On Wed, Dec 04, 2019 at 11:00:11AM +0100, Daniel Vetter wrote:
> Both locking and especially sequencing of nonblocking commits have
> evolved a lot. The details are all there, but I noticed that the big
> picture and connections have fallen behind a bit. Apply polish.
>
> Motivated by some review discussions with Thierry.
>
> Cc: Thierry Reding <treding@xxxxxxxxxx>
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> ---
> Documentation/gpu/drm-kms.rst | 11 ++++++-
> drivers/gpu/drm/drm_atomic.c | 10 ++++---
> drivers/gpu/drm/drm_atomic_helper.c | 46 ++++++++++++++++++-----------
> include/drm/drm_atomic.h | 13 ++++++--
> 4 files changed, 56 insertions(+), 24 deletions(-)
Hi Daniel,
"drm/atomic" in the subject. A couple more minor things I noticed below.
>
> diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
> index c68588ce4090..b9330343d1bc 100644
> --- a/Documentation/gpu/drm-kms.rst
> +++ b/Documentation/gpu/drm-kms.rst
> @@ -260,7 +260,8 @@ Taken all together there's two consequences for the atomic design:
> drm_connector_state <drm_connector_state>` for connectors. These are the only
> objects with userspace-visible and settable state. For internal state drivers
> can subclass these structures through embeddeding, or add entirely new state
> - structures for their globally shared hardware functions.
> + structures for their globally shared hardware functions, see :c:type:`struct
> + drm_private_state<drm_private_state>`.
>
> - An atomic update is assembled and validated as an entirely free-standing pile
> of structures within the :c:type:`drm_atomic_state <drm_atomic_state>`
> @@ -269,6 +270,14 @@ Taken all together there's two consequences for the atomic design:
> to the driver and modeset objects. This way rolling back an update boils down
> to releasing memory and unreferencing objects like framebuffers.
>
> +Locking of atomic state structures is internally using :c:type:`struct
> +drm_modeset_lock <drm_modeset_lock>`. As a general rule the locking shouldn't be
> +exposed to drivers, instead the right locks should be automatically acquired by
> +any function that duplicates or peeks into a state, like e.g.
> +:c:func:`drm_atomic_get_crtc_state()`. Locking only protects the software data
> +structure, ordering of committing state changes to hardware is sequenced using
> +:c:type:`struct drm_crtc_commit <drm_crtc_commit>`.
> +
> Read on in this chapter, and also in :ref:`drm_atomic_helper` for more detailed
> coverage of specific topics.
>
> diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> index a351a9a39530..5b4787e33f0d 100644
> --- a/drivers/gpu/drm/drm_atomic.c
> +++ b/drivers/gpu/drm/drm_atomic.c
> @@ -688,10 +688,12 @@ static void drm_atomic_plane_print_state(struct drm_printer *p,
> * associated state struct &drm_private_state.
> *
> * Similar to userspace-exposed objects, private state structures can be
> - * acquired by calling drm_atomic_get_private_obj_state(). Since this function
> - * does not take care of locking, drivers should wrap it for each type of
> - * private state object they have with the required call to drm_modeset_lock()
> - * for the corresponding &drm_modeset_lock.
> + * acquired by calling drm_atomic_get_private_obj_state(). This also takes care
> + * of locking, hence drivers should not have a need to call drm_modeset_lock()
> + * directly. Sequence of the actual hardware state commit is not handled,
> + * drivers might need to keep track of struct drm_crtc_commit within subclassed
> + * structure of &drm_private_state as necessary, e.g. similar to
> + * &drm_plane_state.commit. See also &drm_atomic_state.fake_commit.
> *
> * All private state structures contained in a &drm_atomic_state update can be
> * iterated using for_each_oldnew_private_obj_in_state(),
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index 711801b9d4f1..10d62f726b22 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -1827,17 +1827,21 @@ EXPORT_SYMBOL(drm_atomic_helper_commit);
> /**
> * DOC: implementing nonblocking commit
> *
> - * Nonblocking atomic commits have to be implemented in the following sequence:
> + * Nonblocking atomic commits should use struct &drm_crtc_commit to sequence
> + * different operations against each another. Locks, especially struct
> + * &drm_modeset_lock, should not be held in worker threads or any other
> + * asynchronous context used to commit the hardware state.
> *
> - * 1. Run drm_atomic_helper_prepare_planes() first. This is the only function
> - * which commit needs to call which can fail, so we want to run it first and
> + * drm_atomic_helper_commit() implements the recommended sequence for
> + * nonblocking commits, using drm_atomic_helper_setup_commit() internally:
> + *
> + * 1. Run drm_atomic_helper_prepare_planes(). Since this can fail and we
> + * need to propagate out of memory/VRAM errors to userspace, it must be called
> * synchronously.
> *
> * 2. Synchronize with any outstanding nonblocking commit worker threads which
> - * might be affected the new state update. This can be done by either cancelling
> - * or flushing the work items, depending upon whether the driver can deal with
> - * cancelled updates. Note that it is important to ensure that the framebuffer
> - * cleanup is still done when cancelling.
> + * might be affected the new state update. This is handled by
"affected _by_ the new state update"?
> + * drm_atomic_helper_setup_commit().
