drm/i915: Restore GT performance in headless mode with DMC loaded

It seems that the DMC likes to transition between the DC states a lot when
there are no connected displays (no active power domains) during command
submission.

This activity on DC states has a negative impact on the performance of the
chip with huge latencies observed in the interrupt handlers and elsewhere.
Simple tests like igt/gem_latency -n 0 are slowed down by a factor of
eight.

Work around it by introducing a new power domain named,
POWER_DOMAIN_GT_IRQ, associtated with the "DC off" power well, which is
held for the duration of command submission activity.

CNL has the same problem which will be addressed as a follow-up. Doing
that requires a fix for a DC6 context corruption problem in the CNL DMC
firmware which is yet to be released.

v2:
 * Add commit text as comment in i915_gem_mark_busy. (Chris Wilson)
 * Protect macro body with braces. (Jani Nikula)

v3:
 * Add dedicated power domain for clarity. (Chris, Imre)
 * Commit message and comment text updates.
 * Apply to all big-core GEN9 parts apart for Skylake which is pending DMC
   firmware release.

v4:
 * Power domain should be inner to device runtime pm. (Chris)
 * Simplify NEEDS_CSR_GT_PERF_WA macro. (Chris)
 * Handle async DMC loading by moving the GT_IRQ power domain logic into
   intel_runtime_pm. (Daniel, Chris)
 * Include small core GEN9 as well. (Imre)

v5
 * Special handling for async DMC load is not needed since on failure the
   power domain reference is kept permanently taken. (Imre)

v6:
 * Drop the NEEDS_CSR_GT_PERF_WA macro since all firmwares have now been
   deployed. (Imre, Chris)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100572
Testcase: igt/gem_exec_nop/headless
Cc: Imre Deak <imre.deak@intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk> (v2)
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v5)
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[Imre: Add note about applying the WA on CNL as a follow-up]
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171205132854.26380-1-tvrtko.ursulin@linux.intel.com

drivers/gpu/drm/i915/i915_drv.h

@@ -398,6 +398,7 @@ enum intel_display_power_domain {
 	POWER_DOMAIN_AUX_D,
 	POWER_DOMAIN_GMBUS,
 	POWER_DOMAIN_MODESET,
+	POWER_DOMAIN_GT_IRQ,
 	POWER_DOMAIN_INIT,
 
 	POWER_DOMAIN_NUM,

drivers/gpu/drm/i915/i915_gem.c

@@ -3386,6 +3386,9 @@ i915_gem_idle_work_handler(struct work_struct *work)
 
 	if (INTEL_GEN(dev_priv) >= 6)
 		gen6_rps_idle(dev_priv);
+
+	intel_display_power_put(dev_priv, POWER_DOMAIN_GT_IRQ);
+
 	intel_runtime_pm_put(dev_priv);
 out_unlock:
 	mutex_unlock(&dev_priv->drm.struct_mutex);

drivers/gpu/drm/i915/i915_gem_request.c

@@ -252,6 +252,20 @@ static void mark_busy(struct drm_i915_private *i915)
 	GEM_BUG_ON(!i915->gt.active_requests);
 
 	intel_runtime_pm_get_noresume(i915);
+
+	/*
+	 * It seems that the DMC likes to transition between the DC states a lot
+	 * when there are no connected displays (no active power domains) during
+	 * command submission.
+	 *
+	 * This activity has negative impact on the performance of the chip with
+	 * huge latencies observed in the interrupt handler and elsewhere.
+	 *
+	 * Work around it by grabbing a GT IRQ power domain whilst there is any
+	 * GT activity, preventing any DC state transitions.
+	 */
+	intel_display_power_get(i915, POWER_DOMAIN_GT_IRQ);
+
 	i915->gt.awake = true;
 
 	intel_enable_gt_powersave(i915);

drivers/gpu/drm/i915/intel_runtime_pm.c

@@ -130,6 +130,8 @@ intel_display_power_domain_str(enum intel_display_power_domain domain)
 		return "INIT";
 	case POWER_DOMAIN_MODESET:
 		return "MODESET";
+	case POWER_DOMAIN_GT_IRQ:
+		return "GT_IRQ";
 	default:
 		MISSING_CASE(domain);
 		return "?";
@@ -1705,6 +1707,7 @@ void intel_display_power_put(struct drm_i915_private *dev_priv,
 	BIT_ULL(POWER_DOMAIN_INIT))
 #define SKL_DISPLAY_DC_OFF_POWER_DOMAINS (		\
 	SKL_DISPLAY_POWERWELL_2_POWER_DOMAINS |		\
+	BIT_ULL(POWER_DOMAIN_GT_IRQ) |			\
 	BIT_ULL(POWER_DOMAIN_MODESET) |			\
 	BIT_ULL(POWER_DOMAIN_AUX_A) |			\
 	BIT_ULL(POWER_DOMAIN_INIT))
@@ -1727,6 +1730,7 @@ void intel_display_power_put(struct drm_i915_private *dev_priv,
 	BIT_ULL(POWER_DOMAIN_INIT))
 #define BXT_DISPLAY_DC_OFF_POWER_DOMAINS (		\
 	BXT_DISPLAY_POWERWELL_2_POWER_DOMAINS |		\
+	BIT_ULL(POWER_DOMAIN_GT_IRQ) |			\
 	BIT_ULL(POWER_DOMAIN_MODESET) |			\
 	BIT_ULL(POWER_DOMAIN_AUX_A) |			\
 	BIT_ULL(POWER_DOMAIN_INIT))
@@ -1785,6 +1789,7 @@ void intel_display_power_put(struct drm_i915_private *dev_priv,
 	BIT_ULL(POWER_DOMAIN_INIT))
 #define GLK_DISPLAY_DC_OFF_POWER_DOMAINS (		\
 	GLK_DISPLAY_POWERWELL_2_POWER_DOMAINS |		\
+	BIT_ULL(POWER_DOMAIN_GT_IRQ) |			\
 	BIT_ULL(POWER_DOMAIN_MODESET) |			\
 	BIT_ULL(POWER_DOMAIN_AUX_A) |			\
 	BIT_ULL(POWER_DOMAIN_INIT))