We are working with Shearwater Research on a slightly odd problem when downloading data from their Shearwater Petrel dive computer. From what we have been able to analyze (with generous help and support from Shearwater Research) it appears that the current firmware version of the Petrel causes a strange interaction with the libdivecomputer routines used to download divelog information from the Petrel.
At this point there appears to be no easy to implement workaround -- Shearwater Research is working on a firmware update for the Petrel that will fix the issue, but a release date for that update is not yet known. In the meantime we are working on implementing a different downloading algorithm for the Petrel that should also work around the issue, but we don’t have an educated guess when that will be finished, either.
So the disappointing status right now is that Petrels running firmware v8 and v11 are not supported in Subsurface.