drm: Document drm_encoder/crtc_helper_funcs

Mostly this is about all the callbacks used for modesets by both legacy
CRTC helpers and atomic helpers and I figured it doesn't make all that
much sense to split this up.

v2: Suggestions from Thierry.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-28-git-send-email-daniel.vetter@ffwll.ch
Reviewed-by: Thierry Reding <treding@nvidia.com>

1 files changed, 401 insertions, 48 deletions

diff --git a/include/drm/drm_modeset_helper_vtables.h b/include/drm/drm_modeset_helper_vtables.h
index 4b3869bd49b5..29e0dc50031d 100644
--- a/include/drm/drm_modeset_helper_vtables.h
+++ b/include/drm/drm_modeset_helper_vtables.h
@@ -51,58 +51,236 @@ enum mode_set_atomic;
 
 /**
  * struct drm_crtc_helper_funcs - helper operations for CRTCs
- * @dpms: set power state
- * @prepare: prepare the CRTC, called before @mode_set
- * @commit: commit changes to CRTC, called after @mode_set
- * @mode_fixup: try to fixup proposed mode for this CRTC
- * @mode_set: set this mode
- * @mode_set_nofb: set mode only (no scanout buffer attached)
- * @mode_set_base: update the scanout buffer
- * @mode_set_base_atomic: non-blocking mode set (used for kgdb support)
- * @load_lut: load color palette
- * @disable: disable CRTC when no longer in use
- * @enable: enable CRTC
  *
- * The helper operations are called by the mid-layer CRTC helper.
- *
- * Note that with atomic helpers @dpms, @prepare and @commit hooks are
- * deprecated. Used @enable and @disable instead exclusively.
- *
- * With legacy crtc helpers there's a big semantic difference between @disable
- * and the other hooks: @disable also needs to release any resources acquired in
- * @mode_set (like shared PLLs).
+ * These hooks are used by the legacy CRTC helpers, the transitional plane
+ * helpers and the new atomic modesetting helpers.
  */
 struct drm_crtc_helper_funcs {
-        /*
-         * Control power levels on the CRTC.  If the mode passed in is
-         * unsupported, the provider must use the next lowest power level.
+        /**
+         * @dpms:
+         *
+         * Callback to control power levels on the CRTC.  If the mode passed in
+         * is unsupported, the provider must use the next lowest power level.
+         * This is used by the legacy CRTC helpers to implement DPMS
+         * functionality in drm_helper_connector_dpms().
+         *
+         * This callback is also used to disable a CRTC by calling it with
+         * DRM_MODE_DPMS_OFF if the @disable hook isn't used.
+         *
+         * This callback is used by the legacy CRTC helpers.  Atomic helpers
+         * also support using this hook for enabling and disabling a CRTC to
+         * facilitate transitions to atomic, but it is deprecated. Instead
+         * @enable and @disable should be used.
          */
         void (*dpms)(struct drm_crtc *crtc, int mode);
+
+        /**
+         * @prepare:
+         *
+         * This callback should prepare the CRTC for a subsequent modeset, which
+         * in practice means the driver should disable the CRTC if it is
+         * running. Most drivers ended up implementing this by calling their
+         * @dpms hook with DRM_MODE_DPMS_OFF.
+         *
+         * This callback is used by the legacy CRTC helpers.  Atomic helpers
+         * also support using this hook for disabling a CRTC to facilitate
+         * transitions to atomic, but it is deprecated. Instead @disable should
+         * be used.
+         */
         void (*prepare)(struct drm_crtc *crtc);
+
+        /**
+         * @commit:
+         *
+         * This callback should commit the new mode on the CRTC after a modeset,
+         * which in practice means the driver should enable the CRTC.  Most
+         * drivers ended up implementing this by calling their @dpms hook with
+         * DRM_MODE_DPMS_ON.
+         *
+         * This callback is used by the legacy CRTC helpers.  Atomic helpers
+         * also support using this hook for enabling a CRTC to facilitate
+         * transitions to atomic, but it is deprecated. Instead @enable should
+         * be used.
+         */
         void (*commit)(struct drm_crtc *crtc);
 
