scripts: kernel-doc: parse next structs/unions

There are several places within the Kernel tree with nested
structs/unions, like this one:

  struct ingenic_cgu_clk_info {
    const char *name;
    enum {
      CGU_CLK_NONE = 0,
      CGU_CLK_EXT = BIT(0),
      CGU_CLK_PLL = BIT(1),
      CGU_CLK_GATE = BIT(2),
      CGU_CLK_MUX = BIT(3),
      CGU_CLK_MUX_GLITCHFREE = BIT(4),
      CGU_CLK_DIV = BIT(5),
      CGU_CLK_FIXDIV = BIT(6),
      CGU_CLK_CUSTOM = BIT(7),
    } type;
    int parents[4];
    union {
      struct ingenic_cgu_pll_info pll;
      struct {
        struct ingenic_cgu_gate_info gate;
        struct ingenic_cgu_mux_info mux;
        struct ingenic_cgu_div_info div;
        struct ingenic_cgu_fixdiv_info fixdiv;
      };
      struct ingenic_cgu_custom_info custom;
    };
  };

Currently, such struct is documented as:

	**Definition**

	::
	struct ingenic_cgu_clk_info {
	    const char * name;
	};

	**Members**

	``name``
	  name of the clock

With is obvioulsy wrong. It also generates an error:
	drivers/clk/ingenic/cgu.h:169: warning: No description found for parameter 'enum'

However, there's nothing wrong with this kernel-doc markup: everything
is documented there.

It makes sense to document all fields there. So, add a
way for the core to parse those structs.

With this patch, all documented fields will properly generate
documentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/doc-guide/kernel-doc.rst

@@ -281,6 +281,52 @@ comment block.
 The kernel-doc data structure comments describe each member of the structure,
 in order, with the member descriptions.
 
+Nested structs/unions
+~~~~~~~~~~~~~~~~~~~~~
+
+It is possible to document nested structs unions, like::
+
+      /**
+       * struct nested_foobar - a struct with nested unions and structs
+       * @arg1: - first argument of anonymous union/anonymous struct
+       * @arg2: - second argument of anonymous union/anonymous struct
+       * @arg3: - third argument of anonymous union/anonymous struct
+       * @arg4: - fourth argument of anonymous union/anonymous struct
+       * @bar.st1.arg1 - first argument of struct st1 on union bar
+       * @bar.st1.arg2 - second argument of struct st1 on union bar
+       * @bar.st2.arg1 - first argument of struct st2 on union bar
+       * @bar.st2.arg2 - second argument of struct st2 on union bar
+      struct nested_foobar {
+        /* Anonymous union/struct*/
+        union {
+          struct {
+            int arg1;
+            int arg2;
+	  }
+          struct {
+            void *arg3;
+            int arg4;
+	  }
+	}
+	union {
+          struct {
+            int arg1;
+            int arg2;
+	  } st1;
+          struct {
+            void *arg1;
+            int arg2;
+	  } st2;
+	} bar;
+      };
+
+.. note::
+
+   #) When documenting nested structs or unions, if the struct/union ``foo``
+      is named, the argument ``bar`` inside it should be documented as
+      ``@foo.bar:``
+   #) When the nested struct/union is anonymous, the argument ``bar`` on it
+      should be documented as ``@bar:``
 
 Typedef documentation
 ---------------------