Sometimes they're still necessary. E.g. many devices need particular sequencing in their power-up between multiple rails. The Intel 82598 Ethernet Controller da

Sometimes they're still necessary. E.g. many devices need particular sequencing in their power-up between multiple rails. The Intel 82598 Ethernet Controller datasheet[1] page 129-132 shows this nicely. If you're writing a driver you have to test that the appropriate GPIO control calls happen in the right sequence (and with the right timings). Of course it's an implementation detail, different ethernet controllers have different power-up sequence requirements, and thus different drivers. Of course you don't want to have to use real hardware for these tests, especially because with some devices incorrect power-up sequencing can cause permanent damage. So you write mocks for the various hardware functions and test the driver with those, and use their counters & the test harness's logging to assert that the sequencing and timing is correct. [1] https://www.intel.com/content/dam/www/public/us/en/documents/datasheets/82598-10-gbe-controller-datasheet.pdf https://www.intel.com/content/dam/www/public/us/en/documents...