msm_rbmemptrs
sizeof(struct msm_rbmemptrs) * nr_rings,
memptrs += sizeof(struct msm_rbmemptrs);
memptrs_iova += sizeof(struct msm_rbmemptrs);
((ring)->memptrs_iova + offsetof(struct msm_rbmemptrs, member))
struct msm_rbmemptrs *memptrs;