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    SubjectRe: [PATCH 1/5] drm: don't block fb changes for async plane updates
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    On 3/11/19 6:06 AM, Boris Brezillon wrote:
    > Hello Nicholas,
    >
    > On Mon, 4 Mar 2019 15:46:49 +0000
    > "Kazlauskas, Nicholas" <Nicholas.Kazlauskas@amd.com> wrote:
    >
    >> On 3/4/19 9:49 AM, Helen Koike wrote:
    >>> In the case of a normal sync update, the preparation of framebuffers (be
    >>> it calling drm_atomic_helper_prepare_planes() or doing setups with
    >>> drm_framebuffer_get()) are performed in the new_state and the respective
    >>> cleanups are performed in the old_state.
    >>>
    >>> In the case of async updates, the preparation is also done in the
    >>> new_state but the cleanups are done in the new_state (because updates
    >>> are performed in place, i.e. in the current state).
    >>>
    >>> The current code blocks async udpates when the fb is changed, turning
    >>> async updates into sync updates, slowing down cursor updates and
    >>> introducing regressions in igt tests with errors of type:
    >>>
    >>> "CRITICAL: completed 97 cursor updated in a period of 30 flips, we
    >>> expect to complete approximately 15360 updates, with the threshold set
    >>> at 7680"
    >>>
    >>> Fb changes in async updates were prevented to avoid the following scenario:
    >>>
    >>> - Async update, oldfb = NULL, newfb = fb1, prepare fb1, cleanup fb1
    >>> - Async update, oldfb = fb1, newfb = fb2, prepare fb2, cleanup fb2
    >>> - Non-async commit, oldfb = fb2, newfb = fb1, prepare fb1, cleanup fb2 (wrong)
    >>> Where we have a single call to prepare fb2 but double cleanup call to fb2.
    >>>
    >>> To solve the above problems, instead of blocking async fb changes, we
    >>> place the old framebuffer in the new_state object, so when the code
    >>> performs cleanups in the new_state it will cleanup the old_fb and we
    >>> will have the following scenario instead:
    >>>
    >>> - Async update, oldfb = NULL, newfb = fb1, prepare fb1, no cleanup
    >>> - Async update, oldfb = fb1, newfb = fb2, prepare fb2, cleanup fb1
    >>> - Non-async commit, oldfb = fb2, newfb = fb1, prepare fb1, cleanup fb2
    >>>
    >>> Where calls to prepare/cleanup are ballanced.
    >>>
    >>> Cc: <stable@vger.kernel.org> # v4.14+: 25dc194b34dd: drm: Block fb changes for async plane updates
    >>> Fixes: 25dc194b34dd ("drm: Block fb changes for async plane updates")
    >>> Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
    >>> Signed-off-by: Helen Koike <helen.koike@collabora.com>
    >>>
    >>> ---
    >>> Hello,
    >>>
    >>> As mentioned in the cover letter,
    >>> I tested on the rockchip and on i915 (with a patch I am still working on for
    >>> replacing cursors by async update), with igt plane_cursor_legacy and
    >>> kms_cursor_legacy and I didn't see any regressions.
    >>> I couldn't test on MSM and AMD because I don't have the hardware (and I am
    >>> having some issues testing on vc4) and I would appreciate if anyone could help
    >>> me testing those.
    >>>
    >>> I also think it would be a better solution if, instead of having async
    >>> to do in-place updates in the current state, the async path should be
    >>> equivalent to a syncronous update, i.e., modifying new_state and
    >>> performing a flip
    >>> IMHO, the only difference between sync and async should be that async update
    >>> doesn't wait for vblank and applies the changes immeditally to the hw,
    >>> but the code path could be almost the same.
    >>> But for now I think this solution is ok (swaping new_fb/old_fb), and
    >>> then we can adjust things little by little, what do you think?
    >>>
    >>> Thanks!
    >>> Helen
    >>>
    >>> drivers/gpu/drm/drm_atomic_helper.c | 20 ++++++++++----------
    >>> 1 file changed, 10 insertions(+), 10 deletions(-)
    >>>
    >>> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
    >>> index 540a77a2ade9..e7eb96f1efc2 100644
    >>> --- a/drivers/gpu/drm/drm_atomic_helper.c
    >>> +++ b/drivers/gpu/drm/drm_atomic_helper.c
    >>> @@ -1608,15 +1608,6 @@ int drm_atomic_helper_async_check(struct drm_device *dev,
    >>> old_plane_state->crtc != new_plane_state->crtc)
    >>> return -EINVAL;
    >>>
    >>> - /*
    >>> - * FIXME: Since prepare_fb and cleanup_fb are always called on
    >>> - * the new_plane_state for async updates we need to block framebuffer
    >>> - * changes. This prevents use of a fb that's been cleaned up and
    >>> - * double cleanups from occuring.
    >>> - */
    >>> - if (old_plane_state->fb != new_plane_state->fb)
    >>> - return -EINVAL;
    >>> -
    >>> funcs = plane->helper_private;
    >>> if (!funcs->atomic_async_update)
    >>> return -EINVAL;
    >>> @@ -1657,6 +1648,9 @@ void drm_atomic_helper_async_commit(struct drm_device *dev,
    >>> int i;
    >>>
    >>> for_each_new_plane_in_state(state, plane, plane_state, i) {
    >>> + struct drm_framebuffer *new_fb = plane_state->fb;
    >>> + struct drm_framebuffer *old_fb = plane->state->fb;
    >>> +
    >>> funcs = plane->helper_private;
    >>> funcs->atomic_async_update(plane, plane_state);
    >>>
    >>> @@ -1665,11 +1659,17 @@ void drm_atomic_helper_async_commit(struct drm_device *dev,
    >>> * plane->state in-place, make sure at least common
    >>> * properties have been properly updated.
    >>> */
    >>> - WARN_ON_ONCE(plane->state->fb != plane_state->fb);
    >>> + WARN_ON_ONCE(plane->state->fb != new_fb);
    >>> WARN_ON_ONCE(plane->state->crtc_x != plane_state->crtc_x);
    >>> WARN_ON_ONCE(plane->state->crtc_y != plane_state->crtc_y);
    >>> WARN_ON_ONCE(plane->state->src_x != plane_state->src_x);
    >>> WARN_ON_ONCE(plane->state->src_y != plane_state->src_y);
    >>> +
    >>> + /*
    >>> + * Make sure the FBs have been swapped so that cleanups in the
    >>> + * new_state performs a cleanup in the old FB.
    >>> + */
    >>> + WARN_ON_ONCE(plane_state->fb != old_fb);
    >>
    >> I personally think this approach is fine and the WARN_ON s are good for
    >> catching drivers that want to use these in the future.
    >
    > Well, I agree this change is the way to go for a short-term solution
    > to relax the old_fb == new_fb constraint, but I keep thinking this whole
    > "update plane_state in place" is a recipe for trouble and just make
    > things more complicated for drivers for no obvious reasons. Look at the
    > VC4 implem [1] if you need a proof that things can get messy pretty
    > quickly.
    >
    > All this state-fields-copying steps could be skipped if the core was
    > simply swapping the old/new states as is done in the sync update path.
    >
    > [1]https://elixir.bootlin.com/linux/v5.0-rc7/source/drivers/gpu/drm/vc4/vc4_plane.c#L878

