On Wed, May 12, 2021 at 1:23 AM Pekka Paalanen ppaalanen@gmail.com wrote:
On Tue, 11 May 2021 18:44:17 +0200 Daniel Vetter daniel@ffwll.ch 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@ffwll.ch wrote:
On Mon, May 10, 2021 at 6:51 PM Rob Clark robdclark@gmail.com wrote:
On Mon, May 10, 2021 at 9:14 AM Daniel Vetter daniel@ffwll.ch wrote:
On Sat, May 08, 2021 at 12:56:38PM -0700, Rob Clark wrote: > From: Rob Clark robdclark@chromium.org > > 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@chromium.org
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.
jfwiw, there is a lot of hw that just can't do tearing pageflips.. I think this probably includes most arm hw. What is done instead is to skip the pageflip and render directly to the front-buffer.
EGL_KHR_mutable_render_buffer is a thing you might be interested in.. it is wired up for android on i965 and there is a WIP MR[1] for mesa/st (gallium):
Possibly it could be useful to add support for platform_wayland?
[1] https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10685
BR, -R
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