what is the purpose of GeodeCalculatePitchBytes function?

[Cui, Hunk]

Hi, all,

	Ask one question...
	In
http://cgit.freedesktop.org/xorg/driver/xf86-video-geode/tree/src/gx_dri
ver.c#n1560, this is the function " GeodeCalculatePitchBytes", in the
process of calculate the pitch, why does the compression require a power
of 2, what is the purpose? 
	Is it a pitch structure alignment?

Thanks,
Hunk Cui