On 06/06, Maxime Ripard wrote:
> Hi,
>
> On Thu, May 12, 2022 at 09:44:42AM -0100, Melissa Wen wrote:
> > On 05/09, Melissa Wen wrote:
> > > O 05/09, Melissa Wen wrote:
> > > > On 05/03, Maxime Ripard wrote:
> > > > > When doing an asynchronous page flip (PAGE_FLIP ioctl with the
> > > > > DRM_MODE_PAGE_FLIP_ASYNC flag set), the current code waits for the
> > > > > possible GPU buffer being rendered through a call to
> > > > > vc4_queue_seqno_cb().
> > > > >
> > > > > On the BCM2835-37, the GPU driver is part of the vc4 driver and that
> > > > > function is defined in vc4_gem.c to wait for the buffer to be
> > > > > rendered,
> > > > > and once it's done, call a callback.
> > > > >
> > > > > However, on the BCM2711 used on the RaspberryPi4, the GPU driver is
> > > > > separate (v3d) and that function won't do anything. This was working
> > > > > because we were going into a path, due to uninitialized variables,
> > > > > that
> > > > > was always scheduling the callback.
> > > > >
> > > > > However, we were never actually waiting for the buffer to be rendered
> > > > > which was resulting in frames being displayed out of order.
> > > > >
> > > > > The generic API to signal those kind of completion in the kernel are
> > > > > the
> > > > > DMA fences, and fortunately the v3d drivers supports them and signal
> > > > > when its job is done. That API also provides an equivalent function
> > > > > that
> > > > > allows to have a callback being executed when the fence is signalled
> > > > > as
> > > > > done.
> > > > >
> > > > > Let's change our driver a bit to rely on the previous function for the
> > > > > older SoCs, and on DMA fences for the BCM2711.
> > > > >
> > > > > Signed-off-by: Maxime Ripard <max...@cerno.tech>
> > > > > ---
> > > > > drivers/gpu/drm/vc4/vc4_crtc.c | 41
> > > > > ++++++++++++++++++++++++++++++++--
> > > > > 1 file changed, 39 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/drivers/gpu/drm/vc4/vc4_crtc.c
> > > > > b/drivers/gpu/drm/vc4/vc4_crtc.c
> > > > > index e0ae7bef08fa..8e1369fca937 100644
> > > > > --- a/drivers/gpu/drm/vc4/vc4_crtc.c
> > > > > +++ b/drivers/gpu/drm/vc4/vc4_crtc.c
> > > > > @@ -776,6 +776,7 @@ struct vc4_async_flip_state {
> > > > > struct drm_pending_vblank_event *event;
> > > > >
> > > > > union {
> > > > > + struct dma_fence_cb fence;
> > > > > struct vc4_seqno_cb seqno;
> > > > > } cb;
> > > > > };
> > > > > @@ -835,6 +836,43 @@ static void
> > > > > vc4_async_page_flip_seqno_complete(struct vc4_seqno_cb *cb)
> > > > > vc4_bo_dec_usecnt(bo);
> > > > > }
> > > > >
> > > > > +static void vc4_async_page_flip_fence_complete(struct dma_fence
> > > > > *fence,
> > > > > + struct dma_fence_cb *cb)
> > > > > +{
> > > > > + struct vc4_async_flip_state *flip_state =
> > > > > + container_of(cb, struct vc4_async_flip_state, cb.fence);
> > > > > +
> > > > > + vc4_async_page_flip_complete(flip_state);
> > > > > + dma_fence_put(fence);
> > > > > +}
> > > > > +
> > > > > +static int vc4_async_set_fence_cb(struct drm_device *dev,
> > > > > + struct vc4_async_flip_state
> > > > > *flip_state)
> > > > > +{
> > > > > + struct drm_framebuffer *fb = flip_state->fb;
> > > > > + struct drm_gem_cma_object *cma_bo = drm_fb_cma_get_gem_obj(fb,
> > > > > 0);
> > > > > + struct vc4_dev *vc4 = to_vc4_dev(dev);
> > > > > + struct dma_fence *fence;
> > > > > + int ret;
> > > > > +
> > > > > + if (!vc4->is_vc5) {
> > > > > + struct vc4_bo *bo = to_vc4_bo(&cma_bo->base);
> > > > > +
> > > > > + return vc4_queue_seqno_cb(dev, &flip_state->cb.seqno,
> > > > > bo->seqno,
> > > > > +
> > > > > vc4_async_page_flip_seqno_complete);
> > > > > + }
> > > > > +
> > > > > + ret = dma_resv_get_singleton(cma_bo->base.resv, false, &fence);
> > > + for kernel bot complaint, I replaced false with `DMA_RESV_USAGE_READ`
> > > to run some tests
> > >
> > > > > + if (ret)
> > > > > + return ret;
> > > > > +
> > > > > + if (dma_fence_add_callback(fence, &flip_state->cb.fence,
> > me again :)
> >
> > I was thinking if we should add a check here for !fence and just complete
> > the page flip,
> > instead of letting `dma_fence_add_callback` warns whenever fence is NULL.
> > I think there are situation in which fence is NULL and it is not an
> > issue, right? Does it make sense?
>
> I'm not sure. What situation do you have in mind?I mean, if no implicity fence was attached to this bo, it's safe to just do the page flip. This behaviour will happen anyway, after dma_fence_add_callback() checks fence is NULL and return -EINVAL. But this check will also trigger a warning for something that it's not an issue here, I think. So, if we just check `fence` before calling dma_fence_add_callback(), we keep the same behaviour and prevent the warning. Melissa > > Maxime
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