os_fd.h

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// Copyright 2019, Collabora, Ltd.
// Copyright 2026, Beyley Cardellio
// Copyright 2026, Finn Zeruhn
// SPDX-License-Identifier: BSL-1.0
/*!
 * @file
 * @brief  Reading from file descriptors, shared by the Linux hid and serial code.
 * @author Jakob Bornecrantz <jakob@collabora.com>
 * @author Rylie Pavlik <rylie.pavlik@collabora.com>
 * @author Beyley Cardellio <ep1cm1n10n123@gmail.com>
 * @author Finn Zeruhn <fzeruhn@icloud.com>
 * @ingroup aux_os
 */
 
#pragma once
 
#include "xrt/xrt_config_os.h"
 
#ifndef XRT_OS_LINUX
#error Linux-specific file.
#endif
 
#include "os/os_time.h"
 
#include <stdint.h>
#include <errno.h>
#include <poll.h>
#include <unistd.h>
 
 
/*!
 * Waits up to @p milliseconds for @p fd to be readable, then reads from it.
 * A negative @p milliseconds reads without waiting. Retries on EINTR.
 *
 * @return Bytes read, 0 on timeout, -1 on error.
 */
static inline ssize_t
os_fd_read(int fd, uint8_t *data, size_t size, int milliseconds)
{
        ssize_t ret;
 
        if (milliseconds >= 0) {
                int64_t deadline_ns = os_monotonic_get_ns() + (int64_t)milliseconds * U_TIME_1MS_IN_NS;
                int timeout = milliseconds;
                struct pollfd fds = {
                    .fd = fd,
                    .events = POLLIN,
                };
 
                while ((ret = poll(&fds, 1, timeout)) < 0 && errno == EINTR) {
                        int64_t left_ns = deadline_ns - os_monotonic_get_ns();
                        if (left_ns <= 0) {
                                ret = 0;
                                break;
                        }
                        timeout = (int)((left_ns + U_TIME_1MS_IN_NS - 1) / U_TIME_1MS_IN_NS);
                }
 
                if (ret == -1 || ret == 0) {
                        // Error or timeout.
                        return ret;
                }
                if (fds.revents & (POLLERR | POLLHUP | POLLNVAL)) {
                        // Device disconnect?
                        return -1;
                }
        }
 
        do {
                ret = read(fd, data, size);
        } while (ret < 0 && errno == EINTR);
 
        if (ret < 0 && (errno == EAGAIN || errno == EINPROGRESS)) {
                // Process most likely received a signal.
                ret = 0;
        }
 
        return ret;
}