[Intel-gfx] [PATCH] drm/i915: don't track relative-constants-mode

[Dave Gordon]

'relative_constants_mode' has always been tracked per-device, but this
has actually been wrong ever since hardware contexts were introduced, as
the INSTPM register is saved (and automatically restored) as part of the
render ring context. The software per-device value could therefore get
out of sync with the hardware per-context value.

Rather than track this correctly (per-context in LRC modes, per-device
in legacy ringbuffer mode), a simpler solution is just to give up trying
to track it at all, and always prefix each render batch with the single
instruction needed to explicitly set it to the required mode for the
current batch.

Test case (if anyone wants to write it) would be to create two contexts,
submit a batch with a non-default mode in the first context, submit a
batch with the default mode in the other context, submit another batch
in the first context, but this time in default mode. Without this patch,
the driver will fail to insert the instructions to reset INSTPM into
the first context's ringbuffer.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92448

Signed-off-by: Dave Gordon <david.s.gordon at intel.com>
---
 drivers/gpu/drm/i915/i915_drv.h            |  2 --
 drivers/gpu/drm/i915/i915_gem.c            |  3 ---
 drivers/gpu/drm/i915/i915_gem_execbuffer.c | 32 +++++++++++++-----------------
 3 files changed, 14 insertions(+), 23 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 9fdaf23..7938da0 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1728,8 +1728,6 @@ struct drm_i915_private {
 
 	const struct intel_device_info info;
 
-	int relative_constants_mode;
-
 	void __iomem *regs;
 
 	struct intel_uncore uncore;
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index d37b441..8c06778 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4615,9 +4615,6 @@ int i915_gem_init(struct drm_device *dev)
 			  i915_gem_idle_work_handler);
 	init_waitqueue_head(&dev_priv->gpu_error.wait_queue);
 	init_waitqueue_head(&dev_priv->gpu_error.reset_queue);
-
-	dev_priv->relative_constants_mode = I915_EXEC_CONSTANTS_REL_GENERAL;
-
 	init_waitqueue_head(&dev_priv->pending_flip_queue);
 
 	dev_priv->mm.interruptible = true;
diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 601156c..abfb06e 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -1462,30 +1462,28 @@ static void eb_export_fence(struct drm_i915_gem_object *obj,
 			return -EINVAL;
 		}
 
-		if (instp_mode != dev_priv->relative_constants_mode) {
-			if (INTEL_INFO(dev_priv)->gen < 4) {
-				DRM_DEBUG("no rel constants on pre-gen4\n");
-				return -EINVAL;
-			}
-
-			if (INTEL_INFO(dev_priv)->gen > 5 &&
-			    instp_mode == I915_EXEC_CONSTANTS_REL_SURFACE) {
-				DRM_DEBUG("rel surface constants mode invalid on gen5+\n");
-				return -EINVAL;
-			}
+		if (INTEL_INFO(dev_priv)->gen < 4) {
+			DRM_DEBUG("no rel constants on pre-gen4\n");
+			return -EINVAL;
+		}
 
-			/* The HW changed the meaning on this bit on gen6 */
-			if (INTEL_INFO(dev_priv)->gen >= 6)
-				instp_mask &= ~I915_EXEC_CONSTANTS_REL_SURFACE;
+		if (INTEL_INFO(dev_priv)->gen > 5 &&
+		    instp_mode == I915_EXEC_CONSTANTS_REL_SURFACE) {
+			DRM_DEBUG("rel surface constants mode invalid on gen5+\n");
+			return -EINVAL;
 		}
+
+		/* The HW changed the meaning on this bit on gen6 */
+		if (INTEL_INFO(dev_priv)->gen >= 6)
+			instp_mask &= ~I915_EXEC_CONSTANTS_REL_SURFACE;
+
 		break;
 	default:
 		DRM_DEBUG("execbuf with unknown constants: %d\n", instp_mode);
 		return -EINVAL;
 	}
 
-	if (params->engine->id == RCS &&
-	    instp_mode != dev_priv->relative_constants_mode) {
+	if (params->engine->id == RCS) {
 		struct intel_ring *ring = params->request->ring;
 
 		ret = intel_ring_begin(params->request, 4);
@@ -1497,8 +1495,6 @@ static void eb_export_fence(struct drm_i915_gem_object *obj,
 		intel_ring_emit_reg(ring, INSTPM);
 		intel_ring_emit(ring, instp_mask << 16 | instp_mode);
 		intel_ring_advance(ring);
-
-		dev_priv->relative_constants_mode = instp_mode;
 	}
 
 	if (args->flags & I915_EXEC_GEN7_SOL_RESET) {
-- 
1.9.1