drm/atomic-helper: implement ->page_flip

Currently there is no way to implement async flips using atomic, that
essentially requires us to be able to cancel pending requests
mid-flight.

To be able to do that (and I guess we want this since vblank synced
updates which opportunistically cancel still pending updates seem to be
wanted) we'd need to add a mandatory cancellation mode. Depending upon
the exact semantics we decide upon that could mean that userspace will
not get completion events, or will get them all stacked up.

So reject async updates for now. Also async updates usually means not
vblank synced at all, and I guess for drivers which want to support
this they should simply add a special pageflip handler (since usually
you need a special flip cmd to achieve this). That kind of async flip
is pretty much exclusively just used for games and benchmarks where
dropping just one frame means you'll get a headshot or something bad
like that ... And so slight amounts of tearing is acceptable.

v2: Fixup kerneldoc, reported by Paulo.

v3: Use the set_crtc_for_plane function to assign the crtc, since
otherwise the book-keeping is off.

v4: Update crtc->primary->fb since ->page_flip is the only driver
callback where the core won't do this itself. We might want to fix
this inconsistency eventually.

v5: Use set_crtc_for_connector as suggested by Sean.

v6: Daniel Thompson noticed that my error handling is inconsistent
and that in a few cases I didn't handle fatal errors (i.e. not
-EDEADLK). Fix this by consolidate the ww mutex backoff handling
into one check in the fail: block and flatten the error control
flow everywhere else.

v7: Fix spelling mistake in the commit message (Sean).

Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Paulo Zanoni <przanoni@gmail.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_atomic_helper.c

@@ -1620,3 +1620,89 @@ drm_atomic_helper_connector_set_property(struct drm_connector *connector,
 	goto retry;
 }
 EXPORT_SYMBOL(drm_atomic_helper_connector_set_property);
+
+/**
+ * drm_atomic_helper_page_flip - execute a legacy page flip
+ * @crtc: DRM crtc
+ * @fb: DRM framebuffer
+ * @event: optional DRM event to signal upon completion
+ * @flags: flip flags for non-vblank sync'ed updates
+ *
+ * Provides a default page flip implementation using the atomic driver interface.
+ *
+ * Note that for now so called async page flips (i.e. updates which are not
+ * synchronized to vblank) are not supported, since the atomic interfaces have
+ * no provisions for this yet.
+ *
+ * Returns:
+ * Returns 0 on success, negative errno numbers on failure.
+ */
+int drm_atomic_helper_page_flip(struct drm_crtc *crtc,
+				struct drm_framebuffer *fb,
+				struct drm_pending_vblank_event *event,
+				uint32_t flags)
+{
+	struct drm_plane *plane = crtc->primary;
+	struct drm_atomic_state *state;
+	struct drm_plane_state *plane_state;
+	struct drm_crtc_state *crtc_state;
+	int ret = 0;
+
+	if (flags & DRM_MODE_PAGE_FLIP_ASYNC)
+		return -EINVAL;
+
+	state = drm_atomic_state_alloc(plane->dev);
+	if (!state)
+		return -ENOMEM;
+
+	state->acquire_ctx = drm_modeset_legacy_acquire_ctx(crtc);
+retry:
+	crtc_state = drm_atomic_get_crtc_state(state, crtc);
+	if (IS_ERR(crtc_state)) {
+		ret = PTR_ERR(crtc_state);
+		goto fail;
+	}
+	crtc_state->event = event;
+
+	plane_state = drm_atomic_get_plane_state(state, plane);
+	if (IS_ERR(plane_state)) {
+		ret = PTR_ERR(plane_state);
+		goto fail;
+	}
+
+	ret = drm_atomic_set_crtc_for_plane(plane_state, crtc);
+	if (ret != 0)
+		goto fail;
+	plane_state->fb = fb;
+
+	ret = drm_atomic_async_commit(state);
+	if (ret != 0)
+		goto fail;
+
+	/* TODO: ->page_flip is the only driver callback where the core
+	 * doesn't update plane->fb. For now patch it up here. */
+	plane->fb = plane->state->fb;
+
+	/* Driver takes ownership of state on successful async commit. */
+	return 0;
+fail:
+	if (ret == -EDEADLK)
+		goto backoff;
+
+	drm_atomic_state_free(state);
+
+	return ret;
+backoff:
+	drm_atomic_legacy_backoff(state);
+	drm_atomic_state_clear(state);
+
+	/*
+	 * Someone might have exchanged the framebuffer while we dropped locks
+	 * in the backoff code. We need to fix up the fb refcount tracking the
+	 * core does for us.
+	 */
+	plane->old_fb = plane->fb;
+
+	goto retry;
+}
+EXPORT_SYMBOL(drm_atomic_helper_page_flip);

include/drm/drm_atomic_helper.h

@@ -69,5 +69,10 @@ int drm_atomic_helper_plane_set_property(struct drm_plane *plane,
 int drm_atomic_helper_connector_set_property(struct drm_connector *connector,
 					struct drm_property *property,
 					uint64_t val);
+int drm_atomic_helper_page_flip(struct drm_crtc *crtc,
+				struct drm_framebuffer *fb,
+				struct drm_pending_vblank_event *event,
+				uint32_t flags);
+
 
 #endif /* DRM_ATOMIC_HELPER_H_ */