drm/i915/dsb: Allow intel_dsb_chain() to use DSB_WAIT_FOR_VBLANK

Allow intel_dsb_chain() to start the chained DSB
at start of the undelaye vblank. This is slightly
more involved than simply setting the bit as we
must use the DEwake mechanism to eliminate pkgC
latency.

And DSB_ENABLE_DEWAKE itself is problematic in that
it allows us to configure just a single scanline,
and if the current scanline is already past that
DSB_ENABLE_DEWAKE won't do anything, rendering the
whole thing moot.

The current workaround involves checking the pipe's current
scanline with the CPU, and if it looks like we're about to
miss the configured DEwake scanline we set DSB_FORCE_DEWAKE
to immediately assert DEwake. This is somewhat racy since the
hardware is making progress all the while we're checking it on
the CPU.

We can make things less racy by chaining two DSBs and handling
the DSB_FORCE_DEWAKE stuff entirely without CPU involvement:
1. CPU starts the first DSB immediately
2. First DSB configures the second DSB, including its dewake_scanline
3. First DSB starts the second w/ DSB_WAIT_FOR_VBLANK
4. First DSB asserts DSB_FORCE_DEWAKE
5. First DSB waits until we're outside the dewake_scanline-vblank_start
   window
6. First DSB deasserts DSB_FORCE_DEWAKE

That will guarantee that the we are fully awake when the second
DSB starts to actually execute.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240624191032.27333-12-ville.syrjala@linux.intel.comReviewed-by: Animesh Manna <animesh.manna@intel.com>

drivers/gpu/drm/i915/display/intel_dsb.c

@@ -130,8 +130,8 @@ static int dsb_vtotal(struct intel_atomic_state *state,
 		return intel_mode_vtotal(&crtc_state->hw.adjusted_mode);
 }
 
-static int dsb_dewake_scanline(struct intel_atomic_state *state,
-			       struct intel_crtc *crtc)
+static int dsb_dewake_scanline_start(struct intel_atomic_state *state,
+				     struct intel_crtc *crtc)
 {
 	const struct intel_crtc_state *crtc_state = pre_commit_crtc_state(state, crtc);
 	struct drm_i915_private *i915 = to_i915(state->base.dev);
@@ -141,6 +141,14 @@ static int dsb_dewake_scanline(struct intel_atomic_state *state,
 		intel_usecs_to_scanlines(&crtc_state->hw.adjusted_mode, latency);
 }
 
+static int dsb_dewake_scanline_end(struct intel_atomic_state *state,
+				   struct intel_crtc *crtc)
+{
+	const struct intel_crtc_state *crtc_state = pre_commit_crtc_state(state, crtc);
+
+	return intel_mode_vdisplay(&crtc_state->hw.adjusted_mode);
+}
+
 static int dsb_scanline_to_hw(struct intel_atomic_state *state,
 			      struct intel_crtc *crtc, int scanline)
 {
@@ -527,19 +535,44 @@ static void _intel_dsb_chain(struct intel_atomic_state *state,
 			    dsb_error_int_status(display) | DSB_PROG_INT_STATUS |
 			    dsb_error_int_en(display));
 
+	if (ctrl & DSB_WAIT_FOR_VBLANK) {
+		int dewake_scanline = dsb_dewake_scanline_start(state, crtc);
+		int hw_dewake_scanline = dsb_scanline_to_hw(state, crtc, dewake_scanline);
+
+		intel_dsb_reg_write(dsb, DSB_PMCTRL(pipe, chained_dsb->id),
+				    DSB_ENABLE_DEWAKE |
+				    DSB_SCANLINE_FOR_DEWAKE(hw_dewake_scanline));
+	}
+
 	intel_dsb_reg_write(dsb, DSB_HEAD(pipe, chained_dsb->id),
 			    intel_dsb_buffer_ggtt_offset(&chained_dsb->dsb_buf));
 
 	intel_dsb_reg_write(dsb, DSB_TAIL(pipe, chained_dsb->id),
 			    intel_dsb_buffer_ggtt_offset(&chained_dsb->dsb_buf) + tail);
+
+	if (ctrl & DSB_WAIT_FOR_VBLANK) {
+		/*
+		 * Keep DEwake alive via the first DSB, in
+		 * case we're already past dewake_scanline,
+		 * and thus DSB_ENABLE_DEWAKE on the second
+		 * DSB won't do its job.
+		 */
+		intel_dsb_reg_write_masked(dsb, DSB_PMCTRL_2(pipe, dsb->id),
+					   DSB_FORCE_DEWAKE, DSB_FORCE_DEWAKE);
+
+		intel_dsb_wait_scanline_out(state, dsb,
+					    dsb_dewake_scanline_start(state, crtc),
+					    dsb_dewake_scanline_end(state, crtc));
+	}
 }
 
 void intel_dsb_chain(struct intel_atomic_state *state,
 		     struct intel_dsb *dsb,
-		     struct intel_dsb *chained_dsb)
+		     struct intel_dsb *chained_dsb,
+		     bool wait_for_vblank)
 {
 	_intel_dsb_chain(state, dsb, chained_dsb,
-			 0);
+			 wait_for_vblank ? DSB_WAIT_FOR_VBLANK : 0);
 }
 
 static void _intel_dsb_commit(struct intel_dsb *dsb, u32 ctrl,
@@ -697,7 +730,7 @@ struct intel_dsb *intel_dsb_prepare(struct intel_atomic_state *state,
 
 	dsb->chicken = dsb_chicken(state, crtc);
 	dsb->hw_dewake_scanline =
-		dsb_scanline_to_hw(state, crtc, dsb_dewake_scanline(state, crtc));
+		dsb_scanline_to_hw(state, crtc, dsb_dewake_scanline_start(state, crtc));
 
 	return dsb;
 

drivers/gpu/drm/i915/display/intel_dsb.h

@@ -47,7 +47,8 @@ void intel_dsb_wait_scanline_out(struct intel_atomic_state *state,
 				 int lower, int upper);
 void intel_dsb_chain(struct intel_atomic_state *state,
 		     struct intel_dsb *dsb,
-		     struct intel_dsb *chained_dsb);
+		     struct intel_dsb *chained_dsb,
+		     bool wait_for_vblank);
 
 void intel_dsb_commit(struct intel_dsb *dsb,
 		      bool wait_for_vblank);