +/// unsafe impl kernel::transmute::AsBytes for MyStruct{};
>> +///
>> +/// # fn test(alloc: &kernel::dma::CoherentAllocation<MyStruct>) -> Result {
>> +/// let whole = kernel::dma_read!(alloc[2]);
>> +/// let field = kernel::dma_read!(alloc[1].field);
>> +/// # Ok::<(), Error>(()) }
>> +/// ```
>> +#[macro_export]
>> +macro_rules! dma_read {
>> +    ($dma:ident [ $idx:expr ] $($field:tt)* ) => {{
>> +        let item = $dma.item_from_index($idx)?;
>> +        // SAFETY: `item_from_index` ensures that `item` is always a valid pointer and can be
>> +        // dereferenced. The compiler also further validates the expression on whether `field`
>> +        // is a member of `item` when expanded by the macro.
>> +        unsafe {
>> +            let ptr_field = ::core::ptr::addr_of!((*item) $($field)*);
>> +            $dma.field_read(ptr_field)
>> +        }
>> +    }};
>> +}
>> +
>> +/// Writes to a field of an item from an allocated region of structs.
>> +/// # Examples
>> +///
>> +/// ```
>> +/// struct MyStruct { member: u32, }
>> +/// // SAFETY: All bit patterns are acceptable values for MyStruct.
>> +/// unsafe impl kernel::transmute::FromBytes for MyStruct{};
>> +/// // SAFETY: Instances of MyStruct have no uninitialized portions.
>> +/// unsafe impl kernel::transmute::AsBytes for MyStruct{};
>> +///
>> +/// # fn test(alloc: &mut kernel::dma::CoherentAllocation<MyStruct>) -> Result {
>> +/// kernel::dma_write!(alloc[2].member = 0xf);
>> +/// kernel::dma_write!(alloc[1] = MyStruct { member: 0xf });
>> +/// # Ok::<(), Error>(()) }
>> +/// ```
>> +#[macro_export]
>> +macro_rules! dma_write {
>> +    ($dma:ident [ $idx:expr ] $($field:tt)*) => {{
>> +        kernel::dma_write!($dma, $idx, $($field)*);
>> +    }};
>> +    ($dma:ident, $idx: expr, = $val:expr) => {
>> +        let item = $dma.item_from_index($idx)?;
>> +        // SAFETY: `item_from_index` ensures that `item` is always a valid item.
>> +        unsafe { $dma.field_write(item, $val) }
>> +    };
>> +    ($dma:ident, $idx: expr, $(.$field:ident)* = $val:expr) => {
>> +        let item = $dma.item_from_index($idx)?;
>> +        // SAFETY: `item_from_index` ensures that `item` is always a valid pointer and can be
>> +        // dereferenced. The compiler also further validates the expression on whether `field`
>> +        // is a member of `item` when expanded by the macro.
>> +        unsafe {
>> +            let ptr_field = ::core::ptr::addr_of_mut!((*item) $(.$field)*);
>> +            $dma.field_write(ptr_field, $val)
>> +        }
>> +    };
>> +}
>> +
>> +impl<T: AsBytes + FromBytes> Drop for CoherentAllocation<T> {
>> +    fn drop(&mut self) {
>> +        let size = self.count * core::mem::size_of::<T>();
>> +        // SAFETY: the device, cpu address, and the dma handle is valid due to the
>> +        // type invariants on `CoherentAllocation`.
>> +        unsafe {
>> +            bindings::dma_free_attrs(
>> +                self.dev.as_raw(),
>> +                size,
>> +                self.cpu_addr as _,
>> +                self.dma_handle,
>> +                self.dma_attrs.as_raw(),
>> +            )
>> +        }
>> +    }
>> +}
>> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
>> index 496ed32b0911..5081cb66b2f9 100644
>> --- a/rust/kernel/lib.rs
>> +++ b/rust/kernel/lib.rs
>> @@ -44,6 +44,7 @@
>>   pub mod device;
>>   pub mod device_id;
>>   pub mod devres;
>> +pub mod dma;
>>   pub mod driver;
>>   pub mod error;
>>   #[cfg(CONFIG_RUST_FW_LOADER_ABSTRACTIONS)]
>> --
>> 2.43.0