-        /* Provider can fixup or change mode timings before modeset occurs */
+        /**
+         * @mode_fixup:
+         *
+         * This callback is used to validate a mode. The parameter mode is the
+         * display mode that userspace requested, adjusted_mode is the mode the
+         * encoders need to be fed with. Note that this is the inverse semantics
+         * of the meaning for the &drm_encoder and &drm_bridge
+         * ->mode_fixup() functions. If the CRTC cannot support the requested
+         * conversion from mode to adjusted_mode it should reject the modeset.
+         *
+         * This function is used by both legacy CRTC helpers and atomic helpers.
+         * With atomic helpers it is optional.
+         *
+         * NOTE:
+         *
+         * This function is called in the check phase of atomic modesets, which
+         * can be aborted for any reason (including on userspace's request to
+         * just check whether a configuration would be possible). Atomic drivers
+         * MUST NOT touch any persistent state (hardware or software) or data
+         * structures except the passed in adjusted_mode parameter.
+         *
+         * This is in contrast to the legacy CRTC helpers where this was
+         * allowed.
+         *
+         * Atomic drivers which need to inspect and adjust more state should
+         * instead use the @atomic_check callback.
+         *
+         * RETURNS:
+         *
+         * True if an acceptable configuration is possible, false if the modeset
+         * operation should be rejected.
+         */
         bool (*mode_fixup)(struct drm_crtc *crtc,
                            const struct drm_display_mode *mode,
                            struct drm_display_mode *adjusted_mode);
-        /* Actually set the mode */
+
+        /**
+         * @mode_set:
+         *
+         * This callback is used by the legacy CRTC helpers to set a new mode,
+         * position and framebuffer. Since it ties the primary plane to every
+         * mode change it is incompatible with universal plane support. And
+         * since it can't update other planes it's incompatible with atomic
+         * modeset support.
+         *
+         * This callback is only used by CRTC helpers and deprecated.
+         *
+         * RETURNS:
+         *
+         * 0 on success or a negative error code on failure.
+         */
         int (*mode_set)(struct drm_crtc *crtc, struct drm_display_mode *mode,
                         struct drm_display_mode *adjusted_mode, int x, int y,
                         struct drm_framebuffer *old_fb);
-        /* Actually set the mode for atomic helpers, optional */
+
+        /**
+         * @mode_set_nofb:
+         *
+         * This callback is used to update the display mode of a CRTC without
+         * changing anything of the primary plane configuration. This fits the
+         * requirement of atomic and hence is used by the atomic helpers. It is
+         * also used by the transitional plane helpers to implement a
+         * @mode_set hook in drm_helper_crtc_mode_set().
+         *
+         * Note that the display pipe is completely off when this function is
+         * called. Atomic drivers which need hardware to be running before they
+         * program the new display mode (e.g. because they implement runtime PM)
+         * should not use this hook. This is because the helper library calls
+         * this hook only once per mode change and not every time the display
+         * pipeline is suspended using either DPMS or the new "ACTIVE" property.
+         * Which means register values set in this callback might get reset when
+         * the CRTC is suspended, but not restored.  Such drivers should instead
+         * move all their CRTC setup into the @enable callback.
+         *
+         * This callback is optional.
+         */
         void (*mode_set_nofb)(struct drm_crtc *crtc);
 
-        /* Move the crtc on the current fb to the given position *optional* */
+        /**
+         * @mode_set_base:
+         *
+         * This callback is used by the legacy CRTC helpers to set a new
+         * framebuffer and scanout position. It is optional and used as an
+         * optimized fast-path instead of a full mode set operation with all the
+         * resulting flickering. Since it can't update other planes it's
+         * incompatible with atomic modeset support.
+         *
+         * This callback is only used by the CRTC helpers and deprecated.
+         *
+         * RETURNS:
+         *
+         * 0 on success or a negative error code on failure.
+         */
         int (*mode_set_base)(struct drm_crtc *crtc, int x, int y,
                              struct drm_framebuffer *old_fb);
+
+        /**
+         * @mode_set_base_atomic:
+         *
+         * This callback is used by the fbdev helpers to set a new framebuffer
+         * and scanout without sleeping, i.e. from an atomic calling context. It
+         * is only used to implement kgdb support.
+         *
+         * This callback is optional and only needed for kgdb support in the fbdev
+         * helpers.
+         *
+         * RETURNS:
+         *
+         * 0 on success or a negative error code on failure.
+         */
         int (*mode_set_base_atomic)(struct drm_crtc *crtc,
                                     struct drm_framebuffer *fb, int x, int y,
                                     enum mode_set_atomic);
 
