[Intel-gfx] [PATCH] drm/i915/ringbuffer: Delay after invalidating gen6+ xcs

[Chris Wilson]

During stress testing of full-ppgtt (on Baytrail at least), we found
that the invalidation around a context/mm switch was insufficient (writes
would go astray). Adding a second MI_FLUSH_DW barrier prevents this, but
it is unclear as to whether this is merely a delaying tactic or if it is
truly serialising with the TLB invalidation. Either way, it is
empirically required.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107715
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
Cc: Matthew Auld <matthew.william.auld at gmail.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/intel_ringbuffer.c | 101 +++++++++++-------------
 1 file changed, 47 insertions(+), 54 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index d40f55a8dc34..952b6269bab0 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -1944,40 +1944,62 @@ static void gen6_bsd_submit_request(struct i915_request *request)
 	intel_uncore_forcewake_put(dev_priv, FORCEWAKE_ALL);
 }
 
-static int gen6_bsd_ring_flush(struct i915_request *rq, u32 mode)
+static int emit_mi_flush_dw(struct i915_request *rq, u32 mode, u32 invflags)
 {
 	u32 cmd, *cs;
 
-	cs = intel_ring_begin(rq, 4);
-	if (IS_ERR(cs))
-		return PTR_ERR(cs);
+	do {
+		cs = intel_ring_begin(rq, 4);
+		if (IS_ERR(cs))
+			return PTR_ERR(cs);
 
-	cmd = MI_FLUSH_DW;
+		cmd = MI_FLUSH_DW;
 
-	/* We always require a command barrier so that subsequent
-	 * commands, such as breadcrumb interrupts, are strictly ordered
-	 * wrt the contents of the write cache being flushed to memory
-	 * (and thus being coherent from the CPU).
-	 */
-	cmd |= MI_FLUSH_DW_STORE_INDEX | MI_FLUSH_DW_OP_STOREDW;
+		/*
+		 * We always require a command barrier so that subsequent
+		 * commands, such as breadcrumb interrupts, are strictly ordered
+		 * wrt the contents of the write cache being flushed to memory
+		 * (and thus being coherent from the CPU).
+		 */
+		cmd |= MI_FLUSH_DW_STORE_INDEX | MI_FLUSH_DW_OP_STOREDW;
 
-	/*
-	 * Bspec vol 1c.5 - video engine command streamer:
-	 * "If ENABLED, all TLBs will be invalidated once the flush
-	 * operation is complete. This bit is only valid when the
-	 * Post-Sync Operation field is a value of 1h or 3h."
-	 */
-	if (mode & EMIT_INVALIDATE)
-		cmd |= MI_INVALIDATE_TLB | MI_INVALIDATE_BSD;
+		/*
+		 * Bspec vol 1c.3 - blitter engine command streamer:
+		 * "If ENABLED, all TLBs will be invalidated once the flush
+		 * operation is complete. This bit is only valid when the
+		 * Post-Sync Operation field is a value of 1h or 3h."
+		 */
+		if (mode & EMIT_INVALIDATE)
+			cmd |= invflags;
+		*cs++ = cmd;
+		*cs++ = I915_GEM_HWS_SCRATCH_ADDR | MI_FLUSH_DW_USE_GTT;
+		*cs++ = 0;
+		*cs++ = MI_NOOP;
+		intel_ring_advance(rq, cs);
+
+		/*
+		 * Not only do we need a full barrier (post-sync write) after
+		 * invalidating the TLBs, but we need to wait a little bit
+		 * longer. Whether this is merely delaying us, or the
+		 * subsequent flush is a key part of serialising with the
+		 * post-sync op, this extra pass appears vital before a
+		 * mm switch!
+		 */
+		if (!(mode & EMIT_INVALIDATE))
+			break;
+
+		mode &= ~EMIT_INVALIDATE;
+	} while (1);
 
-	*cs++ = cmd;
-	*cs++ = I915_GEM_HWS_SCRATCH_ADDR | MI_FLUSH_DW_USE_GTT;
-	*cs++ = 0;
-	*cs++ = MI_NOOP;
-	intel_ring_advance(rq, cs);
 	return 0;
 }
 
+static int gen6_bsd_ring_flush(struct i915_request *rq, u32 mode)
+{
+	return emit_mi_flush_dw(rq, mode,
+				MI_INVALIDATE_TLB | MI_INVALIDATE_BSD);
+}
+
 static int
 hsw_emit_bb_start(struct i915_request *rq,
 		  u64 offset, u32 len,
@@ -2022,36 +2044,7 @@ gen6_emit_bb_start(struct i915_request *rq,
 
 static int gen6_ring_flush(struct i915_request *rq, u32 mode)
 {
-	u32 cmd, *cs;
-
-	cs = intel_ring_begin(rq, 4);
-	if (IS_ERR(cs))
-		return PTR_ERR(cs);
-
-	cmd = MI_FLUSH_DW;
-
-	/* We always require a command barrier so that subsequent
-	 * commands, such as breadcrumb interrupts, are strictly ordered
-	 * wrt the contents of the write cache being flushed to memory
-	 * (and thus being coherent from the CPU).
-	 */
-	cmd |= MI_FLUSH_DW_STORE_INDEX | MI_FLUSH_DW_OP_STOREDW;
-
-	/*
-	 * Bspec vol 1c.3 - blitter engine command streamer:
-	 * "If ENABLED, all TLBs will be invalidated once the flush
-	 * operation is complete. This bit is only valid when the
-	 * Post-Sync Operation field is a value of 1h or 3h."
-	 */
-	if (mode & EMIT_INVALIDATE)
-		cmd |= MI_INVALIDATE_TLB;
-	*cs++ = cmd;
-	*cs++ = I915_GEM_HWS_SCRATCH_ADDR | MI_FLUSH_DW_USE_GTT;
-	*cs++ = 0;
-	*cs++ = MI_NOOP;
-	intel_ring_advance(rq, cs);
-
-	return 0;
+	return emit_mi_flush_dw(rq, mode, MI_INVALIDATE_TLB);
 }
 
 static void intel_ring_init_semaphores(struct drm_i915_private *dev_priv,
-- 
2.18.0