> This is not true for any upstream Linux kernel driver.

This is very true for at least a portion of upstream Linux kernel drivers.

> They are fully open source

True.

> fully controlled by the community

False (Of course, this claim is based on my belief that source code access is insufficient to do driver development, hardware datasheets are not just nice to have but mandatory for a true free operating system. If you disagree, nothing more can be said).

The upstream Linux kernel has absolutely no requirement that the contributed hardware drivers must include public documentation. In fact, many are written by vendor-hired contractors, or by independent embedded system companies who make computer gadgets based on these chips (so they have a business relationship with the vendor that give them datasheet access).

If you want to do any low-level work on these kinds of drivers independently, it almost always involves trial-and-error and educated guesses.

> and not subject to any NDAs to work on. The vendor can only exert control in the form of reviews and open maintainership. This is the case for both AMD and Intel GPU drivers.

Sure, the drivers themselves are not subject to any NDAs to work on, but hardware vendors often maintain a de-facto control - because they are in possession of technical advantages not available to outsiders, in terms of NDAed information and in-house expertise. Although theoretically everyone is free to participate, and it often happens, but when 80% of the experts are programmers with privileged information, it's difficult although not impossible for an outsider to join the development effort (it's no surprise that most outsider contributions are high-level changes that do not directly affect hardware operations, like moving away from a deprecated kernel API, fixing memory leaks, rather than, say, changing the PLL divider of the hardware's reference clock).

> For numbers, Arch Linux bundles some ~7.5k loadable kernel drivers (which is a subset of all available, but AUR only has 8 out-of-tree drivers, out of which two are proprietary and 2-3 are community-led. "vendor-controlled" drivers are an extreme outlier on Linux.

As a contributor to the Linux kernel, my personal experience is, drivers that contains code similar to the following are not rare:

uint8_t registers[] = {
0xFF, 0xFE, 0xEA, 0x3C, 0x5A, 0x6A,
0x01, 0x02, 0x3A, 0x4D, 0x55, 0x66
/* many lines */
};

/* initialize hardware */
write_registers(device_ctx, registers);

I bet at least 10% of device drivers (or as high as 25%?) are written with privileged information and they're almost impossible for independent developers to work with (other than making educated guesses) due to incomplete or NDAed documentation. Datasheet access is the only way to interpret the intentions of this kind of drivers. Even with macro or bitfields definitions - which is better than none, the exact effect of a register is often subtle. Not to mention that hardware bugs are common and their workarounds often involve very particular register-write sequences - and that tend not to be publicly documented. Sometimes, even firmware binary blobs are embedded into the source code this way, and the code is almost always contributed by a vendor who actually knows what the blob is doing. If I was asked to do some development on top of an existing driver but without hardware documentation, I would refuse it every single time.