drm_atomic_helper_check(9)

📦 version 7.3.1, in nixpkgs unstable

💅 drm_atomic_helper_check(9) 💅

okay so this is a page from the Kernel API Manual, and honestly? it's a bop. it's drm_atomic_helper_check, and it's here to tell you whether the state you're asking for is even physically possible or if you're delulu. vibe check for your atomic commit, basically. 🦋

✨ NAME ✨

drm_atomic_helper_check — bestie, it validates the state object. that's the whole job. 💖

🌸 SYNOPSIS 🌸

int drm_atomic_helper_check (struct drm_device *dev , struct
       drm_atomic_commit *state );

🎀 ARGUMENTS 🎀

  • dev — the DRM device. the main character of this whole situation. 👑
  • state — the driver state object. the thing we're checking. the moment of truth. ✨

💫 DESCRIPTION 💫

so what does it actually do? it checks the state object to see if the requested state is physically possible. fr fr. only CRTCs and planes have check callbacks, so if a driver needs any extra (global) checking, it can just wrap that around this function. and drivers with no such needs? they can directly use this as their drm_mode_config_funcs.atomic_check callback. slay, no extra work. 💅

this just wraps the two parts of the state checking for planes and modeset state, in the default order: first it calls drm_atomic_helper_check_modeset and then drm_atomic_helper_check_planes. the assumption here (iykyk) is that the drm_plane_helper_funcs.atomic_check and drm_crtc_helper_funcs.atomic_check functions depend upon an updated adjusted_mode.clock to e.g. properly compute watermarks. the way that clock needs to be updated before anyone does math on it 😭

and now, the plot twist: note that zpos normalization will add all enable planes to the state, which might not be desired for some drivers. for example, enable/disable of a cursor plane which have fixed zpos value would trigger all other enabled planes to be forced to the state change. lowkey chaotic but that's the tea. 🍵

IMPORTANT: since this function calls drm_atomic_helper_check_modeset internally, its restrictions also apply too, no cap. drivers which set drm_crtc_state.mode_changed (like, e.g. in their drm_plane_helper_funcs.atomic_check hooks if a plane update can't be done without a full modeset) _must_ call drm_atomic_helper_check_modeset function again after that change. like. you have to. it's not optional. 🚨

🔥 RETURN 🔥

zero for success, or -errno. that's it, that's the whole vibe check result. 💀

🌈 SEE ALSO 🌈

okay bestie, this list is long, but every single one of these is iconic in its own right, so we're listing them all. no skipping. periodt. 👑

🫶 the cute sign-off 🫶

this one lives in ./drivers/gpu/drm, dated October 2026, and it's filed under drm_atomic_helper_check(9) in case you need to find it again later. go forth and validate your state objects, bestie. you're gonna slay. 💖✨🎀

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