forked from len0rd/rockbox
ec72d5a58a
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@2785 a1c6a512-1295-4272-9138-f99709370657
174 lines
7.2 KiB
Text
174 lines
7.2 KiB
Text
Rockbox From A Technical Angle
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==============================
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Background
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Björn Stenberg started this venture back in the late year 2001. The first
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Rockbox code was committed to CVS end of March 2002. Rockbox 1.0 was
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released in June.
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Booting and (De)Scrambling
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The built-in firmware in the Archos Jukebox reads a file from disk into
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memory, descrambles it, verifies the checksum and then runs it as code. When
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we build Rockbox images, we scramble the result file to use the same kind of
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scrambling that the original Archos firmware uses so that it can be loaded
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by the built-in firmware.
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1) The built-in firmware starts
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2) It looks in the root directory for a file called "archos.mod" (player)
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or "ajbrec.ajz" (recorder)
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3) If it finds one, it loads the file, descrambles it and runs it
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CPU
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The CPU in use is a SH7034 from Hitachi, running at 11.0592MHz (recorder)
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or 12MHz (player).
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Most single instructions are executed in 1 cycle. There is a 4KB internal RAM
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and a 2MB external RAM.
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Memory Usage
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All Archos Jukebox models have only 2MB RAM. The RAM is used for everything,
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including code, graphics and config. To be able to play as long as possible
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without having to load more data, the size of the mpeg playing buffer must
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remain as big as possible. Also, since we need to be able to do almost
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everything in Rockbox simultaneously, we use no dynamic memory allocation
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system at all. All sub-parts that needs memory must allocate their needs
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staticly. This puts a great responsibility on all coders.
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Playing MPEG
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The MPEG decoding is performed by an external circuit, MAS3507D (for the
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Player/Studio models) or MAS3587F (for the Recorder models).
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The CPU has a serial connection to the MAS for MP3 playback, using serial
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port 0 at approx. 1mbit/s. The MAS has a handshake signal called DEMAND,
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that informs the CPU when it wants more MP3 data. Whenever the DEMAND
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signal goes high, it wants data sent over the serial line, and it wants it
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quickly, within ~1ms. When the MAS has received enough data, it negates the
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DEMAND signal and expects the incoming data stream to stop within 1ms.
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The DEMAND signal is connected to a port pin on the CPU which can generate
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an IRQ, but only on the falling edge. That means that the mpeg driver code
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must poll the DEMAND signal every ms to keep the MAS happy. The mpeg code
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does use the IRQ to detect the falling edge when the MAS is "full".
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Unfortunately, the serial port on the CPU sends the LSB first, and the MAS
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expects the MSB first. Therefore we have to revers the bit order in every
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byte in the loaded MP3 data. This is referred to as "bit swapping" in the
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Rockbox code.
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The internal DMA controller is used to feed the serial port with data. The
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driver works roughly like this:
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1) Load MP3 data into the RAM buffer
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2) Bitswap the data
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3) Load the DMA source pointer to the next 64Kbyte block to be transferred
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4) Wait until DEMAND is high
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5) Enable the DMA
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6) Wait until the falling DEMAND pin generates an IRQ
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7) Disable the DMA
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8) Go to 4
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The DMA generates an IRQ when the 64Kbyte block is transferred, and the
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IRQ handler updates the DMA source pointer.
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_____________________________
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DEMAND __________| |_____________
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_ _ _ _ _ _ _ _ _
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SC0 _____________/ \/ \/ \/ \/ \/ \/ \/ \/ \____________
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\_/\_/\_/\_/\_/\_/\_/\_/\_/
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^ ^
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Poll sees the DEMAND The DEMAND pin generates
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signal go high and an IRQ that in turn disables
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enables the DMA the DMA again
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Spinning The Disk Up/Down
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To save battery, the spinning of the harddrive must be kept at a minimum.
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Rockbox features a timeout, so that if no action has been performed within N
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seconds, the disk will spin-down automaticly. However, if the disk was used
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for mpeg-loading for music playback, the spin-down will be almost immediate
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as then there's no point in timing out. The N second timer is thus only used
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when the disk-activity is trigged by a user.
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FAT and Mounting
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Rockbox scans the partitions of the disk and tries to mount them as fat32
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filesystems at boot.
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Directory Buffer
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When using the "dir browser" in Rockbox to display a single directory, it
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loads all entries in the directory into memory first, then sorts them and
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presents them on screen. The buffer used for all file entries is limited to
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maximum 16K or 400 entries. If the file names are longish, the 16K will run
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out before 400 entries have been used.
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This rather limited buffer size is of course again related to the necessity
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to keep the footprint small to keep the mpeg buffer as large as possible.
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Playlist Concepts
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One of the most obvious limitations in the firmware Rockbox tries to
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outperform, was the way playlists were dealt with.
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When loading a playlist (which is a plain text file with file names
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separated by newlines), Rockbox will scan through the file and store indexes
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to all file names in an array. The array itself has a 10000-entry limit (for
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memory size reasons).
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To play a specific song from the playlist, Rockbox checks the index and then
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seeks to that position in the original file on disk and gets the file name
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from there. This way, very little memory is wasted and yet very large
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playlists are supported.
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Playing a Directory
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Playing a full directory is using the same technique as with playlists. The
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difference is that the playlist is not a file on disk, but is the directory
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buffer.
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Shuffle
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Since the playlist is a an array of indexes to where to read the file name,
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shuffle modifies the order of these indexes in the array. The algorithm is
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pretty much like shuffling a deck of cards, and it uses a pseudo random
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generator called the Mersenne Twister. The randomness is identical for the
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same random seed. This is the secret to good resume. Even when you've shut
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down your unit and re-starts it, using the same random seed as the previous
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time will give exactly the same random order.
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Saving Config Data
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The Player/Studio models have no battery-backuped memory while the Recorder
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models have 44 bytes battery-backuped.
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To save data to be persistent and around even after reboots, Rockbox uses
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harddisk sector 63, which is outside the FAT32 filesystem. (Recorder models
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also get some data stored in the battery-backuped area).
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The config is only saved when the disk is spinning. This is important to
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realize, as if you change a config setting and then immediately shuts your
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unit down, the new config is not saved.
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DEVELOPERS:
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The config checksum includes a header with a version number. This version
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number must be increased when the config structure becomes incompatible.
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This makes the checksum check fail, and the settings are reset to default
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values.
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Resume Explained
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...
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Charging
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(Charging concerns Recorder models only, the other models have hardware-
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controlled charging that Rockbox can't affect.)
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...
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