scsi: target: tcmu: Make pgr_support and alua_support attributes writable
Currently in tcmu reservation commands are handled by core's pr implementation (default) or completely rejected (emulate_pr set to 0). We additionally want to be able to do full reservation handling in userspace. Therefore we need a way to set TRANSPORT_FLAG_PASSTHROUGH_PGR. The inverted flag is displayed by attribute pgr_support. Since we moved the flag from transport/backend to se_device in the previous commit, we now can make it changeable per device by allowing to write the attribute. The new field transport_flags_changeable in transport/backend is used to reject writing if not allowed for a backend. Regarding ALUA we also want to be able to passthrough commands to userspace in tcmu. Therefore we need TRANSPORT_FLAG_PASSTHROUGH_ALUA to be changeable, because by setting it we can switch off all ALUA checks in core. So we also set TRANSPORT_FLAG_PASSTHROUGH_ALUA in tcmu's transport_flags_changeable. Of course, ALUA and reservation handling in userspace will work only, if session/nexus information is sent to userspace along with every command. This will be object of a patch series announced by Mike Christie. Link: https://lore.kernel.org/r/20200427150823.15350-5-bstroesser@ts.fujitsu.com Reviewed-by:Mike Christie <mchristi@redhat.com> Signed-off-by: