My naive assumption is that code reuse across platforms is a good thing, I'd love to understand why this isn't the case here or what the concrete arguments are against it.
A driver is inherently platform-specific. It's glue that ties the hardware to the operating system. The only "correct" way to have one driver work on multiple operating systems is for the operating systems to all use the same driver model.
The ugly way is to create your own hardware abstraction layer and then write a translation layer between that and each operating system, because that's complicated and hideous.
But it's especially silly because Linux accepts suitable contributed code, so you could instead use the native Linux model as your "intermediary layer" and fix Linux if it isn't suitable in some way. And then translate that to what the closed operating system you can't modify uses.
The result is that the Linux people are happier and you have one less translation layer to maintain.
One might ask whether it is desirable to avoid the GPL, and there are a lot of arguments on both sides there, but it's certainly easy to run into issues when you have a GPL licensed module designed to be linked into a proprietary program (kernel).
Isn't the point supposed to be to not have other versions of your driver, so you can use the same one on every platform?
I can't speak to the legal status of GPL drivers for Windows, but several seem to exist already (e.g. Windows ext4 driver), and if they were actually worried about it they could always get explicit permission from the copyright holders of the relevant code. Either they say yes and you're fine or they say no you know what pieces of code to replace.
But Linux repesents a tiny portion of the gaming community, so that approach would make no sense at all for a GPU vendor. C'mon.
I know that Linux people really really just want the kernel to take one for the team so they can have GPUs because that's just the goal, and clearly the goal is good and the means don't matter at all and everything else is irrelevant. 100,000 lines of crap code, 200k? 500k? Who cares, it's all in the name of GPUs clearly. It's obviously worth it no matter what.
But the kernel developers do not see it that way, and for good reason -- because once it's in tree, they are all on the hook for it and they all have to deal with the swamp, the added complexity, the maintenance, the un-fucking of this entire HAL, etc etc.
Having worked on a large open source project, I can assure you, it sucks when you have to say "This isn't acceptable and we aren't merging it", even when it's a feature the users want, and one someone worked on for a long time. It is also, almost always, the right thing to do in the long run (and several of those features did come back, in acceptable ways, in our case).
The growth market for GPUs is GPGPU and servers. And Linux represents a large portion of the programming and server communities.
More to the point, as soon as you support Linux at all then it doesn't matter who has more share, it's still less work to do the above than have to maintain another translation layer.
Game developers might like to see clean driver source but they don't get to choose what kind of GPU their customers have already bought. And 99% of gamers are not going to choose their GPU based on Linux drivers. So nobody has any leverage and vendors have no incentive to change.
Meanwhile thousands of universities and institutions are each going to be looking for 25,000 GPUs and they can choose what brand they buy based on what makes their internal developers happy. Hosts like Amazon and Google are each going to be buying millions of GPUs, and having better and more transparent drivers so they can more easily e.g. improve power consumption by a small percentage, can save them a million dollars/year in electricity.
Someone like Google could come to each vendor and say "first to have mainline kernel drivers gets all our business" at any point. Or the same result in the other order; once there are clean drivers third parties are more likely to make power consumption and performance improvements that give AMD the edge when the major customers crunch the numbers.
There is a significant competitive advantage in it for AMD to get this right.
And you really believe that the maintainers will be accepting a giant patch that changes the API and subsystem completely (though into something better) that has the risk of causing lots of regressions to existing drivers? And you believe that AMD is supposed to fix all the regressions that are caused in drivers by other vendors that this change causes?
And yes - who else is supposed to fix all the regressions caused by changes that AMD wants? Volunteers who would rather work on something else? If you want a change, you get to support the regressions - and if AMD's work gets merged, then anyone ELSE who wants to make a change in that page needs to support AMD's regressions.
Hence wanting to make sure that the changes from AMD are manageable and flexible enough to allow further changes.
And what about a change to a stable internal kernel API, which the kernel developers refuse?
https://www.kernel.org/doc/Documentation/stable_api_nonsense...
If the driver doesn't really belong in the Linux kernel source for those reasons, it's better to keep it outside the kernel tree.
code re-use between drivers of different vendors but the same kernel/OS,
VS
code re-use between drivers of the same vendor but different kernels/OSes.
At the end of the day, both sides are arguing for code re-use, of sorts.
This might theoretically make sense if the Linux subsystem was very stable over many years. Practice shows that the Windows interfaces are what are a lot more stable over the years and changes in them are communicated for a long time beforehand so that hardware vendors can begin changing their drivers long beforehand.
Linux maintains compatibility by fixing the driver themselves when they break it. Microsoft cannot (actually, can, and does) break their interfaces since they don't control the drivers.
This allows Linux to keep improving without breaking things in production; while Microsoft has to either maintain huge backward compatibility abstractions for changes, go YOLO and break stuff (often unknowingly) or abstain from improving their OS.
However, it is introducing a second API for a very specific subset of hardware into a kernel that is being developed by not just AMD people. Dave Airlie is rightly saying that the second API and hence two different code structures makes the whole DRI infrastructure harder to maintain for everyone else.
And Dave's responsibility is to everyone else, not to AMD.
It is a bad thing for the targets as they implement both the driver functionality and the abstractions required to make the same code work cross platform. The response linked describes the cost of those abstractions to the target (Linux kernel in this case).