1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
use core::{
ptr,
sync::atomic::{AtomicU32, Ordering},
};
use crate::{consts::BUF_SIZE, MODE_BLOCK_IF_FULL, MODE_MASK};
/// RTT Up channel
#[repr(C)]
pub(crate) struct Channel {
/// Name of the channel (null terminated)
pub name: *const u8,
/// Pointer to the RTT buffer.
pub buffer: *mut u8,
/// Size, in bytes, of the RTT buffer
pub size: u32,
/// Written by the target.
pub write: AtomicU32,
/// Written by the host.
pub read: AtomicU32,
/// Channel properties.
///
/// Currently, only the lowest 2 bits are used to set the channel mode (see constants below).
pub flags: AtomicU32,
}
impl Channel {
pub fn write_all(&self, mut bytes: &[u8]) {
// the host-connection-status is only modified after RAM initialization while the device is
// halted, so we only need to check it once before the write-loop
let write = match self.host_is_connected() {
_ if cfg!(feature = "disable-blocking-mode") => Self::nonblocking_write,
true => Self::blocking_write,
false => Self::nonblocking_write,
};
while !bytes.is_empty() {
let consumed = write(self, bytes);
if consumed != 0 {
bytes = &bytes[consumed..];
}
}
}
fn blocking_write(&self, bytes: &[u8]) -> usize {
if bytes.is_empty() {
return 0;
}
// calculate how much space is left in the buffer
let read = self.read.load(Ordering::Relaxed) as usize;
let write = self.write.load(Ordering::Acquire) as usize;
let available = available_buffer_size(read, write);
// abort if buffer is full
if available == 0 {
return 0;
}
self.write_impl(bytes, write, available)
}
fn nonblocking_write(&self, bytes: &[u8]) -> usize {
let write = self.write.load(Ordering::Acquire) as usize;
// NOTE truncate at BUF_SIZE to avoid more than one "wrap-around" in a single `write` call
self.write_impl(bytes, write, BUF_SIZE)
}
fn write_impl(&self, bytes: &[u8], cursor: usize, available: usize) -> usize {
let len = bytes.len().min(available);
// copy `bytes[..len]` to the RTT buffer
unsafe {
if cursor + len > BUF_SIZE {
// split memcpy
let pivot = BUF_SIZE - cursor;
ptr::copy_nonoverlapping(bytes.as_ptr(), self.buffer.add(cursor), pivot);
ptr::copy_nonoverlapping(bytes.as_ptr().add(pivot), self.buffer, len - pivot);
} else {
// single memcpy
ptr::copy_nonoverlapping(bytes.as_ptr(), self.buffer.add(cursor), len);
}
}
// adjust the write pointer, so the host knows that there is new data
self.write.store(
(cursor.wrapping_add(len) % BUF_SIZE) as u32,
Ordering::Release,
);
// return the number of bytes written
len
}
pub fn flush(&self) {
// return early, if host is disconnected
if !self.host_is_connected() {
return;
}
// busy wait, until the read- catches up with the write-pointer
let read = || self.read.load(Ordering::Relaxed);
let write = || self.write.load(Ordering::Relaxed);
while read() != write() {}
}
fn host_is_connected(&self) -> bool {
// we assume that a host is connected if we are in blocking-mode. this is what probe-run does.
self.flags.load(Ordering::Relaxed) & MODE_MASK == MODE_BLOCK_IF_FULL
}
}
/// How much space is left in the buffer?
fn available_buffer_size(read_cursor: usize, write_cursor: usize) -> usize {
if read_cursor > write_cursor {
read_cursor - write_cursor - 1
} else {
BUF_SIZE - write_cursor - 1 + read_cursor
}
}
#[cfg(test)]
mod tests {
use super::available_buffer_size;
use crate::consts::BUF_SIZE;
#[test]
fn test_rtt_available_buffer_size() {
// Helper to simulate RTT buffer state
let avail = |read: usize, write: usize| available_buffer_size(read, write);
// --- Case 1: Buffer is EMPTY (write == read) ---
// Should have maximum available space: BUF_SIZE - 1
assert_eq!(avail(0, 0), BUF_SIZE - 1);
assert_eq!(avail(10, 10), BUF_SIZE - 1);
assert_eq!(avail(BUF_SIZE - 1, BUF_SIZE - 1), BUF_SIZE - 1);
// --- Case 2: Buffer is FULL ---
// Full condition: (write + 1) % BUF_SIZE == read
// i.e., write == (read - 1 + BUF_SIZE) % BUF_SIZE
assert_eq!(avail(0, BUF_SIZE - 1), 0); // write=BUF_SIZE-1, read=0 → full
assert_eq!(avail(5, 4), 0); // write=4, read=5 → full
assert_eq!(avail(1, 0), 0); // write=0, read=1 → full
// --- Case 3: Read ahead of Write (no wrap-around) ---
// e.g., read=10, write=5 → free space = [5..9] → size = 10 - 5 - 1 = 4
assert_eq!(avail(10, 5), 10 - 5 - 1);
assert_eq!(avail(BUF_SIZE - 1, 0), (BUF_SIZE - 1) - 0 - 1); // = BUF_SIZE - 2
// --- Case 4: Write has wrapped around, Read behind (wrap-around case) ---
// e.g., read=5, write=10 → free space = [10..BUF_SIZE-1] + [0..4]
// size = (BUF_SIZE - 10 - 1) + (5) = BUF_SIZE - 10 - 1 + 5
assert_eq!(avail(5, 10), BUF_SIZE - 10 - 1 + 5);
assert_eq!(avail(0, 1), BUF_SIZE - 1 - 1 + 0); // = BUF_SIZE - 2
assert_eq!(avail(1, BUF_SIZE - 1), BUF_SIZE - (BUF_SIZE - 1) - 1 + 1); // = 1
// --- Edge: Single byte free ---
// After filling BUF_SIZE - 2 bytes from empty, 1 byte remains
assert_eq!(avail(1, BUF_SIZE - 1), 1); // one byte free
assert_eq!(avail(2, BUF_SIZE - 1), 2); // one byte free: only position BUF_SIZE-1 is free? No.
// Actually: write=BUF_SIZE-1, read=2 → free = [BUF_SIZE-1] + [0,1] → but [0,1] is 2 bytes?
// Let's recompute: total free = (BUF_SIZE - (BUF_SIZE-1) - 1) + 2 = (0) + 2 = 2 → wait.
// Better: use invariant
// Total data in buffer = (write - read + BUF_SIZE) % BUF_SIZE
// Free = BUF_SIZE - 1 - data
let data_in_buffer = |read: usize, write: usize| (write + BUF_SIZE - read) % BUF_SIZE;
let free_should_be = |read: usize, write: usize| BUF_SIZE - 1 - data_in_buffer(read, write);
// Validate our function against this invariant
for read in 0..BUF_SIZE.min(64) {
for write in 0..BUF_SIZE.min(64) {
let expected = free_should_be(read, write);
let actual = avail(read, write);
assert_eq!(actual, expected, "Mismatch at read={read}, write={write}");
}
}
}
}