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//! A zero-cost abstraction to track various resource states with the //! type-system. //! //! Lets describe the pattern by example. (Familiarity with //! [`token`](crate::token) module may be required.) Imagine that we need to //! implement a DMA driver. The DMA peripheral consists of the common //! functionality, which includes the power switch for the whole peripheral, and //! separate DMA channels. The channels can be used independently in different //! threads. We want to avoid situations where one thread holding the switch //! breaks the other thread holding a channel. Lets see an example of the //! pattern: //! //! ``` //! use core::sync::atomic::{AtomicBool, Ordering}; //! use drone_core::{inventory, inventory::Inventory, token::{simple_token, Token}}; //! use typenum::{U0, U1}; //! //! // Let it be our power switch, so we can easily observe its state. //! static DMA_EN: AtomicBool = AtomicBool::new(false); //! //! // Our drivers map unique resources expressed by tokens. //! simple_token!(pub struct DmaReg); //! simple_token!(pub struct DmaChReg); //! //! // We split the DMA driver in two types: one for disabled state, and the //! // other for enabled state. //! pub struct Dma(Inventory<DmaEn, U0>); //! pub struct DmaEn(DmaReg); //! //! impl Dma { //! // The constructor for the DMA driver. Note that `reg` is a token, so at most //! // one instance of the driver could ever exist. //! pub fn new(reg: DmaReg) -> Self { //! Self(Inventory::new(DmaEn(reg))) //! } //! //! // It is always a good idea to provide a method to free the token passed to //! // the `new()` method above. //! pub fn free(self) -> DmaReg { //! Inventory::free(self.0).0 //! } //! //! // This method takes `self` by reference and returns a scoped guard object. It //! // enables DMA, and the returned guard will automatically disable it on `drop`. //! pub fn enable(&mut self) -> inventory::Guard<'_, DmaEn> { //! self.setup(); //! Inventory::guard(&mut self.0) //! } //! //! // This method takes `self` by value and returns the inventory object with one //! // token taken. It enables DMA, and in order to disable it, one should //! // explicitly call `from_enabled()` method below. //! pub fn into_enabled(self) -> Inventory<DmaEn, U1> { //! self.setup(); //! let (enabled, token) = self.0.share1(); //! // To be recreated in `from_enabled()`. //! drop(token); //! enabled //! } //! //! // This method takes the inventory object with one token taken, restores the //! // token, and disables DMA. //! pub fn from_enabled(enabled: Inventory<DmaEn, U1>) -> Self { //! // Restoring the token dropped in `into_enabled()`. //! let token = unsafe { inventory::Token::new() }; //! let mut enabled = enabled.merge1(token); //! Inventory::teardown(&mut enabled); //! Self(enabled) //! } //! //! // An example method, which can be called only when DMA is disabled. //! pub fn do_something_with_disabled_dma(&self) {} //! //! // A private method that actually enables DMA. //! fn setup(&self) { //! DMA_EN.store(true, Ordering::Relaxed); //! } //! } //! //! impl inventory::Item for DmaEn { //! // A method that disables DMA. Due to its signature it can't be called directly. //! // It is called only by `Guard::drop` or `Inventory::teardown`. //! fn teardown(&mut self, _token: &mut inventory::GuardToken<DmaEn>) { //! DMA_EN.store(false, Ordering::Relaxed); //! } //! } //! //! impl DmaEn { //! // An example method, which can be called only when DMA is enabled. //! fn do_something_with_enabled_dma(&self) {} //! } //! //! // Here we define types for DMA channels. //! pub struct DmaCh(DmaChEn); //! pub struct DmaChEn(DmaChReg); //! //! impl DmaCh { //! // The following two methods are the usual constructor and destructor. //! //! pub fn new(reg: DmaChReg) -> Self { //! Self(DmaChEn(reg)) //! } //! //! pub fn free(self) -> DmaChReg { //! (self.0).0 //! } //! //! // A DMA channel is enabled when the whole DMA is enabled. If we have a token //! // reference, we can safely assume that the channel is already enabled. //! //! pub fn as_enabled(&self, _token: &inventory::Token<DmaEn>) -> &DmaChEn { //! &self.0 //! } //! //! pub fn as_enabled_mut(&mut self, _token: &inventory::Token<DmaEn>) -> &mut DmaChEn { //! &mut self.0 //! } //! //! // If we consume the token, we can assume that the DMA will be enabled //! // infinitely. Or at least until the token will be resurrected. //! pub fn into_enabled(self, token: inventory::Token<DmaEn>) -> DmaChEn { //! // To be recreated in `into_disabled()`. //! drop(token); //! self.0 //! } //! } //! //! impl DmaChEn { //! // The only way to obtain an instance of `DmaChEn` is with `DmaCh::into_enabled` //! // method. So we can claim that the newly created token is the token dropped in //! // `DmaCh::into_enabled`. //! pub fn into_disabled(self) -> (DmaCh, inventory::Token<DmaEn>) { //! // Restore the token dropped in `into_enabled()`. //! let token = unsafe { inventory::Token::new() }; //! (DmaCh(self), token) //! } //! //! // An example method, which can be called only when DMA channel is enabled. //! fn do_something_with_enabled_dma_ch(&self) {} //! } //! //! fn main() { //! // Instantiate the tokens. This is `unsafe` because we can accidentally //! // create more than one instance of a token. //! let dma_reg = unsafe { DmaReg::take() }; //! let dma_ch_reg = unsafe { DmaChReg::take() }; //! //! // Instantiate drivers. Only one instance of each driver can exist, because //! // they depend on the tokens. //! let mut dma = Dma::new(dma_reg); //! let mut dma_ch = DmaCh::new(dma_ch_reg); //! // DMA is disabled now. //! assert!(!DMA_EN.load(Ordering::Relaxed)); //! //! // We can call methods defined for disabled `Dma`. //! dma.do_something_with_disabled_dma(); //! // We can't call methods defined for enabled `Dma`. This won't compile. //! // dma.do_something_with_enabled_dma(); //! //! { //! // Enable DMA. This method returns a guard scoped to the enclosing block. //! let mut dma = dma.enable(); //! assert!(DMA_EN.load(Ordering::Relaxed)); //! //! // We can call methods defined for enabled DMA. //! dma.do_something_with_enabled_dma(); //! // Calls to methods defined for disabled DMA won't compile. //! // dma.do_something_with_disabled_dma(); //! //! // Get enabled DMA channel. Type system ensures that the lifetime of //! // `dma_ch` is always shorter than the lifetime of `dma`. //! let dma_ch = dma_ch.as_enabled(dma.inventory_token()); //! // We can call methods defined for enabled DMA channel. //! dma_ch.do_something_with_enabled_dma_ch(); //! } //! // After exiting the scope above, DMA is automatically disabled. //! assert!(!DMA_EN.load(Ordering::Relaxed)); //! //! // Sometimes we can't use lifetimes to encode resource states. Here is another //! // approach which encodes states in the types. //! //! // Enable DMA while converting our driver to a different type. //! let mut dma = dma.into_enabled(); //! assert!(DMA_EN.load(Ordering::Relaxed)); //! //! // We can call methods defined for enabled types. //! dma.do_something_with_enabled_dma(); //! dma_ch //! .as_enabled(dma.inventory_token()) //! .do_something_with_enabled_dma_ch(); //! //! // Obtain the owned token from `dma`. From now `dma` has a type that can't be //! // disabled. //! let (dma, token) = dma.share1(); //! // Get enabled DMA channel. This method consumes the token. //! let dma_ch = dma_ch.into_enabled(token); //! // We can call methods defined for enabled DMA channel. //! dma_ch.do_something_with_enabled_dma_ch(); //! //! // At this moment DMA can't be disabled. If `dma` is dropped, then the //! // resource will remain enabled. We need to get our token back from `dma_ch`. //! let (dma_ch, token) = dma_ch.into_disabled(); //! let dma = dma.merge1(token); //! // Now DMA can be disabled. //! let dma = Dma::from_enabled(dma); //! assert!