[PATCH 4/9] drm: Add infrastructure to allow seamless mode switches

[José Roberto de Souza]

Intel hardware supports change between modes with different refresh
rates without any glitches or visual artifacts, that feature is called
seamless DRRS.

So far i915 driver was automatically changing between preferred panel
mode and lower refresh rate mode based on idleness but ChromeOS
compositor team is requesting to be in control of the mode switch.
So for a certain types of content it can switch to mode with a lower
refresh rate without user noticing a thing and saving power.

This seamless mode switch will be triggered when user-space dispatch
a atomic commit with the new mode and clears the
DRM_MODE_ATOMIC_ALLOW_MODESET flag.

A driver that don't implement atomic_seamless_mode_switch_check
function will continue to fail in the atomic check phase with
"[CRTC:%d:%s] requires full modeset" debug message.
While a driver that implements it and support the seamless change
between old and new mode will return 0 otherwise it should return
EOPNOTSUPP or the appropried errno for errors.

So here adding basic drm infrastructure to that be supported by i915
and other drivers.

v2:
- only returning early in drm_atomic_helper_check_modeset() in case
of errors different than EOPNOTSUPP

Cc: Vidya Srinivas <vidya.srinivas at intel.com>
Cc: Sean Paul <seanpaul at chromium.org>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza at intel.com>
---
 drivers/gpu/drm/drm_atomic.c              |  1 +
 drivers/gpu/drm/drm_atomic_helper.c       | 15 ++++++++++++++
 drivers/gpu/drm/drm_atomic_state_helper.c |  1 +
 include/drm/drm_crtc.h                    | 25 +++++++++++++++++++++++
 4 files changed, 42 insertions(+)

diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
index 58c0283fb6b0c..21525f9f4b4c1 100644
--- a/drivers/gpu/drm/drm_atomic.c
+++ b/drivers/gpu/drm/drm_atomic.c
@@ -437,6 +437,7 @@ static void drm_atomic_crtc_print_state(struct drm_printer *p,
 	drm_printf(p, "\tself_refresh_active=%d\n", state->self_refresh_active);
 	drm_printf(p, "\tplanes_changed=%d\n", state->planes_changed);
 	drm_printf(p, "\tmode_changed=%d\n", state->mode_changed);
+	drm_printf(p, "\tseamless_mode_changed=%d\n", state->seamless_mode_changed);
 	drm_printf(p, "\tactive_changed=%d\n", state->active_changed);
 	drm_printf(p, "\tconnectors_changed=%d\n", state->connectors_changed);
 	drm_printf(p, "\tcolor_mgmt_changed=%d\n", state->color_mgmt_changed);
diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index 9603193d2fa13..bc4add3d470c6 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -631,6 +631,21 @@ drm_atomic_helper_check_modeset(struct drm_device *dev,
 			drm_dbg_atomic(dev, "[CRTC:%d:%s] mode changed\n",
 				       crtc->base.id, crtc->name);
 			new_crtc_state->mode_changed = true;
+
+			if (!state->allow_modeset &&
+			    crtc->funcs->atomic_seamless_mode_switch_check) {
+				ret = crtc->funcs->atomic_seamless_mode_switch_check(state, crtc);
+
+				if (ret == 0) {
+					new_crtc_state->seamless_mode_changed = true;
+					new_crtc_state->mode_changed = false;
+				} else if (ret == -EOPNOTSUPP) {
+					drm_dbg_atomic(dev, "[CRTC:%d:%s] Seamless mode switch not supported\n",
+						       crtc->base.id, crtc->name);
+				} else if (ret < 0) {
+					return ret;
+				}
+			}
 		}
 
 		if (old_crtc_state->enable != new_crtc_state->enable) {
diff --git a/drivers/gpu/drm/drm_atomic_state_helper.c b/drivers/gpu/drm/drm_atomic_state_helper.c
index 3b6d3bdbd0996..c093073ea6e11 100644
--- a/drivers/gpu/drm/drm_atomic_state_helper.c
+++ b/drivers/gpu/drm/drm_atomic_state_helper.c
@@ -142,6 +142,7 @@ void __drm_atomic_helper_crtc_duplicate_state(struct drm_crtc *crtc,
 	if (state->gamma_lut)
 		drm_property_blob_get(state->gamma_lut);
 	state->mode_changed = false;
+	state->seamless_mode_changed = false;
 	state->active_changed = false;
 	state->planes_changed = false;
 	state->connectors_changed = false;
diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index a70baea0636ca..b7ce378d679d3 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -140,6 +140,16 @@ struct drm_crtc_state {
 	 */
 	bool mode_changed : 1;
 
+	/**
+	 * @seamless_mode_changed: @mode has been changed but user-space
+	 * is requesting to change to the new mode with a fastset and driver
+	 * supports this request.
+	 * To be used by drivers to steer the atomic commit control flow to
+	 * appropriate paths to change mode without any visual corruption.
+	 * Never set together with @mode_changed.
+	 */
+	bool seamless_mode_changed : 1;
+
 	/**
 	 * @active_changed: @active has been toggled. Used by the atomic
 	 * helpers and drivers to steer the atomic commit control flow. See also
@@ -939,6 +949,21 @@ struct drm_crtc_funcs {
 				     int *max_error,
 				     ktime_t *vblank_time,
 				     bool in_vblank_irq);
+
+	/**
+	 * @atomic_seamless_mode_switch_check
+	 *
+	 * Called when user-space wants to change mode without do a modeset.
+	 * Drivers can optionally support do a mode switch without any visual
+	 * corruption when changing between certain modes.
+	 *
+	 * Returns:
+	 * Zero if possible to seamless switch mode, -EOPNOTSUPP if not
+	 * supported seamless mode change or appropriate errno if an error
+	 * happened.
+	 */
+	int (*atomic_seamless_mode_switch_check)(struct drm_atomic_state *state,
+						 struct drm_crtc *crtc);
 };
 
 /**
-- 
2.36.0