// SPDX-License-Identifier: GPL-2.0 //! Abstractions for scatter-gather lists. //! //! C header: [`include/linux/scatterlist.h`](srctree/include/linux/scatterlist.h) //! //! Scatter-gather (SG) I/O is a memory access technique that allows devices to perform DMA //! operations on data buffers that are not physically contiguous in memory. It works by creating a //! "scatter-gather list", an array where each entry specifies the address and length of a //! physically contiguous memory segment. //! //! The device's DMA controller can then read this list and process the segments sequentially as //! part of one logical I/O request. This avoids the need for a single, large, physically contiguous //! memory buffer, which can be difficult or impossible to allocate. //! //! This module provides safe Rust abstractions over the kernel's `struct scatterlist` and //! `struct sg_table` types. //! //! The main entry point is the [`SGTable`] type, which represents a complete scatter-gather table. //! It can be either: //! //! - An owned table ([`SGTable>`]), created from a Rust memory buffer (e.g., [`VVec`]). //! This type manages the allocation of the `struct sg_table`, the DMA mapping of the buffer, and //! the automatic cleanup of all resources. //! - A borrowed reference (&[`SGTable`]), which provides safe, read-only access to a table that was //! allocated by other (e.g., C) code. //! //! Individual entries in the table are represented by [`SGEntry`], which can be accessed by //! iterating over an [`SGTable`]. use crate::{ alloc, alloc::allocator::VmallocPageIter, bindings, device::{Bound, Device}, devres::Devres, dma, error, io::resource::ResourceSize, page, prelude::*, types::{ARef, Opaque}, }; use core::{ops::Deref, ptr::NonNull}; /// A single entry in a scatter-gather list. /// /// An `SGEntry` represents a single, physically contiguous segment of memory that has been mapped /// for DMA. /// /// Instances of this struct are obtained by iterating over an [`SGTable`]. Drivers do not create /// or own [`SGEntry`] objects directly. #[repr(transparent)] pub struct SGEntry(Opaque); // SAFETY: `SGEntry` can be sent to any task. unsafe impl Send for SGEntry {} // SAFETY: `SGEntry` has no interior mutability and can be accessed concurrently. unsafe impl Sync for SGEntry {} impl SGEntry { /// Convert a raw `struct scatterlist *` to a `&'a SGEntry`. /// /// # Safety /// /// Callers must ensure that the `struct scatterlist` pointed to by `ptr` is valid for the /// lifetime `'a`. #[inline] unsafe fn from_raw<'a>(ptr: *mut bindings::scatterlist) -> &'a Self { // SAFETY: The safety requirements of this function guarantee that `ptr` is a valid pointer // to a `struct scatterlist` for the duration of `'a`. unsafe { &*ptr.cast() } } /// Obtain the raw `struct scatterlist *`. #[inline] fn as_raw(&self) -> *mut bindings::scatterlist { self.0.get() } /// Returns the DMA address of this SG entry. /// /// This is the address that the device should use to access the memory segment. #[inline] pub fn dma_address(&self) -> dma::DmaAddress { // SAFETY: `self.as_raw()` is a valid pointer to a `struct scatterlist`. unsafe { bindings::sg_dma_address(self.as_raw()) } } /// Returns the length of this SG entry in bytes. #[inline] pub fn dma_len(&self) -> ResourceSize { #[allow(clippy::useless_conversion)] // SAFETY: `self.as_raw()` is a valid pointer to a `struct scatterlist`. unsafe { bindings::sg_dma_len(self.as_raw()) }.into() } } /// The borrowed generic type of an [`SGTable`], representing a borrowed or externally managed /// table. #[repr(transparent)] pub struct Borrowed(Opaque); // SAFETY: `Borrowed` can be sent to any task. unsafe impl Send for Borrowed {} // SAFETY: `Borrowed` has no interior mutability and can be accessed concurrently. unsafe impl Sync for Borrowed {} /// A scatter-gather table. /// /// This struct is a wrapper around the kernel's `struct sg_table`. It manages a list of DMA-mapped /// memory segments that can be passed to a device for I/O operations. /// /// The generic parameter `T` is used as a generic type to distinguish between owned and borrowed /// tables. /// /// - [`SGTable`]: An owned table created and managed entirely by Rust code. It handles /// allocation, DMA mapping, and cleanup of all associated resources. See [`SGTable::new`]. /// - [`SGTable`} (or simply [`SGTable`]): Represents a table whose lifetime is managed /// externally. It can be used safely via a borrowed reference `&'a SGTable`, where `'a` is the /// external lifetime. /// /// All [`SGTable`] variants can be iterated over the individual [`SGEntry`]s. #[repr(transparent)] #[pin_data] pub struct SGTable { #[pin] inner: T, } impl SGTable { /// Creates a borrowed `&'a SGTable` from a raw `struct sg_table` pointer. /// /// This allows safe access to an `sg_table` that is managed elsewhere (for example, in C code). /// /// # Safety /// /// Callers must ensure that: /// /// - the `struct sg_table` pointed to by `ptr` is valid for the entire lifetime of `'a`, /// - the data behind `ptr` is not modified concurrently for the duration of `'a`. #[inline] pub unsafe fn from_raw<'a>(ptr: *mut bindings::sg_table) -> &'a Self { // SAFETY: The safety requirements of this function guarantee that `ptr` is a valid pointer // to a `struct sg_table` for the duration of `'a`. unsafe { &*ptr.cast() } } #[inline] fn as_raw(&self) -> *mut bindings::sg_table { self.inner.0.get() } /// Returns an [`SGTableIter`] bound to the lifetime of `self`. pub fn iter(&self) -> SGTableIter<'_> { // SAFETY: `self.as_raw()` is a valid pointer to a `struct sg_table`. let nents = unsafe { (*self.as_raw()).nents }; let pos = if nents > 0 { // SAFETY: `self.as_raw()` is a valid pointer to a `struct sg_table`. let ptr = unsafe { (*self.as_raw()).sgl }; // SAFETY: `ptr` is guaranteed to be a valid pointer to a `struct scatterlist`. Some(unsafe { SGEntry::from_raw(ptr) }) } else { None }; SGTableIter { pos, nents } } } /// Represents the DMA mapping state of a `struct sg_table`. /// /// This is used as an inner type of [`Owned`] to manage the DMA mapping lifecycle. /// /// # Invariants /// /// - `sgt` is a valid pointer to a `struct sg_table` for the entire lifetime of the /// [`DmaMappedSgt`]. /// - `sgt` is always DMA mapped. struct DmaMappedSgt { sgt: NonNull, dev: ARef, dir: dma::DataDirection, } // SAFETY: `DmaMappedSgt` can be sent to any task. unsafe impl Send for DmaMappedSgt {} // SAFETY: `DmaMappedSgt` has no interior mutability and can be accessed concurrently. unsafe impl Sync for DmaMappedSgt {} impl DmaMappedSgt { /// # Safety /// /// - `sgt` must be a valid pointer to a `struct sg_table` for the entire lifetime of the /// returned [`DmaMappedSgt`]. /// - The caller must guarantee that `sgt` remains DMA mapped for the entire lifetime of /// [`DmaMappedSgt`]. unsafe fn new( sgt: NonNull, dev: &Device, dir: dma::DataDirection, ) -> Result { // SAFETY: // - `dev.as_raw()` is a valid pointer to a `struct device`, which is guaranteed to be // bound to a driver for the duration of this call. // - `sgt` is a valid pointer to a `struct sg_table`. error::to_result(unsafe { bindings::dma_map_sgtable(dev.as_raw(), sgt.as_ptr(), dir.into(), 0) })?; // INVARIANT: By the safety requirements of this function it is guaranteed that `sgt` is // valid for the entire lifetime of this object instance. Ok(Self { sgt, dev: dev.into(), dir, }) } } impl Drop for DmaMappedSgt { #[inline] fn drop(&mut self) { // SAFETY: // - `self.dev.as_raw()` is a pointer to a valid `struct device`. // - `self.dev` is the same device the mapping has been created for in `Self::new()`. // - `self.sgt.as_ptr()` is a valid pointer to a `struct sg_table` by the type invariants // of `Self`. // - `self.dir` is the same `dma::DataDirection` the mapping has been created with in // `Self::new()`. unsafe { bindings::dma_unmap_sgtable(self.dev.as_raw(), self.sgt.as_ptr(), self.dir.into(), 0) }; } } /// A transparent wrapper around a `struct sg_table`. /// /// While we could also create the `struct sg_table` in the constructor of [`Owned`], we can't tear /// down the `struct sg_table` in [`Owned::drop`]; the drop order in [`Owned`] matters. #[repr(transparent)] struct RawSGTable(Opaque); // SAFETY: `RawSGTable` can be sent to any task. unsafe impl Send for RawSGTable {} // SAFETY: `RawSGTable` has no interior mutability and can be accessed concurrently. unsafe impl Sync for RawSGTable {} impl RawSGTable { /// # Safety /// /// - `pages` must be a slice of valid `struct page *`. /// - The pages pointed to by `pages` must remain valid for the entire lifetime of the returned /// [`RawSGTable`]. unsafe fn new( pages: &mut [*mut bindings::page], size: usize, max_segment: u32, flags: alloc::Flags, ) -> Result { // `sg_alloc_table_from_pages_segment()` expects at least one page, otherwise it // produces a NPE. if pages.is_empty() { return Err(EINVAL); } let sgt = Opaque::zeroed(); // SAFETY: // - `sgt.get()` is a valid pointer to uninitialized memory. // - As by the check above, `pages` is not empty. error::to_result(unsafe { bindings::sg_alloc_table_from_pages_segment( sgt.get(), pages.as_mut_ptr(), pages.len().try_into()?, 0, size, max_segment, flags.as_raw(), ) })?; Ok(Self(sgt)) } #[inline] fn as_raw(&self) -> *mut bindings::sg_table { self.0.get() } } impl Drop for RawSGTable { #[inline] fn drop(&mut self) { // SAFETY: `sgt` is a valid and initialized `struct sg_table`. unsafe { bindings::sg_free_table(self.0.get()) }; } } /// The [`Owned`] generic type of an [`SGTable`]. /// /// A [`SGTable`] signifies that the [`SGTable`] owns all associated resources: /// /// - The backing memory pages. /// - The `struct sg_table` allocation (`sgt`). /// - The DMA mapping, managed through a [`Devres`]-managed `DmaMappedSgt`. /// /// Users interact with this type through the [`SGTable`] handle and do not need to manage /// [`Owned`] directly. #[pin_data] pub struct Owned