> *
> * Asynchronous workers need to have sufficient parallelism to be able to run
> * different atomic commits on different CRTCs in parallel. The simplest way to
> @@ -1848,21 +1852,29 @@ EXPORT_SYMBOL(drm_atomic_helper_commit);
> * must be done as one global operation, and enabling or disabling a CRTC can
> * take a long time. But even that is not required.
> *
> + * IMPORTANT: A &drm_atomic_state update for multiple CRTCs is sequenced
> + * against all CRTCs therein. Therefor for atomic state updates which only flip
I think "therefor" has a slightly different meaning than "therefore",
and I think you actually want the latter in this case.
> + * planes the driver must not get the struct &drm_crtc_state of unrelated CRTCs
> + * in its atomic check codee: This would prevent committing of atomic updates to
"code"
> + * multiple CRTCs in parallel. In general, adding additional state structures
> + * should be avoided as much as possible, because this reduces parallism in
"parallelism"
> + * (nonblocking) commits, both due to locking and due to commit sequencing
> + * requirements.
> + *
> * 3. The software state is updated synchronously with
> * drm_atomic_helper_swap_state(). Doing this under the protection of all modeset
> - * locks means concurrent callers never see inconsistent state. And doing this
> - * while it's guaranteed that no relevant nonblocking worker runs means that
> - * nonblocking workers do not need grab any locks. Actually they must not grab
> - * locks, for otherwise the work flushing will deadlock.
> + * locks means concurrent callers never see inconsistent state. Note that commit
> + * workers do not hold any locks, their access is only coordinated through
I stumbled across this a couple of times when reading it. I think it
becomes clearer when you replace the comma by a colon:
Note that commit workers do not hold any locks: their access is only
coordinated through ordering.
Or perhaps replace the comma with "and"?
Other than that, looks good to me:
Reviewed-by: Thierry Reding <treding@xxxxxxxxxx>
> + * ordering. If workers would access state only through the pointers in the
> + * free-standing state objects (currently not the case for any driver) then even
> + * multiple pending commits could be in-flight at the same time.
> *
> * 4. Schedule a work item to do all subsequent steps, using the split-out
> * commit helpers: a) pre-plane commit b) plane commit c) post-plane commit and
> * then cleaning up the framebuffers after the old framebuffer is no longer
> - * being displayed.
> - *
> - * The above scheme is implemented in the atomic helper libraries in
> - * drm_atomic_helper_commit() using a bunch of helper functions. See
> - * drm_atomic_helper_setup_commit() for a starting point.
> + * being displayed. The scheduled work should synchronize against other workers
> + * using the &drm_crtc_commit infrastructure as needed. See
> + * drm_atomic_helper_setup_commit() for more details.
> */
>
> static int stall_checks(struct drm_crtc *crtc, bool nonblock)
> @@ -2085,7 +2097,7 @@ EXPORT_SYMBOL(drm_atomic_helper_setup_commit);
> *
> * This function waits for all preceeding commits that touch the same CRTC as
> * @old_state to both be committed to the hardware (as signalled by
> - * drm_atomic_helper_commit_hw_done) and executed by the hardware (as signalled
> + * drm_atomic_helper_commit_hw_done()) and executed by the hardware (as signalled
> * by calling drm_crtc_send_vblank_event() on the &drm_crtc_state.event).
> *
> * This is part of the atomic helper support for nonblocking commits, see
> diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
> index b6c73fd9f55a..5923819dcd68 100644
> --- a/include/drm/drm_atomic.h
> +++ b/include/drm/drm_atomic.h
> @@ -60,8 +60,8 @@
> * wait for flip_done <----
> * clean up atomic state
> *
> - * The important bit to know is that cleanup_done is the terminal event, but the
> - * ordering between flip_done and hw_done is entirely up to the specific driver
> + * The important bit to know is that &cleanup_done is the terminal event, but the
> + * ordering between &flip_done and &hw_done is entirely up to the specific driver
> * and modeset state change.
> *
> * For an implementation of how to use this look at
> @@ -92,6 +92,9 @@ struct drm_crtc_commit {
> * commit is sent to userspace, or when an out-fence is singalled. Note
> * that for most hardware, in most cases this happens after @hw_done is
> * signalled.
> + *
> + * Completion of this stage is signalled implicitly by calling
> + * drm_crtc_send_vblank_event() on &drm_crtc_state.event.
> */
> struct completion flip_done;
>
> @@ -107,6 +110,9 @@ struct drm_crtc_commit {
> * Note that this does not need to include separately reference-counted
> * resources like backing storage buffer pinning, or runtime pm
> * management.
> + *
> + * Drivers should call drm_atomic_helper_commit_hw_done() to signal
> + * completion of this stage.
> */
> struct completion hw_done;
>
> @@ -118,6 +124,9 @@ struct drm_crtc_commit {
> * a vblank wait completed it might be a bit later. This completion is
> * useful to throttle updates and avoid hardware updates getting ahead
> * of the buffer cleanup too much.
> + *
> + * Drivers should call drm_atomic_helper_commit_cleanup_done() to signal
> + * completion of this stage.
> */
> struct completion cleanup_done;
>
> --
> 2.24.0
>
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