On Fri, Aug 14, 2020 at 7:17 PM Daniel Stone <daniel@xxxxxxxxxxxxx> wrote: > > Hi, > > On Fri, 14 Aug 2020 at 17:22, Thierry Reding <thierry.reding@xxxxxxxxx> wrote: > > I suspect that the reason why this works in X but not in Wayland is > > because X passes the right usage flags, whereas Weston may not. But I'll > > have to investigate more in order to be sure. > > Weston allocates its own buffers for displaying the result of > composition through GBM with USE_SCANOUT, which is definitely correct. > > Wayland clients (common to all compositors, in Mesa's > src/egl/drivers/dri2/platform_wayland.c) allocate with USE_SHARED but > _not_ USE_SCANOUT, which is correct in that they are guaranteed to be > shared, but not guaranteed to be scanned out. The expectation is that > non-scanout-compatible buffers would be rejected by gbm_bo_import if > not drmModeAddFB2. > > One difference between Weston and all other compositors (GNOME Shell, > KWin, Sway, etc) is that Weston uses KMS planes for composition when > it can (i.e. when gbm_bo_import from dmabuf + drmModeAddFB2 from > gbm_bo handle + atomic check succeed), but the other compositors only > use the GPU. So if you have different assumptions about the layout of > imported buffers between the GPU and KMS, that would explain a fair > bit. Yeah non-modifiered multi-gpu (of any kind) is pretty much hopeless I think. I guess the only option is if the tegra mesa driver forces linear and an extra copy on everything that's USE_SHARED or USE_SCANOUT. > > Perhaps we can go and release X 1.21.0 with that modifier enablement > > patch and that'll motivate desktops to adopt it as well as the default? > > Unfortunately we don't really have a good way out of this one. They > were disabled because the non-modifier path on Intel can be linear or > X-tiled (row-major), whereas the modifier path enables Y-tiled > (column-major) and compressed layouts. Y-tiled is the most efficient, > but Intel could only spare about six transistors for the global FIFO > shared between all their plane fetch engines, and Y-tiled blows > straight through it. Both X and Shell would thus fail to enable high > resolutions or many heads (2x 4K is enough even on modern platforms > IIRC), so they just turned modifiers off. > > The best solution would be to do a global atomic_check across all > outputs and just blacklist modifiers until you find one which works, > but Shell doesn't yet have that code, and -modesetting ... well, > no-one's volunteered to do that yet, or probably ever. Yeah best bet for modifiered X is Xwayland on top of weston right now :-/ -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
