Shady Dell laptop practices

bitrex · 2018-01-08T15:03:50+00:00

On some of the inexpensive Dell laptops they implement a hardware-lockout system intended to prevent you from using aftermarket charger bricks to charge the battery. The way they do this is there's a chip in the Dell charger that contains a unique identifier which a chip on the laptop mobo requests from it on startup over a one-wire interface. If on boot the BIOS can't confirm that the lockout chip on the mobo has asserted the "OEM OK" pin or whatever the BIOS then proceeds to muck around with the processor MSRs (model-specific registers) and alter their state, specifically the BD_PROCHOT register, which is a flag bit that is usually controlled by the motherboard temperature sensor. Flipping that makes the processor think the motherboard is overheating, which disables battery charging and has the additional nice "feature" of throttling the processor down to around 400MHz from 2.8 GHz or whatever. So not only can you not charge the battery you can't really use the laptop on an aftermarket brick even when plugged into the wall. Also there's then no way to tell if the laptop mobo is _actually_ overheating in that state. Good news is that the MSRs are accessible via software, so it's not hard to write a script that runs after the OS start to flip the bit back, and everything is back to normal.