On Wed, 3 Apr 2024 17:32:46 -0400 Leo Li <sunpeng.li@xxxxxxx> wrote: > On 2024-03-28 10:33, Pekka Paalanen wrote: > > On Fri, 15 Mar 2024 13:09:56 -0400 > > <sunpeng.li@xxxxxxx> wrote: > > > >> From: Leo Li <sunpeng.li@xxxxxxx> > >> > >> These patches aim to make the amdgpgu KMS driver play nicer with compositors > >> when building multi-plane scanout configurations. They do so by: > >> > >> 1. Making cursor behavior more sensible. > >> 2. Allowing placement of DRM OVERLAY planes underneath the PRIMARY plane for > >> 'underlay' configurations (perhaps more of a RFC, see below). > >> > >> Please see the commit messages for details. > >> > >> > >> For #2, the simplest way to accomplish this was to increase the value of the > >> immutable zpos property for the PRIMARY plane. This allowed OVERLAY planes with > >> a mutable zpos range of (0-254) to be positioned underneath the PRIMARY for an > >> underlay scanout configuration. > >> > >> Technically speaking, DCN hardware does not have a concept of primary or overlay > >> planes - there are simply 4 general purpose hardware pipes that can be maped in > >> any configuration. So the immutable zpos restriction on the PRIMARY plane is > >> kind of arbitrary; it can have a mutable range of (0-254) just like the > >> OVERLAYs. The distinction between PRIMARY and OVERLAY planes is also somewhat > >> arbitrary. We can interpret PRIMARY as the first plane that should be enabled on > >> a CRTC, but beyond that, it doesn't mean much for amdgpu. > >> > >> Therefore, I'm curious about how compositors devs understand KMS planes and > >> their zpos properties, and how we would like to use them. It isn't clear to me > >> how compositors wish to interpret and use the DRM zpos property, or > >> differentiate between OVERLAY and PRIMARY planes, when it comes to setting up > >> multi-plane scanout. > > > > You already quoted me on the Weston link, so I don't think I have > > anything to add. Sounds fine to me, and we don't have a standard plane > > arrangement algorithm that the kernel could optimize zpos ranges > > against, yet. > > > >> Ultimately, what I'd like to answer is "What can we do on the KMS driver and DRM > >> plane API side, that can make building multi-plane scanout configurations easier > >> for compositors?" I'm hoping we can converge on something, whether that be > >> updating the existing documentation to better define the usage, or update the > >> API to provide support for something that is lacking. > > > > I think there probably should be a standardised plane arrangement > > algorithm in userspace, because the search space suffers from > > permutational explosion. Either there needs to be very few planes (max > > 4 or 5 at-all-possible per CRTC, including shareable ones) for an > > exhaustive search to be feasible, or all planes should be more or less > > equal in capabilities and userspace employs some simplified or > > heuristic search. > > > > If the search algorithm is fixed, then drivers could optimize zpos > > ranges to have the algorithm find a solution faster. > > > > My worry is that userspace already has heuristic search algorithms that > > may start failing if drivers later change their zpos ranges to be more > > optimal for another algorithm. > > > > OTOH, as long as exhaustive search is feasible, then it does not matter > > how DRM drivers set up the zpos ranges. > > > > In any case, the zpos ranges should try to allow all possible plane > > arrangements while minimizing the number of arrangements that won't > > work. The absolute values of zpos are pretty much irrelevant, so I > > think setting one plane to have an immutable zpos is a good idea, even > > if it's not necessary by the driver. That is one less moving part, and > > only the relative ordering between the planes matters. > > > > > > Thanks, > > pq > > Right, thanks for your thoughts! I agree that there should be a common plane > arrangement algorithm. I think libliftoff is the most obvious candidate here. It > only handles overlay arrangements currently, but mixed-mode arrangements is > something I've been trying to look at. > > Taking the driver's reported zpos into account could narrow down the search > space for mixed arrangements. We could tell whether underlay, or overlay, or > both, is supported by looking at the allowed zpos ranges. > > I also wonder if it'll make underlay assignments easier. libliftoff has an > assumption that the PRIMARY plane has the lowest zpos (which now I realize, is > not always true). Therefore, the underlay buffer has to be placed on the > PRIMARY, with the render buffer on a higher OVERLAY. Swapping buffers between > planes when testing mixed-arrangements is kind of awkward, and simply setting > the OVERLAY's zpos to be lower or higher than the PRIMARY's sounds simpler. > > Currently only gamescope makes use of libliftoff, but I'm curious if patches > hooking it up to Weston would be welcomed? If there are other ways to have a > common arrangement algorithm, I'd be happy to hear that as well. A natural thing would be to document such an algorithm with the KMS UAPI. I don't know libliftoff well enough to say how welcome it would be in Weston. I have no fundamental or policy reason to keep an independent implementation in Weston though, so it's plausible at least. It would need investigation, and perhaps also extending Weston test suite a lot more towards VKMS to verify plane assignments. Currently all plane assignment testing is manual on real hardware. > Note that libliftoff's algorithm is more complex than weston, since it searches > harder, and suffers from that permutational explosion. But it solves that by > trying high benefit arrangements first (offloading surfaces that update > frequently), and bailing out once the search reaches a hard-coded deadline. > Since it's currently overlay-only, the goal could be to "simply" have no > regressions. Ensuring no regressions would indeed need to be taken care of by extending the VKMS-based automated testing. Thanks, pq > > > >> Some links to provide context and details: > >> * What is underlay?: https://gitlab.freedesktop.org/emersion/libliftoff/-/issues/76 > >> * Discussion on how to implement underlay on Weston: https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/1258#note_2325164 > >> > >> Cc: Joshua Ashton <joshua@xxxxxxxxx> > >> Cc: Michel Dänzer <mdaenzer@xxxxxxxxxx> > >> Cc: Chao Guo <chao.guo@xxxxxxx> > >> Cc: Xaver Hugl <xaver.hugl@xxxxxxxxx> > >> Cc: Vikas Korjani <Vikas.Korjani@xxxxxxx> > >> Cc: Robert Mader <robert.mader@xxxxxxxxx> > >> Cc: Pekka Paalanen <pekka.paalanen@xxxxxxxxxxxxx> > >> Cc: Sean Paul <sean@xxxxxxxxxx> > >> Cc: Simon Ser <contact@xxxxxxxxxxx> > >> Cc: Shashank Sharma <shashank.sharma@xxxxxxx> > >> Cc: Harry Wentland <harry.wentland@xxxxxxx> > >> Cc: Sebastian Wick <sebastian.wick@xxxxxxxxxx> > >> > >> Leo Li (2): > >> drm/amd/display: Introduce overlay cursor mode > >> drm/amd/display: Move PRIMARY plane zpos higher > >> > >> .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 405 ++++++++++++++++-- > >> .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h | 7 + > >> .../amd/display/amdgpu_dm/amdgpu_dm_crtc.c | 1 + > >> .../amd/display/amdgpu_dm/amdgpu_dm_plane.c | 28 +- > >> 4 files changed, 391 insertions(+), 50 deletions(-) > >> > >
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