1 // SPDX-License-Identifier: GPL-2.0 2 3 //! Types and functions to work with pointers and addresses. 4 5 use core::mem::align_of; 6 use core::num::NonZero; 7 8 use crate::build_assert; 9 10 /// Type representing an alignment, which is always a power of two. 11 /// 12 /// It is used to validate that a given value is a valid alignment, and to perform masking and 13 /// alignment operations. 14 /// 15 /// This is a temporary substitute for the [`Alignment`] nightly type from the standard library, 16 /// and to be eventually replaced by it. 17 /// 18 /// [`Alignment`]: https://github.com/rust-lang/rust/issues/102070 19 /// 20 /// # Invariants 21 /// 22 /// An alignment is always a power of two. 23 #[repr(transparent)] 24 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)] 25 pub struct Alignment(NonZero<usize>); 26 27 impl Alignment { 28 /// Validates that `ALIGN` is a power of two at build-time, and returns an [`Alignment`] of the 29 /// same value. 30 /// 31 /// A build error is triggered if `ALIGN` is not a power of two. 32 /// 33 /// # Examples 34 /// 35 /// ``` 36 /// use kernel::ptr::Alignment; 37 /// 38 /// let v = Alignment::new::<16>(); 39 /// assert_eq!(v.as_usize(), 16); 40 /// ``` 41 #[inline(always)] new<const ALIGN: usize>() -> Self42 pub const fn new<const ALIGN: usize>() -> Self { 43 build_assert!( 44 ALIGN.is_power_of_two(), 45 "Provided alignment is not a power of two." 46 ); 47 48 // INVARIANT: `align` is a power of two. 49 // SAFETY: `align` is a power of two, and thus non-zero. 50 Self(unsafe { NonZero::new_unchecked(ALIGN) }) 51 } 52 53 /// Validates that `align` is a power of two at runtime, and returns an 54 /// [`Alignment`] of the same value. 55 /// 56 /// Returns [`None`] if `align` is not a power of two. 57 /// 58 /// # Examples 59 /// 60 /// ``` 61 /// use kernel::ptr::Alignment; 62 /// 63 /// assert_eq!(Alignment::new_checked(16), Some(Alignment::new::<16>())); 64 /// assert_eq!(Alignment::new_checked(15), None); 65 /// assert_eq!(Alignment::new_checked(1), Some(Alignment::new::<1>())); 66 /// assert_eq!(Alignment::new_checked(0), None); 67 /// ``` 68 #[inline(always)] new_checked(align: usize) -> Option<Self>69 pub const fn new_checked(align: usize) -> Option<Self> { 70 if align.is_power_of_two() { 71 // INVARIANT: `align` is a power of two. 72 // SAFETY: `align` is a power of two, and thus non-zero. 73 Some(Self(unsafe { NonZero::new_unchecked(align) })) 74 } else { 75 None 76 } 77 } 78 79 /// Returns the alignment of `T`. 80 /// 81 /// This is equivalent to [`align_of`], but with the return value provided as an [`Alignment`]. 82 #[inline(always)] of<T>() -> Self83 pub const fn of<T>() -> Self { 84 #![allow(clippy::incompatible_msrv)] 85 // This cannot panic since alignments are always powers of two. 86 // 87 // We unfortunately cannot use `new` as it would require the `generic_const_exprs` feature. 88 const { Alignment::new_checked(align_of::<T>()).unwrap() } 89 } 90 91 /// Returns this alignment as a [`usize`]. 92 /// 93 /// It is guaranteed to be a power of two. 94 /// 95 /// # Examples 96 /// 97 /// ``` 98 /// use kernel::ptr::Alignment; 99 /// 100 /// assert_eq!(Alignment::new::<16>().as_usize(), 16); 101 /// ``` 102 #[inline(always)] as_usize(self) -> usize103 pub const fn as_usize(self) -> usize { 104 self.as_nonzero().get() 105 } 106 107 /// Returns this alignment as a [`NonZero`]. 108 /// 109 /// It is guaranteed to be a power of two. 110 /// 111 /// # Examples 112 /// 113 /// ``` 114 /// use kernel::ptr::Alignment; 115 /// 116 /// assert_eq!