On Tue, 11 May 2021 18:44:17 +0200 Daniel Vetter <daniel@xxxxxxxx> wrote: > On Mon, May 10, 2021 at 12:06:05PM -0700, Rob Clark wrote: > > On Mon, May 10, 2021 at 10:44 AM Daniel Vetter <daniel@xxxxxxxx> wrote: > > > > > > On Mon, May 10, 2021 at 6:51 PM Rob Clark <robdclark@xxxxxxxxx> wrote: > > > > > > > > On Mon, May 10, 2021 at 9:14 AM Daniel Vetter <daniel@xxxxxxxx> wrote: > > > > > > > > > > On Sat, May 08, 2021 at 12:56:38PM -0700, Rob Clark wrote: > > > > > > From: Rob Clark <robdclark@xxxxxxxxxxxx> > > > > > > > > > > > > drm_atomic_helper_dirtyfb() will end up stalling for vblank on "video > > > > > > mode" type displays, which is pointless and unnecessary. Add an > > > > > > optional helper vfunc to determine if a plane is attached to a CRTC > > > > > > that actually needs dirtyfb, and skip over them. > > > > > > > > > > > > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx> > > > > > > > > > > So this is a bit annoying because the idea of all these "remap legacy uapi > > > > > to atomic constructs" helpers is that they shouldn't need/use anything > > > > > beyond what userspace also has available. So adding hacks for them feels > > > > > really bad. > > > > > > > > I suppose the root problem is that userspace doesn't know if dirtyfb > > > > (or similar) is actually required or is a no-op. > > > > > > > > But it is perhaps less of a problem because this essentially boils > > > > down to "x11 vs wayland", and it seems like wayland compositors for > > > > non-vsync'd rendering just pageflips and throws away extra frames from > > > > the app? > > > > > > Yeah it's about not adequately batching up rendering and syncing with > > > hw. bare metal x11 is just especially stupid about it :-) > > > > > > > > Also I feel like it's not entirely the right thing to do here either. > > > > > We've had this problem already on the fbcon emulation side (which also > > > > > shouldn't be able to peek behind the atomic kms uapi curtain), and the fix > > > > > there was to have a worker which batches up all the updates and avoids any > > > > > stalls in bad places. > > > > > > > > I'm not too worried about fbcon not being able to render faster than > > > > vblank. OTOH it is a pretty big problem for x11 > > > > > > That's why we'd let the worker get ahead at most one dirtyfb. We do > > > the same with fbcon, which trivially can get ahead of vblank otherwise > > > (if sometimes flushes each character, so you have to pile them up into > > > a single update if that's still pending). > > > > > > > > Since this is for frontbuffer rendering userspace only we can probably get > > > > > away with assuming there's only a single fb, so the implementation becomes > > > > > pretty simple: > > > > > > > > > > - 1 worker, and we keep track of a single pending fb > > > > > - if there's already a dirty fb pending on a different fb, we stall for > > > > > the worker to start processing that one already (i.e. the fb we track is > > > > > reset to NULL) > > > > > - if it's pending on the same fb we just toss away all the updates and go > > > > > with a full update, since merging the clip rects is too much work :-) I > > > > > think there's helpers so you could be slightly more clever and just have > > > > > an overall bounding box > > > > > > > > This doesn't really fix the problem, you still end up delaying sending > > > > the next back-buffer to mesa > > > > > > With this the dirtyfb would never block. Also glorious frontbuffer > > > tracking corruption is possible, but that's not the kernel's problem. > > > So how would anything get held up in userspace. > > > > the part about stalling if a dirtyfb is pending was what I was worried > > about.. but I suppose you meant the worker stalling, rather than > > userspace stalling (where I had interpreted it the other way around). > > As soon as userspace needs to stall, you're losing again. > > Nah, I did mean userspace stalling, so we can't pile up unlimited amounts > of dirtyfb request in the kernel. > > But also I never expect userspace that uses dirtyfb to actually hit this > stall point (otherwise we'd need to look at this again). It would really > be only there as defense against abuse. > > > > > But we could re-work drm_framebuffer_funcs::dirty to operate on a > > > > per-crtc basis and hoist the loop and check if dirtyfb is needed out > > > > of drm_atomic_helper_dirtyfb() > > > > > > That's still using information that userspace doesn't have, which is a > > > bit irky. We might as well go with your thing here then. > > > > arguably, this is something we should expose to userspace.. for DSI > > command-mode panels, you probably want to make a different decision > > with regard to how many buffers in your flip-chain.. > > > > Possibly we should add/remove the fb_damage_clips property depending > > on the display type (ie. video/pull vs cmd/push mode)? > > I'm not sure whether atomic actually needs this exposed: > - clients will do full flips for every frame anyway, I've not heard of > anyone seriously doing frontbuffer rendering. That may or may not be changing, depending on whether the DRM drivers will actually support tearing flips. There has been a huge amount of debate for needing tearing for Wayland [1], and while I haven't really joined that discussion, using front-buffer rendering (blits) to work around the driver inability to flip-tear might be something some people will want. Personally, what I do agree with is that "tear if late from intended vblank" is a feature that will be needed when VRR cannot be used. However, I would also argue that multiple tearing updates per refresh cycle is not a good idea, and I know people disagree with this because practically all relevant games are using a naive main loop that makes multi-tearing necessary for good input response. I'm not quite sure where this leaves the KMS UAPI usage patterns. Maybe this matters, maybe not? Does it make a difference between using legacy DirtyFB vs. atomic FB_DAMAGE_CLIPS property? Also mind that Wayland compositors would be dynamically switching between "normal flips" and "tearing updates" depending on the scenegraph. This switch should not be considered a "mode set". [1] https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/65 Thanks, pq
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