-        /* reload the current crtc LUT */
+        /**
+         * @load_lut:
+         *
+         * Load a LUT prepared with the @gamma_set functions from
+         * &drm_fb_helper_funcs.
+         *
+         * This callback is optional and is only used by the fbdev emulation
+         * helpers.
+         *
+         * FIXME:
+         *
+         * This callback is functionally redundant with the core gamma table
+         * support and simply exists because the fbdev hasn't yet been
+         * refactored to use the core gamma table interfaces.
+         */
         void (*load_lut)(struct drm_crtc *crtc);
 
+        /**
+         * @disable:
+         *
+         * This callback should be used to disable the CRTC. With the atomic
+         * drivers it is called after all encoders connected to this CRTC have
+         * been shut off already using their own ->disable hook. If that
+         * sequence is too simple drivers can just add their own hooks and call
+         * it from this CRTC callback here by looping over all encoders
+         * connected to it using for_each_encoder_on_crtc().
+         *
+         * This hook is used both by legacy CRTC helpers and atomic helpers.
+         * Atomic drivers don't need to implement it if there's no need to
+         * disable anything at the CRTC level. To ensure that runtime PM
+         * handling (using either DPMS or the new "ACTIVE" property) works
+         * @disable must be the inverse of @enable for atomic drivers.
+         *
+         * NOTE:
+         *
+         * With legacy CRTC helpers there's a big semantic difference between
+         * @disable and other hooks (like @prepare or @dpms) used to shut down a
+         * CRTC: @disable is only called when also logically disabling the
+         * display pipeline and needs to release any resources acquired in
+         * @mode_set (like shared PLLs, or again release pinned framebuffers).
+         *
+         * Therefore @disable must be the inverse of @mode_set plus @commit for
+         * drivers still using legacy CRTC helpers, which is different from the
+         * rules under atomic.
+         */
         void (*disable)(struct drm_crtc *crtc);
+
+        /**
+         * @enable:
+         *
+         * This callback should be used to enable the CRTC. With the atomic
+         * drivers it is called before all encoders connected to this CRTC are
+         * enabled through the encoder's own ->enable hook.  If that sequence is
+         * too simple drivers can just add their own hooks and call it from this
+         * CRTC callback here by looping over all encoders connected to it using
+         * for_each_encoder_on_crtc().
+         *
+         * This hook is used only by atomic helpers, for symmetry with @disable.
+         * Atomic drivers don't need to implement it if there's no need to
+         * enable anything at the CRTC level. To ensure that runtime PM handling
+         * (using either DPMS or the new "ACTIVE" property) works
+         * @enable must be the inverse of @disable for atomic drivers.
+         */
         void (*enable)(struct drm_crtc *crtc);
 
         /**
@@ -212,53 +390,228 @@ static inline void drm_crtc_helper_add(struct drm_crtc *crtc,
 
 /**
  * struct drm_encoder_helper_funcs - helper operations for encoders
- * @dpms: set power state
- * @mode_fixup: try to fixup proposed mode for this connector
- * @prepare: part of the disable sequence, called before the CRTC modeset
- * @commit: called after the CRTC modeset
- * @mode_set: set this mode, optional for atomic helpers
- * @get_crtc: return CRTC that the encoder is currently attached to
- * @detect: connection status detection
- * @disable: disable encoder when not in use (overrides DPMS off)
- * @enable: enable encoder
- * @atomic_check: check for validity of an atomic update
- *
- * The helper operations are called by the mid-layer CRTC helper.
  *
- * Note that with atomic helpers @dpms, @prepare and @commit hooks are
- * deprecated. Used @enable and @disable instead exclusively.
- *
- * With legacy crtc helpers there's a big semantic difference between @disable
- * and the other hooks: @disable also needs to release any resources acquired in
- * @mode_set (like shared PLLs).
+ * These hooks are used by the legacy CRTC helpers, the transitional plane
+ * helpers and the new atomic modesetting helpers.
  */
 struct drm_encoder_helper_funcs {
+        /**
+         * @dpms:
+         *
+         * Callback to control power levels on the encoder.  If the mode passed in
+         * is unsupported, the provider must use the next lowest power level.
+         * This is used by the legacy encoder helpers to implement DPMS
+         * functionality in drm_helper_connector_dpms().
+         *
+         * This callback is also used to disable an encoder by calling it with
+         * DRM_MODE_DPMS_OFF if the @disable hook isn't used.
+         *
+         * This callback is used by the legacy CRTC helpers.  Atomic helpers
+         * also support using this hook for enabling and disabling an encoder to
+         * facilitate transitions to atomic, but it is deprecated. Instead
+         * @enable and @disable should be used.
+         */
         void (*dpms)(struct drm_encoder *encoder, int mode);
 
