(unknown charset) dsp mmap change

I would like to ask for a review and/or testing of the following patch:
http://people.freebsd.org/~avg/dev_dsp_mmap.diff

It's supposed to fix an issue described here:
http://lists.freebsd.org/pipermail/freebsd-multimedia/2011-February/011691.html

In short, the following pseudo-code should do the right thing:
fd = open(/dev/dsp, O_RDWR);
mmap(PROT_READ, fd);
mmap(PROT_WRITE, fd);

Thank you!
-- 
Andriy Gapon