[PATCH btf-next v3] libbpf: Add sphinx code documentation comments

[grantseltzer <grantseltzer@gmail.com>]

From: Grant Seltzer <grantseltzer@gmail.com>

This adds comments above five functions in btf.h which document
their uses. These comments are of a format that doxygen and sphinx
can pick up and render. These are rendered by libbpf.readthedocs.org

Signed-off-by: Grant Seltzer <grantseltzer@gmail.com>
---
 tools/lib/bpf/btf.h | 67 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)

diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index 4a711f990904..bfbc5e780e0f 100644
--- a/tools/lib/bpf/btf.h
+++ b/tools/lib/bpf/btf.h
@@ -1,5 +1,6 @@
 /* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
 /* Copyright (c) 2018 Facebook */
+/*! \file */
 
 #ifndef __LIBBPF_BTF_H
 #define __LIBBPF_BTF_H
@@ -30,11 +31,77 @@ enum btf_endianness {
 	BTF_BIG_ENDIAN = 1,
 };
 
+/**
+ * @brief **btf__free()** frees all data of a BTF object
+ * @param btf BTF object to free
+ */
 LIBBPF_API void btf__free(struct btf *btf);
 
+/**
+ * @brief **btf__new()** creates a new instance of a BTF object
+ * from the raw bytes of an ELF's BTF section
+ * @param data raw bytes
+ * @param size number of bytes passed in `data`
+ * @return new instance of BTF object which has to be eventually freed
+ * with **btf__free()**
+ *
+ * On error, error-code-encoded-as-pointer is returned, not a NULL. To
+ * extract error code from such a pointer `libbpf_get_error()` should be
+ * used. If `libbpf_set_strict_mode(LIBBPF_STRICT_CLEAN_PTRS)` is
+ * enabled, NULL is returned on error instead. In both cases thread-local
+ * `errno` variable is always set to error code as well.
+ */
 LIBBPF_API struct btf *btf__new(const void *data, __u32 size);
+
+/**
+ * @brief **btf__new_split()** create a new instance of a BTF object from 
+ * the provided raw data bytes. It takes another BTF instance, **base_btf**, 
+ * which serves as a base BTF, which is extended by types in a newly created
+ * BTF instance
+ * @param data raw bytes
+ * @param size length of raw bytes
+ * @param base_btf the base btf object
+ * @return new instance of BTF object which has to be eventually freed
+ * with **btf__free()**
+ *
+ * On error, error-code-encoded-as-pointer is returned, not a NULL. To
+ * extract error code from such a pointer `libbpf_get_error()` should be
+ * used. If `libbpf_set_strict_mode(LIBBPF_STRICT_CLEAN_PTRS)` is
+ * enabled, NULL is returned on error instead. In both cases thread-local
+ * `errno` variable is always set to error code as well.
+ */
 LIBBPF_API struct btf *btf__new_split(const void *data, __u32 size, struct btf *base_btf);
+
+/**
+ * @brief **btf__new_empty()** creates an unpopulated BTF object
+ * Use `btf__add_*()` to populate such BTF object.
+ * @return new instance of BTF object which has to be eventually freed
+ * with **btf__free()**
+ *
+ * On error, error-code-encoded-as-pointer is returned, not a NULL. To
+ * extract error code from such a pointer `libbpf_get_error()` should be
+ * used. If `libbpf_set_strict_mode(LIBBPF_STRICT_CLEAN_PTRS)` is
+ * enabled, NULL is returned on error instead. In both cases thread-local
+ * `errno` variable is always set to error code as well.
+ */
 LIBBPF_API struct btf *btf__new_empty(void);
+
+/**
+ * @brief **btf__new_empty_split()** creates an unpopulated
+ * BTF object from an ELF BTF section except with a base BTF
+ * on top of which split BTF should be based
+ * @return new instance of BTF object which has to be eventually freed 
+ * with **btf__free()**
+ *
+ * If *base_btf* is NULL, `btf__new_empty_split()` is equivalent to
+ * `btf__new_empty()` and creates non-split BTF.
+ *
+ * On error, error-code-encoded-as-pointer is returned, not a NULL. To
+ * extract error code from such a pointer `libbpf_get_error()` should be
+ * used. If `libbpf_set_strict_mode(LIBBPF_STRICT_CLEAN_PTRS)` is
+ * enabled, NULL is returned on error instead. In both cases thread-local
+ * `errno` variable is always set to error code as well.
+ */
 LIBBPF_API struct btf *btf__new_empty_split(struct btf *base_btf);
 
 LIBBPF_API struct btf *btf__parse(const char *path, struct btf_ext **btf_ext);
-- 
2.31.1