Re: [PATCH 24/28] drm: use new iterator in drm_gem_plane_helper_prepare_fb v2

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Oct 19, 2021 at 05:51:38PM +0200, Christian König wrote:
> Am 19.10.21 um 16:30 schrieb Daniel Vetter:
> > On Tue, Oct 19, 2021 at 03:02:26PM +0200, Christian König wrote:
> > > Am 13.10.21 um 16:23 schrieb Daniel Vetter:
> > > > On Tue, Oct 05, 2021 at 01:37:38PM +0200, Christian König wrote:
> > > > > Makes the handling a bit more complex, but avoids the use of
> > > > > dma_resv_get_excl_unlocked().
> > > > > 
> > > > > v2: improve coding and documentation
> > > > > 
> > > > > Signed-off-by: Christian König <christian.koenig@amd.com>
> > > > > ---
> > > > >    drivers/gpu/drm/drm_gem_atomic_helper.c | 13 +++++++++++--
> > > > >    1 file changed, 11 insertions(+), 2 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/gpu/drm/drm_gem_atomic_helper.c b/drivers/gpu/drm/drm_gem_atomic_helper.c
> > > > > index e570398abd78..8534f78d4d6d 100644
> > > > > --- a/drivers/gpu/drm/drm_gem_atomic_helper.c
> > > > > +++ b/drivers/gpu/drm/drm_gem_atomic_helper.c
> > > > > @@ -143,6 +143,7 @@
> > > > >     */
> > > > >    int drm_gem_plane_helper_prepare_fb(struct drm_plane *plane, struct drm_plane_state *state)
> > > > >    {
> > > > > +	struct dma_resv_iter cursor;
> > > > >    	struct drm_gem_object *obj;
> > > > >    	struct dma_fence *fence;
> > > > > @@ -150,9 +151,17 @@ int drm_gem_plane_helper_prepare_fb(struct drm_plane *plane, struct drm_plane_st
> > > > >    		return 0;
> > > > >    	obj = drm_gem_fb_get_obj(state->fb, 0);
> > > > > -	fence = dma_resv_get_excl_unlocked(obj->resv);
> > > > > -	drm_atomic_set_fence_for_plane(state, fence);
> > > > > +	dma_resv_iter_begin(&cursor, obj->resv, false);
> > > > > +	dma_resv_for_each_fence_unlocked(&cursor, fence) {
> > > > > +		/* TODO: We only use the first write fence here and need to fix
> > > > > +		 * the drm_atomic_set_fence_for_plane() API to accept more than
> > > > > +		 * one. */
> > > > I'm confused, right now there is only one write fence. So no need to
> > > > iterate, and also no need to add a TODO. If/when we add more write fences
> > > > then I think this needs to be revisited, and ofc then we do need to update
> > > > the set_fence helpers to carry an entire array of fences.
> > > Well could be that I misunderstood you, but in your last explanation it
> > > sounded like the drm_atomic_set_fence_for_plane() function needs fixing
> > > anyway because a plane could have multiple BOs.
> > > 
> > > So in my understanding what we need is a
> > > drm_atomic_add_dependency_for_plane() function which records that a certain
> > > fence needs to be signaled before a flip.
> > Yeah that's another issue, but in practice there's no libva which decodes
> > into planar yuv with different fences between the planes. So not a bug in
> > practice.
> > 
> > But this is entirely orthogonal to you picking up the wrong fence here if
> > there's not exclusive fence set:
> > 
> > - old code: Either pick the exclusive fence, or not fence if the exclusive
> >    one is not set.
> > 
> > - new code: Pick the exclusive fence or the first shared fence
> 
> Hui what?
> 
> We use "dma_resv_iter_begin(&cursor, obj->resv, *false*);" here which means
> that only the exclusive fence is returned and no shared fences whatsoever.
> 
> My next step is to replace the boolean with a bunch of use case describing
> enums. I hope that will make it much clearer what's going on here.

Yeah I got that entirely wrong, which is kinda bad since that's about the
only thing worth checking in these conversions :-/

I'll go recheck them again and slap some more r-b on stuff.
-Daniel

> 
> Christian.
> 
> > New behaviour is busted, because scanning out and reading from a buffer at
> > the same time (for the next frame, e.g. to copy over damaged areas or some
> > other tricks) is very much a supported thing. Atomic _only_ wants to look
> > at the exclusive fence slot, which mean "there is an implicitly synced
> > write to this buffers". Implicitly synced reads _must_ be ignored.
> 
> 
> > 
> > Now amdgpu doesn't have this distinction in its uapi, but many drivers do.
> > -Daniel
> > 
> > > Support for more than one write fence then comes totally naturally.
> > > 
> > > Christian.
> > > 
> > > > -Daniel
> > > > 
> > > > > +		dma_fence_get(fence);
> > > > > +		break;
> > > > > +	}
> > > > > +	dma_resv_iter_end(&cursor);
> > > > > +	drm_atomic_set_fence_for_plane(state, fence);
> > > > >    	return 0;
> > > > >    }
> > > > >    EXPORT_SYMBOL_GPL(drm_gem_plane_helper_prepare_fb);
> > > > > -- 
> > > > > 2.25.1
> > > > > 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch