devices/src/bus.rs

// Copyright 2017 The Chromium OS Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

//! Handles routing to devices in an address space.

use std::cmp::{Ord, PartialOrd, PartialEq, Ordering};
use std::collections::btree_map::BTreeMap;
use std::result;
use std::sync::{Arc, Mutex};

/// Trait for devices that respond to reads or writes in an arbitrary address space.
///
/// The device does not care where it exists in address space as each method is only given an offset
/// into its allocated portion of address space.
#[allow(unused_variables)]
pub trait BusDevice: Send {
    /// Reads at `offset` from this device
    fn read(&mut self, offset: u64, data: &mut [u8]) {}
    /// Writes at `offset` into this device
    fn write(&mut self, offset: u64, data: &[u8]) {}
}

#[derive(Debug)]
pub enum Error {
    /// The insertion failed because the new device overlapped with an old device.
    Overlap,
}

pub type Result<T> = result::Result<T, Error>;

#[derive(Debug, Copy, Clone)]
struct BusRange(u64, u64);

impl Eq for BusRange {}

impl PartialEq for BusRange {
    fn eq(&self, other: &BusRange) -> bool {
        self.0 == other.0
    }
}

impl Ord for BusRange {
    fn cmp(&self, other: &BusRange) -> Ordering {
        self.0.cmp(&other.0)
    }
}

impl PartialOrd for BusRange {
    fn partial_cmp(&self, other: &BusRange) -> Option<Ordering> {
        self.0.partial_cmp(&other.0)
    }
}

/// A device container for routing reads and writes over some address space.
///
/// This doesn't have any restrictions on what kind of device or address space this applies to. The
/// only restriction is that no two devices can overlap in this address space.
#[derive(Clone)]
pub struct Bus {
    devices: BTreeMap<BusRange, Arc<Mutex<BusDevice>>>,
}

impl Bus {
    /// Constructs an a bus with an empty address space.
    pub fn new() -> Bus {
        Bus { devices: BTreeMap::new() }
    }

    fn first_before(&self, addr: u64) -> Option<(BusRange, &Mutex<BusDevice>)> {
        // for when we switch to rustc 1.17: self.devices.range(..addr).iter().rev().next()
        for (range, dev) in self.devices.iter().rev() {
            if range.0 <= addr {
                return Some((*range, dev));
            }
        }
        None
    }

    fn get_device(&self, addr: u64) -> Option<(u64, &Mutex<BusDevice>)> {
        if let Some((BusRange(start, len), dev)) = self.first_before(addr) {
            let offset = addr - start;
            if offset < len {
                return Some((offset, dev));
            }
        }
        None
    }

    /// Puts the given device at the given address space.
    pub fn insert(&mut self, device: Arc<Mutex<BusDevice>>, base: u64, len: u64) -> Result<()> {
        if len == 0 {
            return Err(Error::Overlap);
        }

        // Reject all cases where the new device's base is within an old device's range.
        if self.get_device(base).is_some() {
            return Err(Error::Overlap);
        }

        // The above check will miss an overlap in which the new device's base address is before the
        // range of another device. To catch that case, we search for a device with a range before
        // the new device's range's end. If there is no existing device in that range that starts
        // after the new device, then there will be no overlap.
        if let Some((BusRange(start, _), _)) = self.first_before(base + len - 1) {
            // Such a device only conflicts with the new device if it also starts after the new
            // device because of our initial `get_device` check above.
            if start >= base {
                return Err(Error::Overlap);
            }
        }

        if self.devices
               .insert(BusRange(base, len), device)
               .is_some() {
            return Err(Error::Overlap);
        }

        Ok(())
    }

    /// Reads data from the device that owns the range containing `addr` and puts it into `data`.
    ///
    /// Returns true on success, otherwise `data` is untouched.
    pub fn read(&self, addr: u64, data: &mut [u8]) -> bool {
        if let Some((offset, dev)) = self.get_device(addr) {
            dev.lock().unwrap().read(offset, data);
            true
        } else {
            false
        }
    }

    /// Writes `data` to the device that owns the range containing `addr`.
    ///
    /// Returns true on success, otherwise `data` is untouched.
    pub fn write(&self, addr: u64, data: &[u8]) -> bool {
        if let Some((offset, dev)) = self.get_device(addr) {
            dev.lock().unwrap().write(offset, data);
            true
        } else {
            false
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    struct DummyDevice;
    impl BusDevice for DummyDevice {}

    struct ConstantDevice;
    impl BusDevice for ConstantDevice {
        fn read(&mut self, offset: u64, data: &mut [u8]) {
            for (i, v) in data.iter_mut().enumerate() {
                *v = (offset as u8) + (i as u8);
            }
        }

        fn write(&mut self, offset: u64, data: &[u8]) {
            for (i, v) in data.iter().enumerate() {
                assert_eq!(*v, (offset as u8) + (i as u8))
            }
        }
    }

    #[test]
    fn bus_insert() {
        let mut bus = Bus::new();
        let dummy = Arc::new(Mutex::new(DummyDevice));
        assert!(bus.insert(dummy.clone(), 0x10, 0).is_err());
        assert!(bus.insert(dummy.clone(), 0x10, 0x10).is_ok());
        assert!(bus.insert(dummy.clone(), 0x0f, 0x10).is_err());
        assert!(bus.insert(dummy.clone(), 0x10, 0x10).is_err());
        assert!(bus.insert(dummy.clone(), 0x10, 0x15).is_err());
        assert!(bus.insert(dummy.clone(), 0x12, 0x15).is_err());
        assert!(bus.insert(dummy.clone(), 0x12, 0x01).is_err());
        assert!(bus.insert(dummy.clone(), 0x0, 0x20).is_err());
        assert!(bus.insert(dummy.clone(), 0x20, 0x05).is_ok());
        assert!(bus.insert(dummy.clone(), 0x25, 0x05).is_ok());
        assert!(bus.insert(dummy.clone(), 0x0, 0x10).is_ok());
    }

    #[test]
    fn bus_read_write() {
        let mut bus = Bus::new();
        let dummy = Arc::new(Mutex::new(DummyDevice));
        assert!(bus.insert(dummy.clone(), 0x10, 0x10).is_ok());
        assert!(bus.read(0x10, &mut [0, 0, 0, 0]));
        assert!(bus.write(0x10, &[0, 0, 0, 0]));
        assert!(bus.read(0x11, &mut [0, 0, 0, 0]));
        assert!(bus.write(0x11, &[0, 0, 0, 0]));
        assert!(bus.read(0x16, &mut [0, 0, 0, 0]));
        assert!(bus.write(0x16, &[0, 0, 0, 0]));
        assert!(!bus.read(0x20, &mut [0, 0, 0, 0]));
        assert!(!bus.write(0x20, &mut [0, 0, 0, 0]));
        assert!(!bus.read(0x06, &mut [0, 0, 0, 0]));
        assert!(!bus.write(0x06, &mut [0, 0, 0, 0]));
    }

    #[test]
    fn bus_read_write_values() {
        let mut bus = Bus::new();
        let dummy = Arc::new(Mutex::new(ConstantDevice));
        assert!(bus.insert(dummy.clone(), 0x10, 0x10).is_ok());

        let mut values = [0, 1, 2, 3];
        assert!(bus.read(0x10, &mut values));
        assert_eq!(values, [0, 1, 2, 3]);
        assert!(bus.write(0x10, &values));
        assert!(bus.read(0x15, &mut values));
        assert_eq!(values, [5, 6, 7, 8]);
        assert!(bus.write(0x15, &values));
    }
}