+        /**
+         * @mode_fixup:
+         *
+         * This callback is used to validate and adjust a mode. The parameter
+         * mode is the display mode that should be fed to the next element in
+         * the display chain, either the final &drm_connector or a &drm_bridge.
+         * The parameter adjusted_mode is the input mode the encoder requires. It
+         * can be modified by this callback and does not need to match mode.
+         *
+         * This function is used by both legacy CRTC helpers and atomic helpers.
+         * With atomic helpers it is optional.
+         *
+         * NOTE:
+         *
+         * This function is called in the check phase of atomic modesets, which
+         * can be aborted for any reason (including on userspace's request to
+         * just check whether a configuration would be possible). Atomic drivers
+         * MUST NOT touch any persistent state (hardware or software) or data
+         * structures except the passed in adjusted_mode parameter.
+         *
+         * This is in contrast to the legacy CRTC helpers where this was
+         * allowed.
+         *
+         * Atomic drivers which need to inspect and adjust more state should
+         * instead use the @atomic_check callback.
+         *
+         * RETURNS:
+         *
+         * True if an acceptable configuration is possible, false if the modeset
+         * operation should be rejected.
+         */
         bool (*mode_fixup)(struct drm_encoder *encoder,
                            const struct drm_display_mode *mode,
                            struct drm_display_mode *adjusted_mode);
+
+        /**
+         * @prepare:
+         *
+         * This callback should prepare the encoder for a subsequent modeset,
+         * which in practice means the driver should disable the encoder if it
+         * is running. Most drivers ended up implementing this by calling their
+         * @dpms hook with DRM_MODE_DPMS_OFF.
+         *
+         * This callback is used by the legacy CRTC helpers.  Atomic helpers
+         * also support using this hook for disabling an encoder to facilitate
+         * transitions to atomic, but it is deprecated. Instead @disable should
+         * be used.
+         */
         void (*prepare)(struct drm_encoder *encoder);
+
+        /**
+         * @commit:
+         *
+         * This callback should commit the new mode on the encoder after a modeset,
+         * which in practice means the driver should enable the encoder.  Most
+         * drivers ended up implementing this by calling their @dpms hook with
+         * DRM_MODE_DPMS_ON.
+         *
+         * This callback is used by the legacy CRTC helpers.  Atomic helpers
+         * also support using this hook for enabling an encoder to facilitate
+         * transitions to atomic, but it is deprecated. Instead @enable should
+         * be used.
+         */
         void (*commit)(struct drm_encoder *encoder);
+
+        /**
+         * @mode_set:
+         *
+         * This callback is used to update the display mode of an encoder.
+         *
+         * Note that the display pipe is completely off when this function is
+         * called. Drivers which need hardware to be running before they program
+         * the new display mode (because they implement runtime PM) should not
+         * use this hook, because the helper library calls it only once and not
+         * every time the display pipeline is suspend using either DPMS or the
+         * new "ACTIVE" property. Such drivers should instead move all their
+         * encoder setup into the ->enable() callback.
+         *
+         * This callback is used both by the legacy CRTC helpers and the atomic
+         * modeset helpers. It is optional in the atomic helpers.
+         */
         void (*mode_set)(struct drm_encoder *encoder,
                          struct drm_display_mode *mode,
                          struct drm_display_mode *adjusted_mode);
+
+        /**
+         * @get_crtc:
+         *
+         * This callback is used by the legacy CRTC helpers to work around
+         * deficiencies in its own book-keeping.
+         *
+         * Do not use, use atomic helpers instead, which get the book keeping
+         * right.
+         *
+         * FIXME:
+         *
+         * Currently only nouveau is using this, and as soon as nouveau is
+         * atomic we can ditch this hook.
+         */
         struct drm_crtc *(*get_crtc)(struct drm_encoder *encoder);
-        /* detect for DAC style encoders */
+
+        /**
+         * @detect:
+         *
+         * This callback can be used by drivers who want to do detection on the
+         * encoder object instead of in connector functions.
+         *
+         * It is not used by any helper and therefore has purely driver-specific
+         * semantics. New drivers shouldn't use this and instead just implement
+         * their own private callbacks.
+         *
+         * FIXME:
+         *
+         * This should just be converted into a pile of driver vfuncs.
+         * Currently radeon, amdgpu and nouveau are using it.
+         */
         enum drm_connector_status (*detect)(struct drm_encoder *encoder,
                                             struct drm_connector *connector);
+
+        /**
+         * @disable:
+         *
+         * This callback should be used to disable the encoder. With the atomic
+         * drivers it is called before this encoder's CRTC has been shut off
+         * using the CRTC's own ->disable hook.  If that sequence is too simple
+         * drivers can just add their own driver private encoder hooks and call
+         * them from CRTC's callback by looping over all encoders connected to
+         * it using for_each_encoder_on_crtc().
+         *
+         * This hook is used both by legacy CRTC helpers and atomic helpers.
+         * Atomic drivers don't need to implement it if there's no need to
+         * disable anything at the encoder level. To ensure that runtime PM
+         * handling (using either DPMS or the new "ACTIVE" property) works
+         * @disable must be the inverse of @enable for atomic drivers.
+         *
+         * NOTE:
+         *
+         * With legacy CRTC helpers there's a big semantic difference between
+         * @disable and other hooks (like @prepare or @dpms) used to shut down a
+         * encoder: @disable is only called when also logically disabling the
+         * display pipeline and needs to release any resources acquired in
+         * @mode_set (like shared PLLs, or again release pinned framebuffers).
+         *
+         * Therefore @disable must be the inverse of @mode_set plus @commit for
+         * drivers still using legacy CRTC helpers, which is different from the
+         * rules under atomic.
+         */
         void (*disable)(struct drm_encoder *encoder);
 
