[PATCH 0/2] drm/amdgpu/display: Make multi-plane configurations more flexible

Thu Apr 4 10:24:11 UTC 2024

On Wed, 3 Apr 2024 17:32:46 -0400
Leo Li <sunpeng.li at amd.com> wrote:

> On 2024-03-28 10:33, Pekka Paalanen wrote:
> > On Fri, 15 Mar 2024 13:09:56 -0400
> > <sunpeng.li at amd.com> wrote:
> >   
> >> From: Leo Li <sunpeng.li at amd.com>
> >>
> >> 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 at froggi.es>
> >> Cc: Michel Dänzer <mdaenzer at redhat.com>
> >> Cc: Chao Guo <chao.guo at nxp.com>
> >> Cc: Xaver Hugl <xaver.hugl at gmail.com>
> >> Cc: Vikas Korjani <Vikas.Korjani at amd.com>
> >> Cc: Robert Mader <robert.mader at posteo.de>
> >> Cc: Pekka Paalanen <pekka.paalanen at collabora.com>
> >> Cc: Sean Paul <sean at poorly.run>
> >> Cc: Simon Ser <contact at emersion.fr>
> >> Cc: Shashank Sharma <shashank.sharma at amd.com>
> >> Cc: Harry Wentland <harry.wentland at amd.com>
> >> Cc: Sebastian Wick <sebastian.wick at redhat.com>
> >>
> >> 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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