drm/i915/migrate: Account for the reserved_space

If the ring is nearly full when calling into emit_pte(), we might
incorrectly trample the reserved_space when constructing the packet to
emit the PTEs. This then triggers the GEM_BUG_ON(rq->reserved_space >
ring->space) when later submitting the request, since the request itself
doesn't have enough space left in the ring to emit things like
workarounds, breadcrumbs etc.

v2: Fix the whitespace errors

Testcase: igt@i915_selftests@live_emit_pte_full_ring
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/7535
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/6889
Fixes: cf586021 ("drm/i915/gt: Pipelined page migration")
Signed-off-by: Chris Wilson <chris.p.wilson@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Cc: <stable@vger.kernel.org> # v5.15+
Tested-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221202122844.428006-1-matthew.auld@intel.com

drivers/gpu/drm/i915/gt/intel_migrate.c

@@ -342,6 +342,16 @@ static int emit_no_arbitration(struct i915_request *rq)
 	return 0;
 }
 
+static int max_pte_pkt_size(struct i915_request *rq, int pkt)
+{
+	struct intel_ring *ring = rq->ring;
+
+	pkt = min_t(int, pkt, (ring->space - rq->reserved_space) / sizeof(u32) + 5);
+	pkt = min_t(int, pkt, (ring->size - ring->emit) / sizeof(u32) + 5);
+
+	return pkt;
+}
+
 static int emit_pte(struct i915_request *rq,
 		    struct sgt_dma *it,
 		    enum i915_cache_level cache_level,
@@ -388,8 +398,7 @@ static int emit_pte(struct i915_request *rq,
 		return PTR_ERR(cs);
 
 	/* Pack as many PTE updates as possible into a single MI command */
-	pkt = min_t(int, dword_length, ring->space / sizeof(u32) + 5);
-	pkt = min_t(int, pkt, (ring->size - ring->emit) / sizeof(u32) + 5);
+	pkt = max_pte_pkt_size(rq, dword_length);
 
 	hdr = cs;
 	*cs++ = MI_STORE_DATA_IMM | REG_BIT(21); /* as qword elements */
@@ -422,8 +431,7 @@ static int emit_pte(struct i915_request *rq,
 				}
 			}
 
-			pkt = min_t(int, dword_rem, ring->space / sizeof(u32) + 5);
-			pkt = min_t(int, pkt, (ring->size - ring->emit) / sizeof(u32) + 5);
+			pkt = max_pte_pkt_size(rq, dword_rem);
 
 			hdr = cs;
 			*cs++ = MI_STORE_DATA_IMM | REG_BIT(21);