rtio: Add a context pool

Adds a context pool that can be used when a number of threads may be
dynamically created to use with RTIO. While the pool has a small cost to
it, the cost of verifying a statically declared kobject likely makes it
worth avoiding dynamically allocating these with the typical
kobject_alloc method. Instead this arena style pool of objects can be
used where the kobject validation uses the gperf hash created at build
time.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>

Author: teburd

include/zephyr/rtio/pool.h

@@ -0,0 +1,125 @@
+/*
+ * Copyright (c) 2025 Intel Corporation
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @file
+ * @brief Real-Time IO pool API for maintaining a pool for RTIO contexts
+ */
+
+#ifndef ZEPHYR_INCLUDE_RTIO_POOL_H_
+#define ZEPHYR_INCLUDE_RTIO_POOL_H_
+
+#include <zephyr/rtio.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief RTIO Pool
+ * @defgroup rtio_pool RTIO Pool API
+ * @ingroup rtio
+ * @{
+ */
+
+/* clang-format off */
+
+/** @cond ignore */
+
+#define Z_RTIO_POOL_NAME_N(n, name)                                             \
+	name##_##n
+
+#define Z_RTIO_POOL_DEFINE_N(n, name, sq_sz, cq_sz)				\
+	RTIO_DEFINE(Z_RTIO_POOL_NAME_N(n, name), sq_sz, cq_sz)
+
+#define Z_RTIO_POOL_REF_N(n, name)                                              \
+	&Z_RTIO_POOL_NAME_N(n, name)
+
+/** @endcond */
+
+/* clang-format on */
+
+/**
+ * @brief Pool of RTIO contexts to use with dynamically created threads
+ */
+struct rtio_pool {
+	/** Size of the pool */
+	size_t pool_size;
+
+	/** Array containing contexts of the pool */
+	struct rtio **contexts;
+
+	/** Atomic bitmap to signal a member is used/unused */
+	atomic_t *used;
+};
+
+/* clang-format off */
+
+/**
+ * @brief Statically define and initialize a pool of RTIO contexts
+ *
+ * @param name Name of the RTIO pool
+ * @param pool_sz Number of RTIO contexts to allocate in the pool
+ * @param sq_sz Size of the submission queue entry pool per context
+ * @param cq_sz Size of the completion queue entry pool per context
+ */
+#define RTIO_POOL_DEFINE(name, pool_sz, sq_sz, cq_sz)				\
+	LISTIFY(pool_sz, RTIO_DEFINE_N, (;), name, sq_sz, cq_sz)                \
+	struct rtio *name_##contexts[] = {                                      \
+		LISTIFY(pool_sz, Z_RTIO_POOL_REF_N, (,), name)                  \
+	};                                                                      \
+	ATOMIC_DEFINE(name_##used, pool_sz);                                    \
+	STRUCT_SECTION_ITERABLE(rtio_pool, name) = {                            \
+		.pool_size = pool_sz,                                           \
+		.contexts = name_##contexts,                                    \
+		.used = name_##used,                                            \
+	}
+
+/* clang-format on */
+
+/**
+ * @brief Obtain an RTIO context from a pool
+ */
+__syscall struct rtio *rtio_pool_acquire(struct rtio_pool *pool);
+
+static inline struct rtio *z_impl_rtio_pool_acquire(struct rtio_pool *pool)
+{
+	struct rtio *r = NULL;
+
+	for (size_t i = 0; i < pool->pool_size; i++) {
+		if (atomic_test_and_set_bit(pool->used, i) == 0) {
+			r = pool->contexts[i];
+			break;
+		}
+	}
+
+	return r;
+}
+
+/**
+ * @brief Return an RTIO context to a pool
+ */
+__syscall void rtio_pool_release(struct rtio_pool *pool, struct rtio *r);
+
+static inline void z_impl_rtio_pool_release(struct rtio_pool *pool, struct rtio *r)
+{
+	for (size_t i = 0; i < pool->pool_size; i++) {
+		if (pool->contexts[i] == r) {
+			atomic_clear_bit(pool->used, i);
+			break;
+		}
+	}
+}
+
+/**
+ * @}
+ */
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* ZEPHYR_INCLUDE_RTIO_POOL_H_ */