STRUCT DRM_ENCODER_HELPER_FUNCS(9)

📦 version 7.3.1, in nixpkgs unstable

✨ NAME ✨

struct drm_encoder_helper_funcs — the helper operations for encoders, aka the lil' callback pack your encoder gets to fill in 💅

okay so bestie, quick vibe check: this struct is literally the handbook of "what should happen when my encoder needs to do stuff" — it's the helper ops set for encoders, and yes, the kernel manual name is struct drm_encoder_helper_funcs(9) in case you were wondering 🎀

🦋 SYNOPSIS 🦋

struct drm_encoder_helper_funcs {
    void (*dpms)(struct drm_encoder *encoder, int mode);
    enum drm_mode_status (*mode_valid)(struct drm_encoder *crtc, const struct drm_display_mode *mode);
    bool (*mode_fixup)(struct drm_encoder *encoder, const struct drm_display_mode *mode, struct drm_display_mode *adjusted_mode);
    void (*prepare)(struct drm_encoder *encoder);
    void (*commit)(struct drm_encoder *encoder);
    void (*mode_set)(struct drm_encoder *encoder, struct drm_display_mode *mode, struct drm_display_mode *adjusted_mode);
    void (*atomic_mode_set)(struct drm_encoder *encoder, struct drm_crtc_state *crtc_state, struct drm_connector_state *conn_state);
    enum drm_connector_status (*detect)(struct drm_encoder *encoder, struct drm_connector *connector);
    void (*atomic_disable)(struct drm_encoder *encoder, struct drm_atomic_commit *state);
    void (*atomic_enable)(struct drm_encoder *encoder, struct drm_atomic_commit *state);
    void (*disable)(struct drm_encoder *encoder);
    void (*enable)(struct drm_encoder *encoder);
    int (*atomic_check)(struct drm_encoder *encoder, struct drm_crtc_state *crtc_state, struct drm_connector_state *conn_state);
};

🌸 Members 🌸

💜 dpms

Callback to control power levels on the encoder. If the mode you pass in is unsupported, the provider must use the next lowest power level — no cap, that's the rule. 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 ngl it's deprecated. Instead enable and disable should be used. 👑

🌈 mode_valid

This callback is used to check if a specific mode is valid in this encoder. Bestie, you should implement this if the encoder has some sort of restriction in the modes it can display. Like, for example, a given encoder may be responsible for setting a clock value — and if that clock can't produce all the values for the available modes, then this callback can restrict the number of modes to only the ones that can actually be displayed ✨

This hook is used by the probe helpers to filter the mode list in drm_helper_probe_single_connector_modes, and it's used by the atomic helpers to validate modes supplied by userspace in drm_atomic_helper_check_modeset.

This function is optional.

NOTE 🫶

Since this function is both called from the check phase of an atomic commit, and the mode validation in the probe paths, it is not allowed to look at anything else but the passed-in mode, and validate it against configuration-invariant hardware constraints. Any further limits which depend upon the configuration can only be checked in mode_fixup or atomic_check.

RETURNS 💖

drm_mode_status Enum

🎀 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. See also drm_crtc_state.adjusted_mode for more details 👀

This function is used by both legacy CRTC helpers and atomic helpers. This hook 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. If atomic_check is used, this hook isn't called since atomic_check allows a strict superset of the functionality of mode_fixup.

Also beware that userspace can request its own custom modes — neither core nor helpers filter modes to the list of probe modes reported by the GETCONNECTOR IOCTL and stored in drm_connector.modes. To ensure that modes are filtered consistently, put any encoder constraints and limits checks into mode_valid. 💀

RETURNS 🍓

True if an acceptable configuration is possible, false if the modeset operation should be rejected.

🧚 prepare

This callback should prepare the encoder for a subsequent modeset, which in practice means the driver should disable the encoder if it's 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's deprecated. Instead disable should be used.

⭐ 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's deprecated. Instead enable should be used.

🔥 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.

NOTE 🙈

If the driver uses the atomic modeset helpers and needs to inspect the connector state or connector display info during mode setting, atomic_mode_set can be used instead.

💅 atomic_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 suspended 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 by the atomic modeset helpers in place of the mode_set callback, if set by the driver. It is optional and should be used instead of mode_set if the driver needs to inspect the connector state or display info, since there is no direct way to go from the encoder to the current connector.

🦋 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.

👑 atomic_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 their own drm_crtc_helper_funcs.atomic_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 callback is a variant of disable that provides the atomic state to the driver. If atomic_disable is implemented, disable is not called by the helpers.

This hook is only used by 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, atomic_disable must be the inverse of atomic_enable. 💜

💖 atomic_enable

This callback should be used to enable the encoder. It is called after this encoder's CRTC has been enabled using their own drm_crtc_helper_funcs.atomic_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 callback is a variant of enable that provides the atomic state to the driver. If atomic_enable is implemented, enable is not called by the helpers.

This hook is only used by atomic helpers, it is the opposite of atomic_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 works, atomic_enable must be the inverse of atomic_disable. 🔥

🌈 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 their own drm_crtc_helper_funcs.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. 🫶

okay so for atomic drivers, also consider atomic_disable and honestly save yourself from having to read the NOTE below! 💅

NOTE: With legacy CRTC helpers there's a big semantic difference between disable and other hooks (like prepare or dpms) used to shut down an 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.
  • 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 their own drm_crtc_helper_funcs.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 only used by atomic helpers, it is the opposite of 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. ✨

  • 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.

Since this provides a strict superset of the functionality of mode_fixup (the requested and adjusted modes are both available through the passed in struct drm_crtc_state) mode_fixup is not called when atomic_check is implemented.

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_commit update tracking structure. Also beware that userspace can request its own custom modes, neither core nor helpers filter modes to the list of probe modes reported by the GETCONNECTOR IOCTL and stored in drm_connector.modes. To ensure that modes are filtered consistently put any encoder constraints and limits checks into mode_valid. 👑

💜 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. 😭

✨ Description ✨

These hooks are used by the legacy CRTC helpers and the new atomic modesetting helpers. main character energy for the whole modeset pipeline, fr fr 🌈

🦋 SEE ALSO 🦋

💖 that's the tea, bestie! source/version: ./include/drm · date: October 2026 · struct drm_...helper_funcs(9) 🎀

📦 where this page comes from