Re: [PATCH 1/8] drm: Introduce an atomic_commit_setup function

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Nov 19, 2020 at 10:59:42AM +0100, Thomas Zimmermann wrote:
> Hi
> 
> Am 13.11.20 um 16:29 schrieb Maxime Ripard:
> > Private objects storing a state shared across all CRTCs need to be
> > carefully handled to avoid a use-after-free issue.
> > 
> > The proper way to do this to track all the commits using that shared
> > state and wait for the previous commits to be done before going on with
> > the current one to avoid the reordering of commits that could occur.
> > 
> > However, this commit setup needs to be done after
> > drm_atomic_helper_setup_commit(), because before the CRTC commit
> > structure hasn't been allocated before, and before the workqueue is
> > scheduled, because we would be potentially reordered already otherwise.
> > 
> > That means that drivers currently have to roll their own
> > drm_atomic_helper_commit() function, even though it would be identical
> > if not for the commit setup.
> > 
> > Let's introduce a hook to do so that would be called as part of
> > drm_atomic_helper_commit, allowing us to reuse the atomic helpers.
> > 
> > Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> > Signed-off-by: Maxime Ripard <maxime@cerno.tech>
> > ---
> >   drivers/gpu/drm/drm_atomic_helper.c      |  6 ++++++
> >   include/drm/drm_modeset_helper_vtables.h | 18 ++++++++++++++++++
> >   2 files changed, 24 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> > index ddd0e3239150..7d69c7844dfc 100644
> > --- a/drivers/gpu/drm/drm_atomic_helper.c
> > +++ b/drivers/gpu/drm/drm_atomic_helper.c
> > @@ -2083,8 +2083,11 @@ int drm_atomic_helper_setup_commit(struct drm_atomic_state *state,
> >   	struct drm_plane *plane;
> >   	struct drm_plane_state *old_plane_state, *new_plane_state;
> >   	struct drm_crtc_commit *commit;
> > +	const struct drm_mode_config_helper_funcs *funcs;
> >   	int i, ret;
> > +	funcs = state->dev->mode_config.helper_private;
> > +
> >   	for_each_oldnew_crtc_in_state(state, crtc, old_crtc_state, new_crtc_state, i) {
> >   		commit = kzalloc(sizeof(*commit), GFP_KERNEL);
> >   		if (!commit)
> > @@ -2169,6 +2172,9 @@ int drm_atomic_helper_setup_commit(struct drm_atomic_state *state,
> >   		new_plane_state->commit = drm_crtc_commit_get(commit);
> >   	}
> > +	if (funcs && funcs->atomic_commit_setup)
> > +		return funcs->atomic_commit_setup(state);
> > +
> >   	return 0;
> >   }
> >   EXPORT_SYMBOL(drm_atomic_helper_setup_commit);
> > diff --git a/include/drm/drm_modeset_helper_vtables.h b/include/drm/drm_modeset_helper_vtables.h
> > index f2de050085be..56470baf0513 100644
> > --- a/include/drm/drm_modeset_helper_vtables.h
> > +++ b/include/drm/drm_modeset_helper_vtables.h
> > @@ -1396,6 +1396,24 @@ struct drm_mode_config_helper_funcs {
> >   	 * drm_atomic_helper_commit_tail().
> >   	 */
> >   	void (*atomic_commit_tail)(struct drm_atomic_state *state);
> > +
> > +	/**
> > +	 * @atomic_commit_setup:
> > +	 *
> > +	 * This hook is used by the default atomic_commit() hook implemented in
> > +	 * drm_atomic_helper_commit() together with the nonblocking helpers (see
> > +	 * drm_atomic_helper_setup_commit()) to extend the DRM commit setup. It
> > +	 * is not used by the atomic helpers.
> > +	 *
> > +	 * This function is called at the end of
> > +	 * drm_atomic_helper_setup_commit(), so once the commit has been
> > +	 * properly setup across the generic DRM object states. It allows
> > +	 * drivers to do some additional commit tracking that isn't related to a
> > +	 * CRTC, plane or connector, typically a private object.
> > +	 *
> > +	 * This hook is optional.
> > +	 */
> > +	int (*atomic_commit_setup)(struct drm_atomic_state *state);
> 
> It feels hacky and screwed-on to me. I'd suggest to add an
> atomic_commit_prepare callback that is called by drm_atomic_helper where it
> currently calls drm_atomic_helper_setup_commit(). The default implementation
> would include setup_commit and prepare_planes. Some example code for
> drm_atomic_helper.c
> 
> static int commit_prepare(state)
> {
> 	drm_atomic_helper_setup_commit(state)
> 
> 	drm_atomic_helper_prepare_planes(state)
> }
> 
> int drm_atomic_helper_commit()
> {
> 	if (async_update) {
> 		...
> 	}
> 
> 	if (funcs->atomic_commit_prepare)
> 		funcs->atomic_commit_prepare(state)
> 	else
> 		commit_prepare(state)
> 
> 	/* the rest of the current function below */
> 	INIT_WORK(&state->commit_work, commit_work);
> 	...
> }
> 
> Drivers that implement atomic_commit_prepare would be expected to call
> drm_atomic_helper_setup_commit() and drm_atomic_helper_prepare_planes() or
> their own implementation of them.
> 
> The whole construct mimics how commit tails work.

Yeah what I suggested is a bit much midlayer, but we've done what you
suggested above with all drivers rolling their own atomic_commit. It
wasn't pretty. There's still drivers like vc4 which haven't switched, and
which have their own locking and synchronization.

Hence why I think the callback approach here is better, gives drivers less
excuses to roll their own and make a mess.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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