[seL4] questions about PCIE SMMU for tk1 vmm

Hi, I am confused by the SMMU management for PCIE device in the updated TK1 VMM. The IO ASID for PCIE device is 0. However, the start IO ASID is from 1 as defined in smmu.h. When initialized, the lookup table, smmu_ioasid_to_pd, has no entry for IO asid 0 at all. I don't understand how the page table for PCIE device is generated and used by SMMU? which part source code should I check ?

thanks Peng