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

[Daniel Vetter]

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

Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Cc: Maxime Ripard <mripard at kernel.org>
Cc: Thomas Zimmermann <tzimmermann at suse.de>
Cc: David Airlie <airlied at gmail.com>
Cc: Daniel Vetter <daniel at ffwll.ch>
Cc: Pekka Paalanen <pekka.paalanen at collabora.com>
Cc: Rob Clark <robdclark at gmail.com>
Cc: Simon Ser <contact at emersion.fr>
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 include/drm/drm_atomic.h | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
index cf8e1220a4ac..09bef1b6c170 100644
--- a/include/drm/drm_atomic.h
+++ b/include/drm/drm_atomic.h
@@ -372,8 +372,22 @@ 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().
+	 * not consult this flag, instead looking at the output of
+	 * drm_atomic_crtc_needs_modeset(). The detailed rules are:
+	 *
+	 * - Drivers must not consult @allow_modeset in the atomic commit path,
+	 *   and instead use drm_atomic_crtc_needs_modeset().
+	 *
+	 * - 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.
+	 *
+	 * - Drivers must never change this flag, it is only under the control
+	 *   of userspace.
 	 */
 	bool allow_modeset : 1;
 	/**
-- 
2.40.1