+        /**
+         * @enable:
+         *
+         * This callback should be used to enable the encoder. With the atomic
+         * drivers it is called after this encoder's CRTC has been enabled using
+         * the CRTC's own ->enable hook.  If that sequence is too simple drivers
+         * can just add their own driver private encoder hooks and call them
+         * from CRTC's callback by looping over all encoders connected to it
+         * using for_each_encoder_on_crtc().
+         *
+         * This hook is used only by atomic helpers, for symmetry with @disable.
+         * Atomic drivers don't need to implement it if there's no need to
+         * enable anything at the encoder level. To ensure that runtime PM handling
+         * (using either DPMS or the new "ACTIVE" property) works
+         * @enable must be the inverse of @disable for atomic drivers.
+         */
         void (*enable)(struct drm_encoder *encoder);
 
-        /* atomic helpers */
+        /**
+         * @atomic_check:
+         *
+         * This callback is used to validate encoder state for atomic drivers.
+         * Since the encoder is the object connecting the CRTC and connector it
+         * gets passed both states, to be able to validate interactions and
+         * update the CRTC to match what the encoder needs for the requested
+         * connector.
+         *
+         * This function is used by the atomic helpers, but it is optional.
+         *
+         * NOTE:
+         *
+         * This function is called in the check phase of an atomic update. The
+         * driver is not allowed to change anything outside of the free-standing
+         * state objects passed-in or assembled in the overall &drm_atomic_state
+         * update tracking structure.
+         *
+         * RETURNS:
+         *
+         * 0 on success, -EINVAL if the state or the transition can't be
+         * supported, -ENOMEM on memory allocation failure and -EDEADLK if an
+         * attempt to obtain another state object ran into a &drm_modeset_lock
+         * deadlock.
+         */
         int (*atomic_check)(struct drm_encoder *encoder,
                             struct drm_crtc_state *crtc_state,
                             struct drm_connector_state *conn_state);
 };
 
 /**
- * drm_encoder_helper_add - sets the helper vtable for a encoder
+ * drm_encoder_helper_add - sets the helper vtable for an encoder
  * @encoder: DRM encoder
  * @funcs: helper vtable to set for @encoder
  */