{ // Note: The drop order is relevant; we first have to unmap the `struct sg_table`, then free the // `struct sg_table` and finally free the backing pages. #[pin] dma: Devres, sgt: RawSGTable, _pages: P, } // SAFETY: `Owned` can be sent to any task if `P` can be send to any task. unsafe impl Send for Owned

{} // SAFETY: `Owned` has no interior mutability and can be accessed concurrently if `P` can be // accessed concurrently. unsafe impl Sync for Owned

{} impl

Owned

where for<'a> P: page::AsPageIter = VmallocPageIter<'a>> + 'static, { fn new( dev: &Device, mut pages: P, dir: dma::DataDirection, flags: alloc::Flags, ) -> Result + '_> { let page_iter = pages.page_iter(); let size = page_iter.size(); let mut page_vec: KVec<*mut bindings::page> = KVec::with_capacity(page_iter.page_count(), flags)?; for page in page_iter { page_vec.push(page.as_ptr(), flags)?; } // `dma_max_mapping_size` returns `size_t`, but `sg_alloc_table_from_pages_segment()` takes // an `unsigned int`. // // SAFETY: `dev.as_raw()` is a valid pointer to a `struct device`. let max_segment = match unsafe { bindings::dma_max_mapping_size(dev.as_raw()) } { 0 => u32::MAX, max_segment => u32::try_from(max_segment).unwrap_or(u32::MAX), }; Ok(try_pin_init!(&this in Self { // SAFETY: // - `page_vec` is a `KVec` of valid `struct page *` obtained from `pages`. // - The pages contained in `pages` remain valid for the entire lifetime of the // `RawSGTable`. sgt: unsafe { RawSGTable::new(&mut page_vec, size, max_segment, flags) }?, dma <- { // SAFETY: `this` is a valid pointer to uninitialized memory. let sgt = unsafe { &raw mut (*this.as_ptr()).sgt }.cast(); // SAFETY: `sgt` is guaranteed to be non-null. let sgt = unsafe { NonNull::new_unchecked(sgt) }; // SAFETY: // - It is guaranteed that the object returned by `DmaMappedSgt::new` won't out-live // `sgt`. // - `sgt` is never DMA unmapped manually. Devres::new(dev, unsafe { DmaMappedSgt::new(sgt, dev, dir) }) }, _pages: pages, })) } } impl