    I completely agree with this view FWIW. I had a discussion with Daniel
    about this when I had posted the original block FB changes patch.

    - The plane object needs to be locked in order for async state to be updated
    - Blocking commit work holds the lock for the plane, async update won't
    happen
    - Non-blocking commit work that's still ongoing won't have hw_done
    signaled and drm_atomic_helper_async_check will block the async update

    So this looks safe in theory, with the exception of the call to
    drm_atomic_helper_cleanup_planes occuring after hw_done is signaled.

    I believe that the behavior of this function still remains the same even
    if plane->state is swapped to something else during the call (since
    old_plane_state should never be equal to plane->state if the commit
    succeeded and the plane is in the commit), but I'm not sure that's
    something we'd want to rely on.

    I think other than that issue, you could probably just:

    drm_atomic_helper_prepare_planes(...);
    drm_atomic_helper_swap_state(...);
    drm_atomic_state_get(state);
    drm_atomic_helper_async_commit(...);
    drm_atomic_helper_cleanup_planes(dev, state);

    and it would work as expected. But there still may be other things I'm
    missing or haven't considered here.

    Nicholas Kazlauskas

    >
    >>
    >> I do think it would be good to add something to the function docs that
    >> explains this requirement and the issue that it addresses. It's a little
    >> unintuitive to require that the old fb is placed into the new state, but
    >> it makes sense as a workaround to this problem.
    >>
    >> Nicholas Kazlauskas
    >>
    >>> }
    >>> }
    >>> EXPORT_SYMBOL(drm_atomic_helper_async_commit);
    >>>
    >>
    >

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