On Fri, Jun 19, 2020 at 10:13:35AM +0100, Chris Wilson wrote: > Quoting Daniel Vetter (2020-06-19 09:51:59) > > On Fri, Jun 19, 2020 at 10:25 AM Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote: > > > Forcing a generic primitive to always be part of the same global map is > > > horrible. > > > > And no concrete example or reason for why that's not possible. > > Because frankly it's not horrible, this is what upstream is all about: > > Shared concepts, shared contracts, shared code. > > > > The proposed patches might very well encode the wrong contract, that's > > all up for discussion. But fundamentally questioning that we need one > > is missing what upstream is all about. > > Then I have not clearly communicated, as my opinion is not that > validation is worthless, but that the implementation is enshrining a > global property on a low level primitive that prevents it from being > used elsewhere. And I want to replace completion [chains] with fences, and > bio with fences, and closures with fences, and what other equivalencies > there are in the kernel. The fence is as central a locking construct as > struct completion and deserves to be a foundational primitive provided > by kernel/ used throughout all drivers for discrete problem domains. > > This is narrowing dma_fence whereby adding > struct lockdep_map *dma_fence::wait_map > and annotating linkage, allows you to continue to specify that all > dma_fence used for a particular purpose must follow common rules, > without restricting the primitive for uses outside of this scope. Somewhere else in this thread I had discussions with Jason Gunthorpe about this topic. It might maybe change somewhat depending upon exact rules, but his take is very much "I don't want dma_fence in rdma". Or pretty close to that at least. Similar discussions with habanalabs, they're using dma_fence internally without any of the uapi. Discussion there has also now concluded that it's best if they remove them, and simply switch over to a wait_queue or completion like every other driver does. The next round of the patches already have a paragraph to at least somewhat limit how non-gpu drivers use dma_fence. And I guess actual consensus might be pointing even more strongly at dma_fence being solely something for gpus and closely related subsystem (maybe media) for syncing dma-buf access. So dma_fence as general replacement for completion chains I think just wont happen. What might make sense is if e.g. the lockdep annotations could be reused, at least in design, for wait_queue or completion or anything else really. I do think that has a fair chance compared to the automagic cross-release annotations approach, which relied way too heavily on guessing where barriers are. My experience from just a bit of playing around with these patches here and discussing them with other driver maintainers is that accurately deciding where critical sections start and end is a job for humans only. And if you get it wrong, you will have a false positive. And you're indeed correct that if we'd do annotations for completions and wait queues, then that would need to have a class per semantically equivalent user, like we have lockdep classes for mutexes, not just one overall. But dma_fence otoh is something very specific, which comes with very specific rules attached - it's not a generic wait_queue at all. Originally it did start out as one even, but it is a very specialized wait_queue. So there's imo two cases: - Your completion is entirely orthogonal of dma_fences, and can never ever block a dma_fence. Don't use dma_fence for this, and no problem. It's just another wait_queue somewhere. - Your completion can eventually, maybe through lots of convolutions and depdencies, block a dma_fence. In that case full dma_fence rules apply, and the only thing you can do with a custom annotation is make the rules even stricter. E.g. if a sub-timeline in the scheduler isn't allowed to take certain scheduler locks. But the userspace visible/published fence do take them, maybe as part of command submission or retirement. Entirely hypotethical, no idea any driver actually needs this. Cheers, Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
