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Building a modular sound driver

================================

  The following information is current as of linux-2.1.85. Check the other

readme files, especially README.OSS, for information not specific to

making sound modular.

  First, configure your kernel. This is an idea of what you should be

setting in the sound section:

 Sound card support

 100% Sound Blaster compatibles (SB16/32/64, ESS, Jazz16) support

  I have SoundBlaster. Select your card from the list.

 Generic OPL2/OPL3 FM synthesizer support

 FM synthesizer (YM3812/OPL-3) support

  If you don't set these, you will probably find you can play .wav files

but not .midi. As the help for them says, set them unless you know your

card does not use one of these chips for FM support.

  Once you are configured, make zlilo, modules, modules_install; reboot.

Note that it is no longer necessary or possible to configure sound in the

drivers/sound dir. Now one simply configures and makes one's kernel and

modules in the usual way.

 Then, add to your /etc/modprobe.conf something like:

alias char-major-14-* sb

install sb /sbin/modprobe -i sb && /sbin/modprobe adlib_card

options sb io=0x220 irq=7 dma=1 dma16=5 mpu_io=0x330

options adlib_card io=0x388     # FM synthesizer

 Alternatively, if you have compiled in kernel level ISAPnP support:

alias char-major-14 sb

post-install sb /sbin/modprobe "-k" "adlib_card"

options adlib_card io=0x388

  The effect of this is that the sound driver and all necessary bits and

pieces autoload on demand, assuming you use kerneld (a sound choice) and

autoclean when not in use. Also, options for the device drivers are

set. They will not work without them. Change as appropriate for your card.

If you are not yet using the very cool kerneld, you will have to "modprobe

-k sb" yourself to get things going. Eventually things may be fixed so

that this kludgery is not necessary; for the time being, it seems to work

well.

  Replace 'sb' with the driver for your card, and give it the right

options. To find the filename of the driver, look in

/lib/modules//misc. Mine looks like:

adlib_card.o # This is the generic OPLx driver

opl3.o # The OPL3 driver

sb.o # <>

sound.o # The sound driver

uart401.o # Used by sb, maybe other cards

 Whichever card you have, try feeding it the options that would be the

default if you were making the driver wired, not as modules. You can

look at function referred to by module_init() for the card to see what

args are expected.

 Note that at present there is no way to configure the io, irq and other

parameters for the modular drivers as one does for the wired drivers.. One

needs to pass the modules the necessary parameters as arguments, either

with /etc/modprobe.conf or with command-line args to modprobe, e.g.

modprobe sb io=0x220 irq=7 dma=1 dma16=5 mpu_io=0x330

modprobe adlib_card io=0x388

 recommend using /etc/modprobe.conf.

Persistent DMA Buffers:

The sound modules normally allocate DMA buffers during open() and

deallocate them during close(). Linux can often have problems allocating

DMA buffers for ISA cards on machines with more than 16MB RAM. This is

because ISA DMA buffers must exist below the 16MB boundary and it is quite

possible that we can't find a large enough free block in this region after

the machine has been running for any amount of time. The way to avoid this

problem is to allocate the DMA buffers during module load and deallocate

them when the module is unloaded. For this to be effective we need to load

the sound modules right after the kernel boots, either manually or by an

init script, and keep them around until we shut down. This is a little

wasteful of RAM, but it guarantees that sound always works.

To make the sound driver use persistent DMA buffers we need to pass the

sound.o module a "dmabuf=1" command-line argument. This is normally done

in /etc/modprobe.conf like so:

options sound           dmabuf=1

If you have 16MB or less RAM or a PCI sound card, this is wasteful and

unnecessary. It is possible that machine with 16MB or less RAM will find

this option useful, but if your machine is so memory-starved that it

cannot find a 64K block free, you will be wasting even more RAM by keeping

the sound modules loaded and the DMA buffers allocated when they are not

needed. The proper solution is to upgrade your RAM. But you do also have

this improper solution as well. Use it wisely.

  I'm afraid I know nothing about anything but my setup, being more of a

text-mode guy anyway. If you have options for other cards or other helpful

hints, send them to me, Jim Bray, jb@as220.org, http://as220.org/jb.