Re: [PATCH v3 3/5] drm/msm: fix fb references in async update

[Rob Clark <robdclark@gmail.com>]

On Wed, Mar 13, 2019 at 8:21 PM Helen Koike <helen.koike@collabora.com> wrote:
>
> Async update callbacks are expected to set the old_fb in the new_state
> so prepare/cleanup framebuffers are balanced.
>
> Cc: <stable@vger.kernel.org> # v4.14+
> Fixes: 224a4c970987 ("drm/msm: update cursors asynchronously through atomic")
> Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
> Signed-off-by: Helen Koike <helen.koike@collabora.com>
>
> ---
> Hello,
>
> As mentioned in the cover letter,
> But I couldn't test on MSM because I don't have the hardware and I would
> appreciate if anyone could test it.
>
> In other platforms (VC4, AMD, Rockchip), there is a hidden
> drm_framebuffer_get(new_fb)/drm_framebuffer_put(old_fb) in async_update
> that is wrong, but I couldn't identify those here, not sure if it is hidden
> somewhere else, but if tests fail this is probably the cause.
>
> Thanks!
> Helen

Apologies, I haven't had a chance to test this yet, and it is likely
to be a few weeks before I have a chance..

That said, not refcnt'ing the fb seems suspicious.   I read the
explanation in the other patches, but kind of think it is more clear
to keep the extra refcnt'ing.. or at least have a comment.

I do a bit wonder if we hold the ref to the outgoing cursor long
enough.. I think the hw will actually continue to scan it out until
the next vblank.  But Archit was the one who added async_update
support so maybe that is handled somewhere else.  It's been a while
since I've looked at this.

BR,
-R



>
> Changes in v3: None
> Changes in v2:
> - update CC stable and Fixes tag
>
>  drivers/gpu/drm/msm/disp/mdp5/mdp5_plane.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/drivers/gpu/drm/msm/disp/mdp5/mdp5_plane.c b/drivers/gpu/drm/msm/disp/mdp5/mdp5_plane.c
> index be13140967b4..b854f471e9e5 100644
> --- a/drivers/gpu/drm/msm/disp/mdp5/mdp5_plane.c
> +++ b/drivers/gpu/drm/msm/disp/mdp5/mdp5_plane.c
> @@ -502,6 +502,8 @@ static int mdp5_plane_atomic_async_check(struct drm_plane *plane,
>  static void mdp5_plane_atomic_async_update(struct drm_plane *plane,
>                                            struct drm_plane_state *new_state)
>  {
> +       struct drm_framebuffer *old_fb = plane->state->fb;
> +
>         plane->state->src_x = new_state->src_x;
>         plane->state->src_y = new_state->src_y;
>         plane->state->crtc_x = new_state->crtc_x;
> @@ -524,6 +526,8 @@ static void mdp5_plane_atomic_async_update(struct drm_plane *plane,
>
>         *to_mdp5_plane_state(plane->state) =
>                 *to_mdp5_plane_state(new_state);
> +
> +       new_state->fb = old_fb;
>  }
>
>  static const struct drm_plane_helper_funcs mdp5_plane_helper_funcs = {
> --
> 2.20.1
>