While investigating using the i.MX 8 for the Librem 5 phone we found an issue that would have been problematic for us to obtain the Free Software Foundation’s “Respects Your Freedom” (RYF) hardware endorsement:
- In U-Boot there are a number of firmware blobs that need to be loaded into the DDR PHY so that it can be trained to work with DDR4. This training is done on every boot.
- The normal boot sequence for the i.MX 8 is that the internal ROM loader loads the Secondary Program Loader (SPL) which, in this case, is a small version of U-Boot that can initialize the DDR and load the full U-Boot into DDR to finish the boot process. Very early in the SPL, the training blobs get loaded into the DDR PHY and the training sequence is run. The DDR training procedure is completely un-documented so re-writing the firmware blobs with free/libre or open source versions would be an arduous process.
- We can’t ignore the DDR PHY because it is interface between the i.MX 8 internal buses and the DDR4 chips outside of the SOC. The DDR PHY is also part of the i.MX 8 silicon so we can’t just replace the DDR PHY with a different one. It also appears that all DDR PHY’s required this training to work with DDR4, so going to a different SOC wouldn’t solve it either.
The RYF has a “secondary processor” exclusion that can be granted on a case by case basis. We will leverage this exclusion to load and train the DDR PHY on the i.MX 8. We will use a secondary processor to keep binary blobs out of u-boot and the kernel. Read more