posix: eventfd: fix dependency cycle between net and posix

Until recently, the posix api was purely a consumer of the
network subsystem. However, a dependency cycle was added as
a stop-gap solution for challenges with the native platform.

Specifically,

1. eventfd symbols conflict with those of the host
2. eventfd was excluded from native libc builds via cmake

If any part of the posix were then to select the network
subsystem (which is a legitimate use case, given that networking
is a part of the posix api), we would get a build error due to
the Kconfig dependency cycle.

As usual, with dependency cycles, the cycle can be broken
via a third, mutual dependency.

What is the third mutual dependency? Naturally, it is ZVFS
which was planned some time ago. ZVFS will be where we
collect file-descriptor and FILE-pointer APIs so that we can
ensure consistency for Zephyr users.

This change deprecates EVENTFD_MAX in favour of
ZVFS_EVENTFD_MAX.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>

Author: cfriedt

doc/services/portability/posix/kconfig/index.rst

@@ -9,7 +9,6 @@ implementation of the POSIX API.
 * :kconfig:option:`CONFIG_DYNAMIC_THREAD`
 * :kconfig:option:`CONFIG_DYNAMIC_THREAD_POOL_SIZE`
 * :kconfig:option:`CONFIG_EVENTFD`
-* :kconfig:option:`CONFIG_EVENTFD_MAX`
 * :kconfig:option:`CONFIG_FDTABLE`
 * :kconfig:option:`CONFIG_GETOPT_LONG`
 * :kconfig:option:`CONFIG_MAX_PTHREAD_SPINLOCK_COUNT`
@@ -38,3 +37,4 @@ implementation of the POSIX API.
 * :kconfig:option:`CONFIG_POSIX_SEM_VALUE_MAX`
 * :kconfig:option:`CONFIG_TIMER_CREATE_WAIT`
 * :kconfig:option:`CONFIG_THREAD_STACK_INFO`
+* :kconfig:option:`CONFIG_ZVFS_EVENTFD_MAX`

include/zephyr/posix/sys/eventfd.h

@@ -7,19 +7,16 @@
 #ifndef ZEPHYR_INCLUDE_POSIX_SYS_EVENTFD_H_
 #define ZEPHYR_INCLUDE_POSIX_SYS_EVENTFD_H_
 
-#include <stdint.h>
-
-#include <zephyr/kernel.h>
-#include <zephyr/posix/fcntl.h>
+#include <zephyr/zvfs/eventfd.h>
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-#define EFD_SEMAPHORE 0x2
-#define EFD_NONBLOCK  O_NONBLOCK
+#define EFD_SEMAPHORE ZVFS_EFD_SEMAPHORE
+#define EFD_NONBLOCK  ZVFS_EFD_NONBLOCK
 
-typedef uint64_t eventfd_t;
+typedef zvfs_eventfd_t eventfd_t;
 
 /**
  * @brief Create a file descriptor for event notification

include/zephyr/zvfs/eventfd.h

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2020 Tobias Svehagen
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef ZEPHYR_INCLUDE_ZEPHYR_ZVFS_EVENTFD_H_
+#define ZEPHYR_INCLUDE_ZEPHYR_ZVFS_EVENTFD_H_
+
+#include <stdint.h>
+
+#include <zephyr/kernel.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define ZVFS_EFD_SEMAPHORE 2
+#define ZVFS_EFD_NONBLOCK 0x4000
+
+typedef uint64_t zvfs_eventfd_t;
+
+/**
+ * @brief Create a file descriptor for ZVFS event notification
+ *
+ * The returned file descriptor can be used with POSIX read/write calls or
+ * with the @ref zvfs_eventfd_read or @ref zvfs_eventfd_write functions.
+ *
+ * It also supports polling and by including an eventfd in a call to poll,
+ * it is possible to signal and wake the polling thread by simply writing to
+ * the eventfd.
+ *
+ * When using read() and write() on a ZVFS eventfd, the size must always be at
+ * least 8 bytes or the operation will fail with EINVAL.
+ *
+ * @return New ZVFS eventfd file descriptor on success, -1 on error
+ */
+int zvfs_eventfd(unsigned int initval, int flags);
+
+/**
+ * @brief Read from a ZVFS eventfd
+ *
+ * If call is successful, the value parameter will have the value 1
+ *
+ * @param fd File descriptor
+ * @param value Pointer for storing the read value
+ *
+ * @return 0 on success, -1 on error
+ */
+int zvfs_eventfd_read(int fd, zvfs_eventfd_t *value);
+
+/**
+ * @brief Write to a ZVFS eventfd
+ *
+ * @param fd File descriptor
+ * @param value Value to write
+ *
+ * @return 0 on success, -1 on error
+ */
+int zvfs_eventfd_write(int fd, zvfs_eventfd_t value);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* ZEPHYR_INCLUDE_ZEPHYR_ZVFS_EVENTFD_H_ */

lib/os/CMakeLists.txt

@@ -40,3 +40,5 @@ zephyr_library_include_directories(
   ${ZEPHYR_BASE}/kernel/include
   ${ZEPHYR_BASE}/arch/${ARCH}/include
 )
+
+add_subdirectory_ifdef(CONFIG_ZVFS zvfs)

lib/os/Kconfig

@@ -123,5 +123,6 @@ config POWEROFF
 	  Enable support for system power off.
 
 rsource "Kconfig.cbprintf"
+rsource "zvfs/Kconfig"
 
 endmenu

lib/os/zvfs/CMakeLists.txt

@@ -0,0 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
+
+zephyr_library()
+zephyr_library_sources_ifdef(CONFIG_ZVFS_EVENTFD zvfs_eventfd.c)

lib/os/zvfs/Kconfig

@@ -0,0 +1,36 @@
+# Copyright (c) 2020 Tobias Svehagen
+# Copyright (c) 2023 Meta
+#
+# SPDX-License-Identifier: Apache-2.0
+
+menuconfig ZVFS
+	bool "Zephyr virtual filesystem (ZVFS) support [EXPERIMENTAL]"
+	select FDTABLE
+	select EXPERIMENTAL
+	help
+	  ZVFS is a central, Zephyr-native library that provides a common interoperable API for all
+	  types of file descriptors such as those from the non-virtual FS, sockets, eventfds, FILE *'s
+	  and more. It is designed to be used by all Zephyr subsystems that need to work with files.
+
+if ZVFS
+
+config ZVFS_EVENTFD
+	bool "ZVFS event file descriptor support"
+	select POLL
+	help
+	  Enable support for ZVFS event file descriptors. An eventfd can
+	  be used as an event wait/notify mechanism together with POSIX calls
+	  like read, write and poll.
+
+if ZVFS_EVENTFD
+
+config ZVFS_EVENTFD_MAX
+	int "Maximum number of ZVFS eventfd's"
+	default 1
+	range 1 4096
+	help
+	  The maximum number of supported event file descriptors.
+
+endif # ZVFS_EVENTFD
+
+endif # ZVFS