SGTable> where for<'a> P: page::AsPageIter = VmallocPageIter<'a>> + 'static, { /// Allocates a new scatter-gather table from the given pages and maps it for DMA. /// /// This constructor creates a new [`SGTable`] that takes ownership of `P`. /// It allocates a `struct sg_table`, populates it with entries corresponding to the physical /// pages of `P`, and maps the table for DMA with the specified [`Device`] and /// [`dma::DataDirection`]. /// /// The DMA mapping is managed through [`Devres`], ensuring that the DMA mapping is unmapped /// once the associated [`Device`] is unbound, or when the [`SGTable`] is dropped. /// /// # Parameters /// /// * `dev`: The [`Device`] that will be performing the DMA. /// * `pages`: The entity providing the backing pages. It must implement [`page::AsPageIter`]. /// The ownership of this entity is moved into the new [`SGTable`]. /// * `dir`: The [`dma::DataDirection`] of the DMA transfer. /// * `flags`: Allocation flags for internal allocations (e.g., [`GFP_KERNEL`]). /// /// # Examples /// /// ``` /// use kernel::{ /// device::{Bound, Device}, /// dma, page, /// prelude::*, /// scatterlist::{SGTable, Owned}, /// }; /// /// fn test(dev: &Device) -> Result { /// let size = 4 * page::PAGE_SIZE; /// let pages = VVec::::with_capacity(size, GFP_KERNEL)?; /// /// let sgt = KBox::pin_init(SGTable::new( /// dev, /// pages, /// dma::DataDirection::ToDevice, /// GFP_KERNEL, /// ), GFP_KERNEL)?; /// /// Ok(()) /// } /// ``` pub fn new( dev: &Device, pages: P, dir: dma::DataDirection, flags: alloc::Flags, ) -> impl PinInit + '_ { try_pin_init!(Self { inner <- Owned::new(dev, pages, dir, flags)? }) } } impl

Deref for SGTable> { type Target = SGTable; #[inline] fn deref(&self) -> &Self::Target { // SAFETY: // - `self.inner.sgt.as_raw()` is a valid pointer to a `struct sg_table` for the entire // lifetime of `self`. // - The backing `struct sg_table` is not modified for the entire lifetime of `self`. unsafe { SGTable::from_raw(self.inner.sgt.as_raw()) } } } mod private { pub trait Sealed {} impl Sealed for super::Borrowed {} impl

Sealed for super::Owned

{} } /// An [`Iterator`] over the DMA mapped [`SGEntry`] items of an [`SGTable`]. /// /// Note that the existence of an [`SGTableIter`] does not guarantee that the [`SGEntry`] items /// actually remain DMA mapped; they are prone to be unmapped on device unbind. pub struct SGTableIter<'a> { pos: Option<&'a SGEntry>, /// The number of DMA mapped entries in a `struct sg_table`. nents: c_uint, } impl<'a> Iterator for SGTableIter<'a> { type Item = &'a SGEntry; fn next(&mut self) -> Option { let entry = self.pos?; self.nents = self.nents.saturating_sub(1); // SAFETY: `entry.as_raw()` is a valid pointer to a `struct scatterlist`. let next = unsafe { bindings::sg_next(entry.as_raw()) }; self.pos = (!next.is_null() && self.nents > 0).then(|| { // SAFETY: If `next` is not NULL, `sg_next()` guarantees to return a valid pointer to // the next `struct scatterlist`. unsafe { SGEntry::from_raw(next) } }); Some(entry) } }