drm/atomic: clarify the rules around drm_atomic_state->allow_modeset

msm is automagically upgrading normal commits to full modesets, and
that's a big no-no:

- for one this results in full on->off->on transitions on all these
  crtc, at least if you're using the usual helpers. Which seems to be
  the case, and is breaking uapi

- further even if the ctm change itself would not result in flicker,
  this can hide modesets for other reasons. Which again breaks the
  uapi

v2: I forgot the case of adding unrelated crtc state. Add that case
and link to the existing kerneldoc explainers. This has come up in an
irc discussion with Manasi and Ville about intel's bigjoiner mode.
Also cc everyone involved in the msm irc discussion, more people
joined after I sent out v1.

v3: Wording polish from Pekka and Thomas

Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Simon Ser <contact@emersion.fr>
Cc: Manasi Navare <navaremanasi@google.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Abhinav Kumar <quic_abhinavk@quicinc.com>
Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Simona Vetter <simona.vetter@intel.com>
Signed-off-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20250108172417.160831-1-simona.vetter@ffwll.ch

include/drm/drm_atomic.h

@@ -376,8 +376,27 @@ struct drm_atomic_state {
 	 *
 	 * Allow full modeset. This is used by the ATOMIC IOCTL handler to
 	 * implement the DRM_MODE_ATOMIC_ALLOW_MODESET flag. Drivers should
-	 * never consult this flag, instead looking at the output of
-	 * drm_atomic_crtc_needs_modeset().
+	 * generally not consult this flag, but instead look at the output of
+	 * drm_atomic_crtc_needs_modeset(). The detailed rules are:
+	 *
+	 * - Drivers must not consult @allow_modeset in the atomic commit path.
+	 *   Use drm_atomic_crtc_needs_modeset() instead.
+	 *
+	 * - Drivers must consult @allow_modeset before adding unrelated struct
+	 *   drm_crtc_state to this commit by calling
+	 *   drm_atomic_get_crtc_state(). See also the warning in the
+	 *   documentation for that function.
+	 *
+	 * - Drivers must never change this flag, it is under the exclusive
+	 *   control of userspace.
+	 *
+	 * - Drivers may consult @allow_modeset in the atomic check path, if
+	 *   they have the choice between an optimal hardware configuration
+	 *   which requires a modeset, and a less optimal configuration which
+	 *   can be committed without a modeset. An example would be suboptimal
+	 *   scanout FIFO allocation resulting in increased idle power
+	 *   consumption. This allows userspace to avoid flickering and delays
+	 *   for the normal composition loop at reasonable cost.
 	 */
 	bool allow_modeset : 1;
 	/**