[PATCH] drm/atomc: Update docs around locking and commit sequencing
Daniel Vetter
daniel at ffwll.ch
Tue Dec 10 10:31:38 UTC 2019
On Thu, Dec 05, 2019 at 05:02:24PM +0100, Thierry Reding wrote:
> 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 at nvidia.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > ---
> > 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 at nvidia.com>
All your comments applied and patch merged, thanks for your review.
-Daniel
>
> > + * 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
> >
> > _______________________________________________
> > dri-devel mailing list
> > dri-devel at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
More information about the dri-devel
mailing list