(Alignment::new::<16>().as_nonzero().get(), 16); 117 /// ``` 118 #[inline(always)] as_nonzero(self) -> NonZero<usize>119 pub const fn as_nonzero(self) -> NonZero<usize> { 120 // Allow the compiler to know that the value is indeed a power of two. This can help 121 // optimize some operations down the line, like e.g. replacing divisions by bit shifts. 122 if !self.0.is_power_of_two() { 123 // SAFETY: Per the invariants, `self.0` is always a power of two so this block will 124 // never be reached. 125 unsafe { core::hint::unreachable_unchecked() } 126 } 127 self.0 128 } 129 130 /// Returns the base-2 logarithm of the alignment. 131 /// 132 /// # Examples 133 /// 134 /// ``` 135 /// use kernel::ptr::Alignment; 136 /// 137 /// assert_eq!(Alignment::of::<u8>().log2(), 0); 138 /// assert_eq!(Alignment::new::<16>().log2(), 4); 139 /// ``` 140 #[inline(always)] log2(self) -> u32141 pub const fn log2(self) -> u32 { 142 self.0.ilog2() 143 } 144 145 /// Returns the mask for this alignment. 146 /// 147 /// This is equivalent to `!(self.as_usize() - 1)`. 148 /// 149 /// # Examples 150 /// 151 /// ``` 152 /// use kernel::ptr::Alignment; 153 /// 154 /// assert_eq!(Alignment::new::<0x10>().mask(), !0xf); 155 /// ``` 156 #[inline(always)] mask(self) -> usize157 pub const fn mask(self) -> usize { 158 // No underflow can occur as the alignment is guaranteed to be a power of two, and thus is 159 // non-zero. 160 !(self.as_usize() - 1) 161 } 162 } 163 164 /// Trait for items that can be aligned against an [`Alignment`]. 165 pub trait Alignable: Sized { 166 /// Aligns `self` down to `alignment`. 167 /// 168 /// # Examples 169 /// 170 /// ``` 171 /// use kernel::ptr::{Alignable, Alignment}; 172 /// 173 /// assert_eq!(0x2f_usize.align_down(Alignment::new::<0x10>()), 0x20); 174 /// assert_eq!(0x30usize.align_down(Alignment::new::<0x10>()), 0x30); 175 /// assert_eq!(0xf0u8.align_down(Alignment::new::<0x1000>()), 0x0); 176 /// ``` align_down(self, alignment: Alignment) -> Self177 fn align_down(self, alignment: Alignment) -> Self; 178 179 /// Aligns `self` up to `alignment`, returning `None` if aligning would result in an overflow. 180 /// 181 /// # Examples 182 /// 183 /// ``` 184 /// use kernel::ptr::{Alignable, Alignment}; 185 /// 186 /// assert_eq!(0x4fusize.align_up(Alignment::new::<0x10>()), Some(0x50)); 187 /// assert_eq!(0x40usize.align_up(Alignment::new::<0x10>()), Some(0x40)); 188 /// assert_eq!(0x0usize.align_up(Alignment::new::<0x10>()), Some(0x0)); 189 /// assert_eq!(u8::MAX.align_up(Alignment::new::<0x10>()), None); 190 /// assert_eq!(0x10u8.align_up(Alignment::new::<0x100>()), None); 191 /// assert_eq!(0x0u8.align_up(Alignment::new::<0x100>()), Some(0x0)); 192 /// ``` align_up(self, alignment: Alignment) -> Option<Self>193 fn align_up(self, alignment: Alignment) -> Option<Self>; 194 } 195 196 /// Implement [`Alignable`] for unsigned integer types. 197 macro_rules! impl_alignable_uint { 198 ($($t:ty),*) => { 199 $( 200 impl Alignable for $t { 201 #[inline(always)] 202 fn align_down(self, alignment: Alignment) -> Self { 203 // The operands of `&` need to be of the same type so convert the alignment to 204 // `Self`. This means we need to compute the mask ourselves. 205 ::core::num::NonZero::<Self>::try_from(alignment.as_nonzero()) 206 .map(|align| self & !(align.get() - 1)) 207 // An alignment larger than `Self` always aligns down to `0`. 208 .unwrap_or(0) 209 } 210 211 #[inline(always)] 212 fn align_up(self, alignment: Alignment) -> Option<Self> { 213 let aligned_down = self.align_down(alignment); 214 if self == aligned_down { 215 Some(aligned_down) 216 } else { 217 Self::try_from(alignment.as_usize()) 218 .ok() 219 .and_then(|align| aligned_down.checked_add(align)) 220 } 221 } 222 } 223 )* 224 }; 225 } 226 227 impl_alignable_uint!(u8, u16, u32, u64, usize); 228