(!DMA_EN.load(Ordering::Relaxed)); //! } //! ``` use core::{ marker::PhantomData, ops::{Add, Deref, DerefMut, Sub}, }; use typenum::{Diff, Sum, Unsigned, U0, U1, U2, U3, U4, U5, U6, U7, U8}; /// The inventory wrapper for `T`. Parameter `C` encodes the number of emitted /// tokens. /// /// See [the module-level documentation](self) for details. #[repr(transparent)] pub struct Inventory<T: Item, C: Unsigned> { item: T, _marker: PhantomData<C>, } /// An RAII scoped guard for the inventory item `T`. Will call /// [`Item::teardown`] on `drop`. #[must_use = "if unused the item will immediately teardown"] pub struct Guard<'a, T: Item> { borrow: &'a mut T, token: Token<T>, guard_token: GuardToken<T>, } /// A zero-sized token for [`Item::teardown`]. Cannot be created by the user. pub struct GuardToken<T: Item>(PhantomData<T>); /// A zero-sized token for resource `T`. Having an instance or reference to it, /// guarantees that `T` is in its active state. pub struct Token<T: Item>(PhantomData<T>); /// An inventory item interface. pub trait Item: Sized { /// Sets the inactive state. Called by [`Guard`] on `drop`. fn teardown(&mut self, _token: &mut GuardToken<Self>); } impl<T: Item> Inventory<T, U0> { /// Creates a new [`Inventory`] in the inactive state with zero tokens /// emitted. /// /// `item` should contain some form of token. #[inline] pub fn new(item: T) -> Self { Self { item, _marker: PhantomData } } /// Drops `inventory` and returns the stored item. #[inline] pub fn free(inventory: Self) -> T { inventory.item } /// Creates an RAII scoped guard. /// /// The item should be already in its active state. The returned guard will /// call [`Item::teardown`] on drop. #[inline] pub fn guard(inventory: &mut Self) -> Guard<'_, T> { Guard { borrow: &mut inventory.item, token: Token(PhantomData), guard_token: GuardToken(PhantomData), } } /// Calls [`Item::teardown`] for the stored item. #[inline] pub fn teardown(inventory: &mut Self) { inventory.item.teardown(&mut GuardToken(PhantomData)); } } impl<T: Item, C: Unsigned> Inventory<T, C> { /// Returns a reference to [`Token`]`<T>`. While the reference exists, the /// item is always in its active state. #[allow(clippy::unused_self)] #[inline] pub fn inventory_token(&self) -> &Token<T> { &Token(PhantomData) } } macro_rules! define_methods { ( $($share:ident $inc:ty => $($share_token:ident)*;)* ; $($merge:ident $dec:ty => $($merge_token:ident)*;)* ) => { impl<T: Item, C: Unsigned> Inventory<T, C> { $( /// Returns a token and a new inventory object with increased /// counter in its type. pub fn $share(self) -> (Inventory<T, Sum<C, $inc>>, $(Token<$share_token>),*) where C: Add<$inc>, Sum<C, $inc>: Unsigned, { ( Inventory { item: self.item, _marker: PhantomData }, $(Token(PhantomData::<$share_token>),)* ) } )* $( /// Consumes a token and returns a new inventory object with /// decreased counter in its type. #[allow(clippy::too_many_arguments)] pub fn $merge(self, $($merge_token: Token<T>,)*) -> Inventory<T, Diff<C, $dec>> where C: Sub<$dec>, Diff<C, $dec>: Unsigned, { $(drop($merge_token);)* Inventory { item: self.item, _marker: PhantomData } } )* } }; } define_methods! { share1 U1 => T; share2 U2 => T T; share3 U3 => T T T; share4 U4 => T T T T; share5 U5 => T T T T T; share6 U6 => T T T T T T; share7 U7 => T T T T T T T; share8 U8 => T T T T T T T T; ; merge1 U1 => a; merge2 U2 => a b; merge3 U3 => a b c; merge4 U4 => a b c d; merge5 U5 => a b c d e; merge6 U6 => a b c d e f; merge7 U7 => a b c d e f g; merge8 U8 => a b c d e f g h; } impl<T: Item, C: Unsigned> Deref for Inventory<T, C> { type Target = T; #[inline] fn deref(&self) -> &T { &self.item } } impl<T: Item, C: Unsigned> DerefMut for Inventory<T, C> { #[inline] fn deref_mut(&mut self) -> &mut T { &mut self.item } } impl<T: Item> Token<T> { /// Creates a new [`Token`]. /// /// # Safety /// /// Calling this method is dangerous because it may break the tokens /// counting. #[inline] pub unsafe fn new() -> Self { Self(PhantomData) } } impl<T: Item> Guard<'_, T> { /// Returns a reference to [`Token`]`<T>`. While the reference exists, the /// item is always in its active state. #[inline] pub fn inventory_token(&self) -> &Token<T> { &self.token } } impl<T: Item> Deref for Guard<'_, T> { type Target = T; #[inline] fn deref(&self) -> &T { self.borrow } } impl<T: Item> DerefMut for Guard<'_, T> { #[inline] fn deref_mut(&mut self) -> &mut T { self.borrow } } impl<T: Item> Drop for Guard<'_, T> { #[inline] fn drop(&mut self) { self.borrow.teardown(&mut self.guard_token); } }