The assemble file under the driver folder can not be recognized when the driver is built as module

[Aubrey <aubreylee@gmail.com>]

Hi all,

I've written a framebuffer driver and put it under the folder
"./drivers/video", it consists of two files" mydriver.c and myfun.S".
When I build it into the kernel image, everything is ok.
But when I build it as module, I got the following error message:
===========================================
aubrey@linux:~/cvs/kernel/uClinux-dist> make V=1
--------snip---------
make -f scripts/Makefile.build obj=drivers/video
make -f scripts/Makefile.build obj=drivers/video/backlight
make[3]: *** No rule to make target `drivers/video/rgb2ycbcr.c',
needed by `drivers/video/rgb2ycbcr.o'.  Stop.
make[2]: *** [drivers/video] Error 2
make[1]: *** [drivers] Error 2
make[1]: Leaving directory `/home/aubrey/cvs/kernel/uClinux-dist/linux-2.6.x'
make: *** [linux] Error 1
===========================================

Make ask me for ".c" file. But it's an assemble file indeed.
Is it a bug of kernel script? (I'm using 2.6.16)

Regards,
-Aubrey