Re: [PATCH 1/2] drm: Fix dirtyfb stalls

[Pekka Paalanen <ppaalanen@gmail.com>]

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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.

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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