ALSA: Introduce snd_BUG_ON() macro

Introduced snd_BUG_ON() macro as a replacement of snd_assert() macro.
snd_assert() is pretty ugly as it has the control flow in its argument.
OTOH, snd_BUG_ON() behaves like a normal conditional, thus it's much
easier to read the flow.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>

Documentation/sound/alsa/DocBook/writing-an-alsa-driver.tmpl

@@ -6173,6 +6173,47 @@ struct _snd_pcm_runtime {
 	 When no debug flag is set, this macro is ignored. 
       </para>
     </section>
+
+    <section id="useful-functions-snd-bug-on">
+      <title><function>snd_BUG_ON()</function></title>
+      <para>
+        <function>snd_BUG_ON()</function> macro is similar with
+	<function>WARN_ON()</function> macro. For example,  
+
+        <informalexample>
+          <programlisting>
+<![CDATA[
+  snd_BUG_ON(!pointer);
+]]>
+          </programlisting>
+        </informalexample>
+
+	or it can be used as the condition,
+        <informalexample>
+          <programlisting>
+<![CDATA[
+  if (snd_BUG_ON(non_zero_is_bug))
+          return -EINVAL;
+]]>
+          </programlisting>
+        </informalexample>
+
+      </para>
+
+      <para>
+        The macro takes an conditional expression to evaluate.
+	When <constant>CONFIG_SND_DEBUG</constant>, is set, the
+	expression is actually evaluated. If it's non-zero, it shows
+	the warning message such as
+	<computeroutput>BUG? (xxx)</computeroutput>
+	normally followed by stack trace.  It returns the evaluated
+	value.
+	When no <constant>CONFIG_SND_DEBUG</constant> is set, this
+	macro always returns zero.
+      </para>
+
+    </section>
+
   </chapter>