3D render image on WC AGP aperture BO and then CPU fetch the image from this
bo, in order to achieve performance, after 3D finished rendering, validate
this bo into cached system memory and then read it from system memory. But I
always get garbage from there.
After check TTM bo validate codes, it has dealt with cache coherence
properly, i wonder whether there is something I misunderstand or it is not
allowed to move the WC bo from AGP to system memory for reading?