The RPMB authentication key is derived from the dev_id handed to the RPMB subsystem. OP-TEE implements the eMMC RPMB flow, where the dev_id is the eMMC CID: a fixed 16-byte value the key derivation depends on. The UFS RPMB id is "<device_id>-R<region>", which is variable length and longer than 16 bytes. Handing it to the RPMB subsystem as-is would tie the derived key to a length OP-TEE does not expect and diverge from the fixed-CID eMMC ABI, forcing OP-TEE to be taught about variable-length UFS ids. A fixed 16-byte dev_id is needed so the derived key stays stable and unique per region while matching the eMMC CID layout OP-TEE relies on, keeping the key-derivation ABI identical with no OP-TEE change. The reduction to a fixed 16 bytes must also be reproducible by the bootloaders (such as U-Boot) that derive the same dev_id. Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez@oss.qualcomm.com> Reviewed-by: Bean Huo <beanhuo@micron.com> Reviewed-by: Stanley Jhu <stanleyjhu@google.com> Link: https://patch.msgid.link/20260831154804.719528-3-jorge.ramirez@oss.qualcomm.com Signed-off-by: Martin K. Petersen (Oracle) <mkp@kernel.org>