132 lines
4.7 KiB
Perl
132 lines
4.7 KiB
Perl
#define _PAGE_ Rockbox Developer Conference 2002
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#include "head.t"
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<table align="right"><tr><td><a href="show.cgi?img4083.jpg"><img src="img4083t.jpg" alt="photo" border=0 width=200 height=150></a><br><small><i>Comparison of Recorder and Player</i></small></td></tr></table>
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<p>Well, almost. :-) Björn, Linus, Daniel and Kjell sat down at Linus' house
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friday night (2002-04-19) with our Archoses and had a long and fruitful discussion about software design.
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Here are a few things that we discussed:
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<h2>Application Programming Interfaces</h2>
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<p>We want to try to stick to POSIX where these exist and are practical. The
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reason is simply that many people already know these APIs well. Here are a
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few which haven't already been defined in the code:
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<h3>File operations</h3>
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<ul>
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<li>open
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<li>close
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<li>read
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<li>write
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<li>seek
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<li>unlink
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<li>rename
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</ul>
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<table align="right"><tr><td><a href="show.cgi?img4084.jpg"><img src="img4084t.jpg" alt="photo" border=0 width=200 height=150></a>
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<br><small><i>Contest: Spot the development box!</i></small></td></tr></table>
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<h3>Directory operations</h3>
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<ul>
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<li>opendir
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<li>closedir
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<li>readdir
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</ul>
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<h3>Disk operations</h3>
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<ul>
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<li>readblock
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<li>writeblock
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<li>spindown
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<li>diskinfo
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<li>partitioninfo
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</ul>
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<p>We also decided that we will use the 'newlib' standard C library,
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replacing some functions with smaller variants as we move forward.
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<h2>Multitasking</h2>
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<p>We spent much time discussing and debating task scheduling, or the lack
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thereof. First, we went with the idea that we don't really need "real"
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scheduling. Instead, a simple "tree-task" system would be used: A
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main-loop, a timer tick and a "bottom half" low-priority interrupt, each
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with an event queue.
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<p>Pretty soon we realized that we will want to:
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<ol style="a">
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<li> Use a timer tick to poll disk I/O (assuming we can't get an interrupt)
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<li> Perform slow disk operations in both the MP3->DAC feeder and the user
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interface, sometimes at the same time.
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<li> Not lock up the user interface during I/O.
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</ol>
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<table align="right"><tr><td><a href="show.cgi?img4086.jpg"><img src="img4086t.jpg" alt="photo" border=0 width=200 height=150></a>
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<br><small><i>A stack of "virgins"!</i></small></td></tr></table>
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<p>At the same time, we agreed that we should not walk into the common trap
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of engaging in "job splitting". That is, to split up jobs in small chunks
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so they don't take so long to finish. The problem with job splitting is
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that it makes the code flow very complex.
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<p>After much scratching our collective heads over how to make a primitive
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"three-task" system be able to do everything we wanted without resorting
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to complex job splitting, we finally came to the conclusion that we were
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heading down the wrong road:
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<p><blockquote>
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<b>We need threading.</b>
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</blockquote>
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<p>Even though a scheduler adds complexity, it makes the rest of the code so
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much more straight-forward that the total net result is less overall
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complexity.
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<p>To keep it simple, we decided to use a cooperative scheduler. That is, one
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in which the threads themselves decide when scheduling is performed. The
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big gain from this, apart from making the scheduler itself less complex,
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is that we don't have to worry as much about making all code "multithread
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safe".
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<p>Affording ourselves the luxury of threads, we soon identified four basic
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threads:
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<ul>
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<li>Disk thread, performing all disk operations
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<li>UI thread, handling the user interface
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<li>MP3 feed thread, making sure the MAS is fed with data at all times
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<li>I2C thread, handling the sometimes very relaxed timing of the I2C bus
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</ul>
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<p>Threads use message passing between them and each have a message queue
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associated to it.
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<table align="right"><tr><td><a href="show.cgi?img4089.jpg"><img src="img4089t.jpg" alt="photo" border=0 width=200 height=150></a>
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<br><small><i>There's much fun to be had with these things!</i></small></td></tr></table>
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<p>In addition to the threads, we need a timer interrupt with the ability to
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send messages to threads at specific intervals. This will also be used to
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scan the keys of the jukebox and handle key repeat detection (when a key
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has been pressed for a number of ticks).
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<p>None of these things are, of course, written in stone. Feel free to
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comment, discuss and argue about them!
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<p>We are currently 89 subscribers to this list. If you want to get more
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deeply involved in what's going on, I encourage you to:
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<ul>
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<li>Subscribe to the rockbox-cvs list, to see all code that goes in.
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<li>Join the #rockbox channel on irc.openprojects.net. There are always a
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couple of us in there.
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</ul>
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<p>I have written a set of guidelines for contributing code to the project.
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Take a look at them in CVS or here:
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<a href="http://bjorn.haxx.se/rockbox/firmware/CONTRIBUTING">CONTRIBUTING</a>
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<p>/Björn
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#include "foot.t"
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