bpf: Document BTF_KIND_FLOAT in btf.rst

Also document the expansion of the kind bitfield.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210226202256.116518-11-iii@linux.ibm.com

Documentation/bpf/btf.rst

@@ -84,6 +84,7 @@ sequentially and type id is assigned to each recognized type starting from id
     #define BTF_KIND_FUNC_PROTO     13      /* Function Proto       */
     #define BTF_KIND_VAR            14      /* Variable     */
     #define BTF_KIND_DATASEC        15      /* Section      */
+    #define BTF_KIND_FLOAT          16      /* Floating point       */
 
 Note that the type section encodes debug info, not just pure types.
 ``BTF_KIND_FUNC`` is not a type, and it represents a defined subprogram.
@@ -95,8 +96,8 @@ Each type contains the following common data::
         /* "info" bits arrangement
          * bits  0-15: vlen (e.g. # of struct's members)
          * bits 16-23: unused
-         * bits 24-27: kind (e.g. int, ptr, array...etc)
-         * bits 28-30: unused
+         * bits 24-28: kind (e.g. int, ptr, array...etc)
+         * bits 29-30: unused
          * bit     31: kind_flag, currently used by
          *             struct, union and fwd
          */
@@ -452,6 +453,18 @@ map definition.
   * ``offset``: the in-section offset of the variable
   * ``size``: the size of the variable in bytes
 
+2.2.16 BTF_KIND_FLOAT
+~~~~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+ * ``name_off``: any valid offset
+ * ``info.kind_flag``: 0
+ * ``info.kind``: BTF_KIND_FLOAT
+ * ``info.vlen``: 0
+ * ``size``: the size of the float type in bytes: 2, 4, 8, 12 or 16.
+
+No additional type data follow ``btf_type``.
+
 3. BTF Kernel API
 *****************