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phoenix |
Building a modular sound driver
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The following information is current as of linux-2.1.85. Check the other
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readme files, especially README.OSS, for information not specific to
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making sound modular.
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First, configure your kernel. This is an idea of what you should be
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setting in the sound section:
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Sound card support
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100% Sound Blaster compatibles (SB16/32/64, ESS, Jazz16) support
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I have SoundBlaster. Select your card from the list.
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Generic OPL2/OPL3 FM synthesizer support
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FM synthesizer (YM3812/OPL-3) support
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If you don't set these, you will probably find you can play .wav files
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but not .midi. As the help for them says, set them unless you know your
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card does not use one of these chips for FM support.
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Once you are configured, make zlilo, modules, modules_install; reboot.
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Note that it is no longer necessary or possible to configure sound in the
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drivers/sound dir. Now one simply configures and makes one's kernel and
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modules in the usual way.
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Then, add to your /etc/modules.conf something like:
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alias char-major-14 sb
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post-install sb /sbin/modprobe "-k" "adlib_card"
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options sb io=0x220 irq=7 dma=1 dma16=5 mpu_io=0x330
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options adlib_card io=0x388 # FM synthesizer
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Alternatively, if you have compiled in kernel level ISAPnP support:
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alias char-major-14 sb
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post-install sb /sbin/modprobe "-k" "adlib_card"
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options adlib_card io=0x388
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The effect of this is that the sound driver and all necessary bits and
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pieces autoload on demand, assuming you use kerneld (a sound choice) and
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autoclean when not in use. Also, options for the device drivers are
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set. They will not work without them. Change as appropriate for your card.
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If you are not yet using the very cool kerneld, you will have to "modprobe
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-k sb" yourself to get things going. Eventually things may be fixed so
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that this kludgery is not necessary; for the time being, it seems to work
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well.
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Replace 'sb' with the driver for your card, and give it the right
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options. To find the filename of the driver, look in
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/lib/modules//misc. Mine looks like:
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adlib_card.o # This is the generic OPLx driver
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opl3.o # The OPL3 driver
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sb.o # <>
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sound.o # The sound driver
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uart401.o # Used by sb, maybe other cards
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Whichever card you have, try feeding it the options that would be the
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default if you were making the driver wired, not as modules. You can look
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at the init_module() code for the card to see what args are expected.
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Note that at present there is no way to configure the io, irq and other
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parameters for the modular drivers as one does for the wired drivers.. One
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needs to pass the modules the necessary parameters as arguments, either
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with /etc/modules.conf or with command-line args to modprobe, e.g.
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modprobe -k sb io=0x220 irq=7 dma=1 dma16=5 mpu_io=0x330
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modprobe -k adlib_card io=0x388
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recommend using /etc/modules.conf.
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Persistent DMA Buffers:
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The sound modules normally allocate DMA buffers during open() and
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deallocate them during close(). Linux can often have problems allocating
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DMA buffers for ISA cards on machines with more than 16MB RAM. This is
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because ISA DMA buffers must exist below the 16MB boundary and it is quite
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possible that we can't find a large enough free block in this region after
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the machine has been running for any amount of time. The way to avoid this
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problem is to allocate the DMA buffers during module load and deallocate
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them when the module is unloaded. For this to be effective we need to load
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the sound modules right after the kernel boots, either manually or by an
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init script, and keep them around until we shut down. This is a little
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wasteful of RAM, but it guarantees that sound always works.
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To make the sound driver use persistent DMA buffers we need to pass the
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sound.o module a "dmabuf=1" command-line argument. This is normally done
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in /etc/modules.conf like so:
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options sound dmabuf=1
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If you have 16MB or less RAM or a PCI sound card, this is wasteful and
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unnecessary. It is possible that machine with 16MB or less RAM will find
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this option useful, but if your machine is so memory-starved that it
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cannot find a 64K block free, you will be wasting even more RAM by keeping
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the sound modules loaded and the DMA buffers allocated when they are not
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needed. The proper solution is to upgrade your RAM. But you do also have
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this improper solution as well. Use it wisely.
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I'm afraid I know nothing about anything but my setup, being more of a
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text-mode guy anyway. If you have options for other cards or other helpful
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hints, send them to me, Jim Bray, jb@as220.org, http://as220.org/jb.
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