blob: 7101b2e2eaf5757ed6666b4ba926fd811278379f [file] [log] [blame]
* Copyright (c) 2021 Intel Corporation
* SPDX-License-Identifier: Apache-2.0
#include <stdint.h>
/** @brief Lockless shared memory byte stream IPC
* The sys_winstream utility implements a unidirectional byte stream
* with simple read/write semantics on top of a memory region shared
* by the writer and reader. It requires no locking or
* synchronization mechanisms beyond reliable ordering of memory
* operations, and so is a good fit for use with heterogenous shared
* memory environments (for example, where Zephyr needs to talk to
* other CPUs in the system running their own software).
* This object does not keep track of the last sequence number read: the
* reader must keep that state and provide it on every read
* operation. After reaching "steady state", 'end' and 'start' are one
* byte apart because the buffer is always full.
struct sys_winstream {
uint32_t len; /* Length of data[] in bytes */
uint32_t start; /* Index of first valid byte in data[] */
uint32_t end; /* Index of next byte in data[] to write */
uint32_t seq; /* Mod-2^32 index of 'end' since stream init */
uint8_t data[];
/** @brief Construct a sys_winstream from a region of memory
* This function initializes a sys_winstream in an arbitrarily-sized
* region of memory, returning the resulting object (which is
* guaranteed to be at the same address as the buffer). The memory
* must (obviously) be shared between the reader and writer, and all
* operations to it must be coherent and consistently ordered.
* @param buf Pointer to a region of memory to contain the stream
* @param buflen Length of the buffer, must be large enough to contain
* the struct sys_winstream and at least one byte of
* data.
* @return A pointer to an initialized sys_winstream (same address as
* the buf parameter).
static inline struct sys_winstream *sys_winstream_init(void *buf, int buflen)
struct sys_winstream *ws = buf, tmp = { .len = buflen - sizeof(*ws) };
*ws = tmp;
return ws;
/** @brief Write bytes to a sys_winstream
* This function writes the specified number of bytes into the stream.
* It will always return synchronously, it does not block or engage in
* any kind of synchronization beyond memory write ordering. Any
* bytes passed beyond what can be stored in the buffer will be
* silently dropped, but readers can detect their presence via the
* sequence number.
* @param ws A sys_winstream to which to write
* @param data Pointer to bytes to be written
* @param len Number of bytes to write
void sys_winstream_write(struct sys_winstream *ws,
const char *data, uint32_t len);
/** @brief Read bytes from a sys_winstream
* This function will read bytes from a sys_winstream into a specified
* buffer. It will always return in constant time, it does not block
* or engage in any kind of synchronization beyond memory ordering.
* The number of bytes read into the buffer will be returned, but note
* that it is possible that an underflow can occur if the writer gets
* ahead of our context. That situation can be detected via the
* sequence number returned via a pointer (i.e. if "*seq != old_seq +
* return_value", an underflow occurred and bytes were dropped).
* @param ws A sys_winstream from which to read
* @param seq A pointer to an integer containing the last sequence
* number read from the stream, or zero to indicate "start
* of stream". It is updated in place and returned for
* future calls and for detecting underflows.
* @param buf A buffer into which to store the data read
* @param buflen The length of buf in bytes
* @return The number of bytes written into the buffer
uint32_t sys_winstream_read(struct sys_winstream *ws,
uint32_t *seq, char *buf, uint32_t buflen);