How to enforce 32bit access?

[Joachim Foerster <mls.JOFT@gmx.de>]

Hi ALSA devs,

(based on a Xilinx ML403, PPC405, OPB bus)
I have a custom sound controller, which allows 32bit write accesses to
its IO memory, only. The controller supports S16_LE, S16_BE, U16_LE,
U16_BE with min. and max. 2 channels (stereo), only! => So a frame has
to be 32bits wide and one frame has to be written at one point of time
(= one 32bit bus access).

Now, playing a 16kHz, S16_LE, Mono .wav-file with aplay invokes the
conversion framework of the alsa-lib. The following line of code (of
pcm_plugin.h) is executed:

conv_xx12_xx12: as_u16(dst) = as_u16c(src);

This is a 16bit access and thus fails with a Bus error/Kernel Oops.

Therefore my question is:

In my driver for the controller, how do I tell ALSA, that it is only
allowed to write in 32bit quantities?
Is this possible at all?
Or do I have to use a special .asoundrc file?

Thanks,
 Joachim

PS: 16bit, signed/unsigned, LE/BE, stereo (!) .wav files can be played
without any problem.