dma_alloc_from_coherent

HI: Can anybody point out that from which pool that function will allocate coherent memory for DMA use? I checked the function source code, the comments is as follow: This function should be only called from per-arch dma_alloc_coherent() * to support allocation from per-device coherent memory pools. * * Returns 0 if dma_alloc_coherent should continue with allocating from * generic memory areas, or !0 if dma_alloc_coherent should return @ret. Can anyone elaborate it? What’s per-device coherent memory pools, when are those pools setup and how? Thanks a lot! Rick lei