Added note about the MPU401 chip.

git-svn-id: file:///srv/svn/repos/haiku/trunk/current@7575 a95241bf-73f2-0310-859d-f6bbb57e9c96

docs/develop/midi/stuff.html

@@ -5,9 +5,23 @@
 
 <UL>
 
-<LI><P>Sometimes the midi_server's debug output shows an "Application -1 not 
+<LI><P><B>MPU401 kernel module.</B> If your soundcard supports MIDI input and
-registered" error message. This means it cannot figure out which app an 
+output, chances are that it is powered by an MPU401 chip. Because this 
-incoming BMessage came from. The server ignores those messages.</P>
+interface is so popular, BeOS comes with a kernel module that makes it easy to
+write drivers for the MPU401. Thanks to Greg Crain, we now have an open source
+version of this kernel module.</P>
+
+<P>The mpu401 module lives in <CODE>src/add-ons/kernel/generic/mpu401</CODE>.
+It supports both the v1 and (undocumented) v2 protocols, although v2 is not
+complete since we don't really know how it works. Unfortunately, almost no
+existing drivers use v1; most of the drivers provided by Be require v2. 
+Currently, the module returns B_ERROR when a MIDI device is opened with
+v2.</P></LI>
+
+<LI><P><B>Clients without a BApplication.</B> Sometimes the midi_server's
+debug output shows an "Application -1 not registered" error message. This 
+means it cannot figure out which app an incoming BMessage came from. The
+server ignores those messages.</P>
 
 <P>How can this happen? libmidi2 has two ways of sending messages to the 
 midi_server: it either expects a reply back or not. In the first case, it is