1 // Copyright 2022, The Android Open Source Project
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 //     http://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14 
15 //! Minimal driver for an 8250 UART. This only implements enough to work with the emulated 8250
16 //! provided by crosvm, and won't work with real hardware.
17 
18 use crate::arch::write_volatile_u8;
19 use core::fmt::{self, Write};
20 
21 /// Minimal driver for an 8250 UART. This only implements enough to work with the emulated 8250
22 /// provided by crosvm, and won't work with real hardware.
23 pub struct Uart {
24     base_address: *mut u8,
25 }
26 
27 impl Uart {
28     /// Constructs a new instance of the UART driver for a device at the given base address.
29     ///
30     /// # Safety
31     ///
32     /// The given base address must point to the 8 MMIO control registers of an appropriate UART
33     /// device, which must be mapped into the address space of the process as device memory and not
34     /// have any other aliases.
new(base_address: usize) -> Self35     pub unsafe fn new(base_address: usize) -> Self {
36         Self { base_address: base_address as *mut u8 }
37     }
38 
39     /// Writes a single byte to the UART.
write_byte(&self, byte: u8)40     pub fn write_byte(&self, byte: u8) {
41         // SAFETY: We know that the base address points to the control registers of a UART device
42         // which is appropriately mapped.
43         unsafe { write_volatile_u8(self.base_address, byte) }
44     }
45 }
46 
47 impl Write for Uart {
write_str(&mut self, s: &str) -> fmt::Result48     fn write_str(&mut self, s: &str) -> fmt::Result {
49         for c in s.as_bytes() {
50             self.write_byte(*c);
51         }
52         Ok(())
53     }
54 }
55 
56 // SAFETY: `Uart` just contains a pointer to device memory, which can be accessed from any context.
57 unsafe impl Send for Uart {}
58