[PATCH 4/4] drm/i915/gem: Replace reloc chain with terminator on error unwind

[Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>]

If we hit an error during construction of the reloc chain, we need to
replace the chain into the next batch with the terminator so that upon
flushing the relocations so far, we do not execute a hanging batch.

Reported-by: Pavel Machek <pavel@xxxxxx>
Fixes: 964a9b0f611e ("drm/i915/gem: Use chained reloc batches")
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
Cc: Pavel Machek <pavel@xxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v5.8+
---
 .../gpu/drm/i915/gem/i915_gem_execbuffer.c    | 31 ++++++++++---------
 1 file changed, 16 insertions(+), 15 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
index 24a1486d2dc5..a09f04eee417 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
@@ -972,21 +972,6 @@ static int reloc_gpu_chain(struct reloc_cache *cache)
 	if (err)
 		goto out_pool;
 
-	GEM_BUG_ON(cache->rq_size + RELOC_TAIL > PAGE_SIZE  / sizeof(u32));
-	cmd = cache->rq_cmd + cache->rq_size;
-	*cmd++ = MI_ARB_CHECK;
-	if (cache->gen >= 8)
-		*cmd++ = MI_BATCH_BUFFER_START_GEN8;
-	else if (cache->gen >= 6)
-		*cmd++ = MI_BATCH_BUFFER_START;
-	else
-		*cmd++ = MI_BATCH_BUFFER_START | MI_BATCH_GTT;
-	*cmd++ = lower_32_bits(batch->node.start);
-	*cmd++ = upper_32_bits(batch->node.start); /* Always 0 for gen<8 */
-	i915_gem_object_flush_map(cache->rq_vma->obj);
-	i915_gem_object_unpin_map(cache->rq_vma->obj);
-	cache->rq_vma = NULL;
-
 	err = intel_gt_buffer_pool_mark_active(pool, rq);
 	if (err == 0) {
 		i915_vma_lock(batch);
@@ -999,15 +984,31 @@ static int reloc_gpu_chain(struct reloc_cache *cache)
 	if (err)
 		goto out_pool;
 
+	GEM_BUG_ON(cache->rq_size + RELOC_TAIL > PAGE_SIZE  / sizeof(u32));
+	cmd = cache->rq_cmd + cache->rq_size;
+	*cmd++ = MI_ARB_CHECK;
+	if (cache->gen >= 8)
+		*cmd++ = MI_BATCH_BUFFER_START_GEN8;
+	else if (cache->gen >= 6)
+		*cmd++ = MI_BATCH_BUFFER_START;
+	else
+		*cmd++ = MI_BATCH_BUFFER_START | MI_BATCH_GTT;
+	*cmd++ = lower_32_bits(batch->node.start);
+	*cmd++ = upper_32_bits(batch->node.start); /* Always 0 for gen<8 */
+
 	cmd = i915_gem_object_pin_map(batch->obj,
 				      cache->has_llc ?
 				      I915_MAP_FORCE_WB :
 				      I915_MAP_FORCE_WC);
 	if (IS_ERR(cmd)) {
+		/* We will replace the BBS with BBE upon flushing the rq */
 		err = PTR_ERR(cmd);
 		goto out_pool;
 	}
 
+	i915_gem_object_flush_map(cache->rq_vma->obj);
+	i915_gem_object_unpin_map(cache->rq_vma->obj);
+
 	/* Return with batch mapping (cmd) still pinned */
 	cache->rq_cmd = cmd;
 	cache->rq_size = 0;
-- 
2.20.1