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 <maxime@xxxxxxxxxx>
> ---
> 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);
> + if (ret)
> + return ret;
> +
> + if (dma_fence_add_callback(fence, &flip_state->cb.fence,
> + vc4_async_page_flip_fence_complete))
> + vc4_async_page_flip_fence_complete(fence, &flip_state->cb.fence);
> +
> + return 0;
> +}
> +
> static int
> vc4_async_page_flip_common(struct drm_crtc *crtc,
> struct drm_framebuffer *fb,
> @@ -874,8 +912,7 @@ vc4_async_page_flip_common(struct drm_crtc *crtc,
> */
> drm_atomic_set_fb_for_plane(plane->state, fb);
>
> - vc4_queue_seqno_cb(dev, &flip_state->cb.seqno, bo->seqno,
> - vc4_async_page_flip_seqno_complete);
> + vc4_async_set_fence_cb(dev, flip_state);
I tried to run igt kms_async_flip, but the test still seems very tied to
the i915 driver. So, as far as I could check, I didn't see any red
flags and sync page flip behaviour seems preserved.
>
> /* Driver takes ownership of state on successful async commit. */
> return 0;
> --
> 2.35.1
>
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