The -f- version seems to do more video processing in the firmware rather than in the kernel. These correspond to files somewhere on your device that describe the device and how to operate it. Vc4-fkms-v3d is a module, vc4-kms-v3d is an entirely different module. The dtoverlay parameter (device tree overlay) allows manually inserting device nodes into the device tree loaded by the system. as long as you're OK with the only documentation for some of the hardware being "This Linux driver works with it" and "Well, people have reverse engineered most of it." But they're absolutely insane, architecturally, they're poorly documented, and this doesn't seem widely known. The Pi2 BCM2836 interrupt controller feels a little bit like an intern's project, using the BCM2835 bolted on the front as an input for the rest of the hardware. The reason you see things like "All the interrupts are only ever on a single core" is because the interrupt controllers simply don't allow anything else. In the same way that userland applications make syscalls into the kernel, the kernel running on a Pi makes syscall-like requests into the GPU to do just about anything. The GPU boots the system, allows the ARM core some access to the DRAM (not at the same addresses as the GPU sees things, so be sure which version of memory addresses you're looking at), and most of the hardware control is done by asking the GPU to please do this thing for you through the mailbox interfaces. The Pi1/2/3 are more or less an (undocumented) GPU with an ARM core bolted on the side. They're nice devices, and work well enough, but they are architecturally insane and unlike anything else except the previous Raspberry Pis that were hacked to to make the new ones. Nothing about the Raspberry Pi at all is "normal," and it's a very useful filter for people who have read about the various deep weeds of firmware/OS development and those who have actually gotten their hands dirty on a Pi - because the first group will express admiration about the "open and well documented hardware," and the second group will laugh at the first group and start cursing the various non-and-poorly-documented nonsense that's in the Pis. Nothing about the Raspberry Pi’s video system is normal.
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