Hi Am 19.11.20 um 16:32 schrieb Daniel Vetter: > 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 >>> Signed-off-by: Maxime Ripard >>> --- >>> 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. But it sounds like you'll regret this. As soon as a driver has to do additional stuff at the beginning of the setup function, another helper will be required, and so on. Maybe rather go with the commit_prepare helper and push driver authors to not use it. Best regards Thomas > -Daniel > -- Thomas Zimmermann Graphics Driver Developer SUSE Software Solutions Germany GmbH Maxfeldstr. 5, 90409 Nürnberg, Germany (HRB 36809, AG Nürnberg) Geschäftsführer: Felix Imendörffer