On 2024-02-23 10:34, Christian König wrote: > Am 23.02.24 um 09:11 schrieb Michel Dänzer: >> On 2024-02-23 08:06, Christian König wrote: >>> Am 22.02.24 um 18:28 schrieb Michel Dänzer: >>>> From: Michel Dänzer <mdaenzer@xxxxxxxxxx> >>>> >>>> Pinning the BO storage to VRAM for scanout would make it inaccessible >>>> to non-P2P dma-buf importers. >>> Thinking more about it I don't think we can do this. >>> >>> Using the BO in a ping/pong fashion for scanout and DMA-buf is actually valid, you just can't do both at the same time. >>> >>> And if I'm not completely mistaken we actually have use cases for this at the moment, >> Those use cases don't have P2P & CONFIG_DMABUF_MOVE_NOTIFY? > > Nope, we are basically talking about unit tests and examples for inter device operations. Sounds like the "no user-space regressions" rule might not apply then. > Those render into a shared buffer and then display it to check if the content was rendered/transferred correctly. That can be fixed by dropping the dma-buf attachments from other GPUs before creating the KMS FB. Conversely, tests / examples which do scanout first can be fixed by destroying KMS FBs before sharing the BO with another GPU. > I'm not sure if we still do those test cases, the last time I looked into it was before P2P was even supported, but I also can't rule it out. Sounds too vague to block this series. >>> So rejecting things during CS and atomic commit is the best thing we can do. >> It's problematic for a Wayland compositor: >> >> The CS ioctl failing is awkward. With GL, I'm pretty sure it means the compositor would have to destroy the context and create a new one. Not sure about Vulkan, but I suspect it's painful there as well. >> >> Similarly for the KMS atomic commit ioctl. The compositor can't know why exactly it failed, all it gets is an error code. > > Yeah, but that is not because the kernel is doing anything wrong. > > Sharing, rendering and then doing an atomic commit is a perfectly valid use case. > > You just can't do scanout and sharing at the same time. Per my later follow-up, Xwayland can't really avoid it. >> And there's no other way for the compositor to detect when both things can actually work concurrently. > > That I totally agree with. And IIRC we already have at least a query for the buffer placement. E.g. you can already check if the BO is in GTT or VRAM and shared. > > What's missing is exposing if the device can scanout from GTT or not. Requiring Wayland compositors to have driver-specific knowledge like that baked in isn't great either. -- Earthling Michel Dänzer | https://redhat.com Libre software enthusiast | Mesa and Xwayland developer
