prepare_dma
addr = prepare_dma(host, data);
int (*prepare_dma)(struct loongson2_mmc_host *host, struct mmc_data *data);
return host->pdata->prepare_dma(host, data);
.prepare_dma = loongson2_mmc_prepare_internal_dma,
.prepare_dma = loongson2_mmc_prepare_external_dma,
.prepare_dma = loongson2_mmc_prepare_external_dma,
.prepare_dma = loongson2_mmc_prepare_internal_dma,
retval = prepare_dma(ep, req, GFP_ATOMIC);