[Intel-gfx] [PATCH] drm/i915: avoid "may be used uninitialised" warnings

[Dave Gordon]

gcc is getting false positives in its detection of uninitialised values.
Specifically it thinks 'gtt_entry' can be used in a WARN_ON() macro
without previously being assigned (the assigment is inside a conditional
loop), bu actually the WARN_ON() can only be reached if the assignment
has also been executed at least once.

To avoid the annoying warning, though, this patch reorganises the code a
little and adds an explicit initialisation of the suspect variable.

Signed-off-by: Dave Gordon <david.s.gordon at intel.com>
---
 drivers/gpu/drm/i915/i915_gem_gtt.c | 19 ++++++++++++-------
 drivers/gpu/drm/i915/i915_gem_gtt.h |  1 +
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
index 30da543..90e1cf3 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
@@ -2368,7 +2368,7 @@ static void gen8_ggtt_insert_entries(struct i915_address_space *vm,
 	struct i915_ggtt *ggtt = i915_vm_to_ggtt(vm);
 	struct sgt_iter sgt_iter;
 	gen8_pte_t __iomem *gtt_entries;
-	gen8_pte_t gtt_entry;
+	gen8_pte_t gtt_entry = I915_NULL_PTE;
 	dma_addr_t addr;
 	int rpm_atomic_seq;
 	int i = 0;
@@ -2389,8 +2389,10 @@ static void gen8_ggtt_insert_entries(struct i915_address_space *vm,
 	 * of NUMA access patterns. Therefore, even with the way we assume
 	 * hardware should work, we must keep this posting read for paranoia.
 	 */
-	if (i != 0)
-		WARN_ON(readq(&gtt_entries[i-1]) != gtt_entry);
+	if (i != 0) {
+		gen8_pte_t last_gtt_entry = readq(&gtt_entries[i-1]);
+		WARN_ON(last_gtt_entry != gtt_entry);
+	}
 
 	/* This next bit makes the above posting read even more important. We
 	 * want to flush the TLBs only after we're certain all the PTE updates
@@ -2465,7 +2467,7 @@ static void gen6_ggtt_insert_entries(struct i915_address_space *vm,
 	struct i915_ggtt *ggtt = i915_vm_to_ggtt(vm);
 	struct sgt_iter sgt_iter;
 	gen6_pte_t __iomem *gtt_entries;
-	gen6_pte_t gtt_entry;
+	gen6_pte_t gtt_entry = I915_NULL_PTE;
 	dma_addr_t addr;
 	int rpm_atomic_seq;
 	int i = 0;
@@ -2479,14 +2481,17 @@ static void gen6_ggtt_insert_entries(struct i915_address_space *vm,
 		iowrite32(gtt_entry, &gtt_entries[i++]);
 	}
 
-	/* XXX: This serves as a posting read to make sure that the PTE has
+	/*
+	 * XXX: This serves as a posting read to make sure that the PTE has
 	 * actually been updated. There is some concern that even though
 	 * registers and PTEs are within the same BAR that they are potentially
 	 * of NUMA access patterns. Therefore, even with the way we assume
 	 * hardware should work, we must keep this posting read for paranoia.
 	 */
-	if (i != 0)
-		WARN_ON(readl(&gtt_entries[i-1]) != gtt_entry);
+	if (i != 0) {
+		gen8_pte_t last_gtt_entry = readl(&gtt_entries[i-1]);
+		WARN_ON(last_gtt_entry != gtt_entry);
+	}
 
 	/* This next bit makes the above posting read even more important. We
 	 * want to flush the TLBs only after we're certain all the PTE updates
diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.h b/drivers/gpu/drm/i915/i915_gem_gtt.h
index c4a6579..e088210 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.h
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.h
@@ -54,6 +54,7 @@ typedef uint64_t gen8_ppgtt_pml4e_t;
 #define GEN6_PTE_UNCACHED		(1 << 1)
 #define GEN6_PTE_VALID			(1 << 0)
 
+#define	I915_NULL_PTE			0
 #define I915_PTES(pte_len)		(PAGE_SIZE / (pte_len))
 #define I915_PTE_MASK(pte_len)		(I915_PTES(pte_len) - 1)
 #define I915_PDES			512
-- 
1.9.1