drm/i915: clarify PMINTRMSK/pm_intr_keep usage

No functional changes; just renaming a bit, tweaking a datatype,
prettifying layout, and adding comments, in particular in the
GuC setup code that touches this data.

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473711577-11454-2-git-send-email-david.s.gordon@intel.com
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_drv.h

@@ -1184,6 +1184,7 @@ struct intel_gen6_power_mgmt {
 	bool interrupts_enabled;
 	u32 pm_iir;
 
+	/* PM interrupt bits that should never be masked */
 	u32 pm_intr_keep;
 
 	/* Frequencies are stored in potentially platform dependent multiples.

drivers/gpu/drm/i915/i915_irq.c

@@ -371,7 +371,7 @@ void gen6_disable_rps_interrupts(struct drm_i915_private *dev_priv)
 	spin_lock_irq(&dev_priv->irq_lock);
 	dev_priv->rps.interrupts_enabled = false;
 
-	I915_WRITE(GEN6_PMINTRMSK, gen6_sanitize_rps_pm_mask(dev_priv, ~0));
+	I915_WRITE(GEN6_PMINTRMSK, gen6_sanitize_rps_pm_mask(dev_priv, ~0u));
 
 	__gen6_disable_pm_irq(dev_priv, dev_priv->pm_rps_events);
 	I915_WRITE(gen6_pm_ier(dev_priv), I915_READ(gen6_pm_ier(dev_priv)) &
@@ -4500,7 +4500,7 @@ void intel_irq_init(struct drm_i915_private *dev_priv)
 		dev_priv->rps.pm_intr_keep |= GEN6_PM_RP_UP_EI_EXPIRED;
 
 	if (INTEL_INFO(dev_priv)->gen >= 8)
-		dev_priv->rps.pm_intr_keep |= GEN8_PMINTR_REDIRECT_TO_NON_DISP;
+		dev_priv->rps.pm_intr_keep |= GEN8_PMINTR_REDIRECT_TO_GUC;
 
 	INIT_DELAYED_WORK(&dev_priv->gpu_error.hangcheck_work,
 			  i915_hangcheck_elapsed);

drivers/gpu/drm/i915/i915_reg.h

@@ -7067,7 +7067,7 @@ enum {
 #define VLV_RCEDATA				_MMIO(0xA0BC)
 #define GEN6_RC6pp_THRESHOLD			_MMIO(0xA0C0)
 #define GEN6_PMINTRMSK				_MMIO(0xA168)
-#define   GEN8_PMINTR_REDIRECT_TO_NON_DISP	(1<<31)
+#define   GEN8_PMINTR_REDIRECT_TO_GUC		  (1<<31)
 #define GEN8_MISC_CTRL0				_MMIO(0xA180)
 #define VLV_PWRDWNUPCTL				_MMIO(0xA294)
 #define GEN9_MEDIA_PG_IDLE_HYSTERESIS		_MMIO(0xA0C4)

drivers/gpu/drm/i915/intel_guc_loader.c

@@ -134,13 +134,28 @@ static void direct_interrupts_to_guc(struct drm_i915_private *dev_priv)
 	I915_WRITE(GUC_WD_VECS_IER, ~irqs);
 
 	/*
-	 * If GuC has routed PM interrupts to itself, don't keep it.
-	 * and keep other interrupts those are unmasked by GuC.
-	*/
+	 * The REDIRECT_TO_GUC bit of the PMINTRMSK register directs all
+	 * (unmasked) PM interrupts to the GuC. All other bits of this
+	 * register *disable* generation of a specific interrupt.
+	 *
+	 * 'pm_intr_keep' indicates bits that are NOT to be set when
+	 * writing to the PM interrupt mask register, i.e. interrupts
+	 * that must not be disabled.
+	 *
+	 * If the GuC is handling these interrupts, then we must not let
+	 * the PM code disable ANY interrupt that the GuC is expecting.
+	 * So for each ENABLED (0) bit in this register, we must SET the
+	 * bit in pm_intr_keep so that it's left enabled for the GuC.
+	 *
+	 * OTOH the REDIRECT_TO_GUC bit is initially SET in pm_intr_keep
+	 * (so interrupts go to the DISPLAY unit at first); but here we
+	 * need to CLEAR that bit, which will result in the register bit
+	 * being left SET!
+	 */
 	tmp = I915_READ(GEN6_PMINTRMSK);
-	if (tmp & GEN8_PMINTR_REDIRECT_TO_NON_DISP) {
-		dev_priv->rps.pm_intr_keep |= ~(tmp & ~GEN8_PMINTR_REDIRECT_TO_NON_DISP);
-		dev_priv->rps.pm_intr_keep &= ~GEN8_PMINTR_REDIRECT_TO_NON_DISP;
+	if (tmp & GEN8_PMINTR_REDIRECT_TO_GUC) {
+		dev_priv->rps.pm_intr_keep |= ~tmp;
+		dev_priv->rps.pm_intr_keep &= ~GEN8_PMINTR_REDIRECT_TO_GUC;
 	}
 }