6e1f5a7301
git-svn-id: file:///srv/svn/repos/haiku/trunk/current@7576 a95241bf-73f2-0310-859d-f6bbb57e9c96
49 lines
2.2 KiB
HTML
49 lines
2.2 KiB
HTML
<HTML>
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<BODY>
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<H1>Misc notes</H1>
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<UL>
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<LI><P><B>MPU401 kernel module.</B> If your soundcard supports MIDI input and
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output, chances are that it is powered by an MPU401 chip. Because this
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interface is so popular, BeOS comes with a kernel module that makes it easy to
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write drivers for the MPU401. Thanks to Greg Crain, we now have an open source
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version of this kernel module.</P>
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<P>The mpu401 module lives in <CODE>src/add-ons/kernel/generic/mpu401</CODE>.
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It supports both the v1 and (undocumented) v2 protocols, although v2 is not
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complete since we don't really know how it works. Unfortunately, almost no
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existing drivers use v1; most of the drivers provided by Be require v2.
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Currently, the module returns B_ERROR when a MIDI device is opened with
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v2.</P>
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<P>For an example on how to use the MPU401 module in your own driver, see
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the source code for the "emuxki" driver elsewhere in the source tree.</P></LI>
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<LI><P><B>Clients without a BApplication.</B> Sometimes the midi_server's
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debug output shows an "Application -1 not registered" error message. This
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means it cannot figure out which app an incoming BMessage came from. The
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server ignores those messages.</P>
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<P>How can this happen? libmidi2 has two ways of sending messages to the
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midi_server: it either expects a reply back or not. In the first case, it is
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obvious to the midi_server what the reply address of the message is. In the
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second case, even though it is not necessary for the server to send a message
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back, it still uses the reply address to determine which app the message came
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from. For this, BMessenger uses be_app_messenger of the client app.</P>
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<P>However, if the client app has no BApplication object, there is no
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be_app_messenger either. Now, the midi_server cannot determine where the
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message came from and will ignore it. Is this important? For example, when such
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a client app Release()'s its endpoints, it sends a message to the server
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without a return address. Now the server ignores that message and does not
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remove the endpoint from the roster. Of course, after the client app has died,
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the endpoints will be removed eventually. Does all of this matter? Not really,
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because only trivial apps will have no BApplication object.</P></LI>
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</UL>
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</BODY>
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</HTML>
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