From: Chris Wilson <chris@chris-wilson.co.uk>
Date: Mon, 11 Jun 2018 11:48:08 +0100
Subject: drm/i915/ringbuffer: Brute force context restore
Git-commit: 1fc719d13ac07b082ff9daa8f40254680cade111
Patch-mainline: v4.19-rc1
References: FATE#326289 FATE#326079 FATE#326049 FATE#322398 FATE#326166

An issue encountered with switching mm on gen7 is that the GPU likes to
hang (with the VS unit busy) when told to force restore the current
context. We can simply workaround this by substituting the
MI_FORCE_RESTORE flag with a round-trip through the kernel_context,
forcing the context to be saved and restored; thereby reloading the
PP_DIR registers and updating the modified page directory!

v2: Undo attempted optimisation in caller (Tvrtko)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180611104808.24295-1-chris@chris-wilson.co.uk

Acked-by: Petr Tesarik <ptesarik@suse.com>
---
 drivers/gpu/drm/i915/intel_ringbuffer.c |   27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -1458,6 +1458,7 @@ static inline int mi_set_context(struct
 		(HAS_LEGACY_SEMAPHORES(i915) && IS_GEN7(i915)) ?
 		INTEL_INFO(i915)->num_rings - 1 :
 		0;
+	bool force_restore = false;
 	int len;
 	u32 *cs;
 
@@ -1471,6 +1472,12 @@ static inline int mi_set_context(struct
 	len = 4;
 	if (IS_GEN7(i915))
 		len += 2 + (num_rings ? 4*num_rings + 6 : 0);
+	if (flags & MI_FORCE_RESTORE) {
+		GEM_BUG_ON(flags & MI_RESTORE_INHIBIT);
+		flags &= ~MI_FORCE_RESTORE;
+		force_restore = true;
+		len += 2;
+	}
 
 	cs = intel_ring_begin(rq, len);
 	if (IS_ERR(cs))
@@ -1495,6 +1502,26 @@ static inline int mi_set_context(struct
 		}
 	}
 
+	if (force_restore) {
+		/*
+		 * The HW doesn't handle being told to restore the current
+		 * context very well. Quite often it likes goes to go off and
+		 * sulk, especially when it is meant to be reloading PP_DIR.
+		 * A very simple fix to force the reload is to simply switch
+		 * away from the current context and back again.
+		 *
+		 * Note that the kernel_context will contain random state
+		 * following the INHIBIT_RESTORE. We accept this since we
+		 * never use the kernel_context state; it is merely a
+		 * placeholder we use to flush other contexts.
+		 */
+		*cs++ = MI_SET_CONTEXT;
+		*cs++ = i915_ggtt_offset(to_intel_context(i915->kernel_context,
+							  engine)->state) |
+			MI_MM_SPACE_GTT |
+			MI_RESTORE_INHIBIT;
+	}
+
 	*cs++ = MI_NOOP;
 	*cs++ = MI_SET_CONTEXT;
 	*cs++ = i915_ggtt_offset(rq->hw_context->state) | flags;