1 /*
2 * Copyright (c) 2021 Intel Corporation
3 * SPDX-License-Identifier: Apache-2.0
4 */
5 #ifndef ZEPHYR_INCLUDE_SYS_WINSTREAM_H_
6 #define ZEPHYR_INCLUDE_SYS_WINSTREAM_H_
7
8 #include <stdint.h>
9
10 /** @brief Lockless shared memory byte stream IPC
11 *
12 * The sys_winstream utility implements a unidirectional byte stream
13 * with simple read/write semantics on top of a memory region shared
14 * by the writer and reader. It requires no locking or
15 * synchronization mechanisms beyond reliable ordering of memory
16 * operations, and so is a good fit for use with heterogeneous shared
17 * memory environments (for example, where Zephyr needs to talk to
18 * other CPUs in the system running their own software).
19 *
20 * This object does not keep track of the last sequence number read: the
21 * reader must keep that state and provide it on every read
22 * operation. After reaching "steady state", 'end' and 'start' are one
23 * byte apart because the buffer is always full.
24 */
25 struct sys_winstream {
26 uint32_t len; /* Length of data[] in bytes */
27 uint32_t start; /* Index of first valid byte in data[] */
28 uint32_t end; /* Index of next byte in data[] to write */
29 uint32_t seq; /* Mod-2^32 index of 'end' since stream init */
30 uint8_t data[];
31 };
32
33 /** @brief Construct a sys_winstream from a region of memory
34 *
35 * This function initializes a sys_winstream in an arbitrarily-sized
36 * region of memory, returning the resulting object (which is
37 * guaranteed to be at the same address as the buffer). The memory
38 * must (obviously) be shared between the reader and writer, and all
39 * operations to it must be coherent and consistently ordered.
40 *
41 * @param buf Pointer to a region of memory to contain the stream
42 * @param buflen Length of the buffer, must be large enough to contain
43 * the struct sys_winstream and at least one byte of
44 * data.
45 * @return A pointer to an initialized sys_winstream (same address as
46 * the buf parameter).
47 */
sys_winstream_init(void * buf,int buflen)48 static inline struct sys_winstream *sys_winstream_init(void *buf, int buflen)
49 {
50 struct sys_winstream *ws = buf, tmp = { .len = buflen - sizeof(*ws) };
51
52 *ws = tmp;
53 return ws;
54 }
55
56 /** @brief Write bytes to a sys_winstream
57 *
58 * This function writes the specified number of bytes into the stream.
59 * It will always return synchronously, it does not block or engage in
60 * any kind of synchronization beyond memory write ordering. Any
61 * bytes passed beyond what can be stored in the buffer will be
62 * silently dropped, but readers can detect their presence via the
63 * sequence number.
64 *
65 * @param ws A sys_winstream to which to write
66 * @param data Pointer to bytes to be written
67 * @param len Number of bytes to write
68 */
69 void sys_winstream_write(struct sys_winstream *ws,
70 const char *data, uint32_t len);
71
72 /** @brief Read bytes from a sys_winstream
73 *
74 * This function will read bytes from a sys_winstream into a specified
75 * buffer. It will always return in constant time, it does not block
76 * or engage in any kind of synchronization beyond memory ordering.
77 * The number of bytes read into the buffer will be returned, but note
78 * that it is possible that an underflow can occur if the writer gets
79 * ahead of our context. That situation can be detected via the
80 * sequence number returned via a pointer (i.e. if "*seq != old_seq +
81 * return_value", an underflow occurred and bytes were dropped).
82 *
83 * @param ws A sys_winstream from which to read
84 * @param seq A pointer to an integer containing the last sequence
85 * number read from the stream, or zero to indicate "start
86 * of stream". It is updated in place and returned for
87 * future calls and for detecting underflows.
88 * @param buf A buffer into which to store the data read
89 * @param buflen The length of buf in bytes
90 * @return The number of bytes written into the buffer
91 */
92 uint32_t sys_winstream_read(struct sys_winstream *ws,
93 uint32_t *seq, char *buf, uint32_t buflen);
94
95 #endif /* ZEPHYR_INCLUDE_SYS_WINSTREAM_H_ */
96
