[1/3] drm/i915/pmu: avoid an maybe-uninitialized warning

Message ID 20200527140526.1458215-1-arnd@arndb.de
State New
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  • [1/3] drm/i915/pmu: avoid an maybe-uninitialized warning
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Commit Message

Arnd Bergmann May 27, 2020, 2:05 p.m.
Conditional spinlocks make it hard for gcc and for lockdep to
follow the code flow. This one causes a warning with at least
gcc-9 and higher:

In file included from include/linux/irq.h:14,
                 from drivers/gpu/drm/i915/i915_pmu.c:7:
drivers/gpu/drm/i915/i915_pmu.c: In function 'i915_sample':
include/linux/spinlock.h:289:3: error: 'flags' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  289 |   _raw_spin_unlock_irqrestore(lock, flags); \
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/i915/i915_pmu.c:288:17: note: 'flags' was declared here
  288 |   unsigned long flags;
      |                 ^~~~~

Split out the part between the locks into a separate function
for readability and to let the compiler figure out what the
logic actually is.

Fixes: d79e1bd676f0 ("drm/i915/pmu: Only use exclusive mmio access for gen7")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

---
I have no idea why I see three separate issues like this pop up in i915,
there are not a lot of them elsewhere.

 drivers/gpu/drm/i915/i915_pmu.c | 84 ++++++++++++++++-----------------
 1 file changed, 42 insertions(+), 42 deletions(-)

-- 
2.26.2

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Comments

Chris Wilson May 27, 2020, 3:42 p.m. | #1
Quoting Arnd Bergmann (2020-05-27 15:05:08)
> Conditional spinlocks make it hard for gcc and for lockdep to

> follow the code flow. This one causes a warning with at least

> gcc-9 and higher:

> 

> In file included from include/linux/irq.h:14,

>                  from drivers/gpu/drm/i915/i915_pmu.c:7:

> drivers/gpu/drm/i915/i915_pmu.c: In function 'i915_sample':

> include/linux/spinlock.h:289:3: error: 'flags' may be used uninitialized in this function [-Werror=maybe-uninitialized]

>   289 |   _raw_spin_unlock_irqrestore(lock, flags); \

>       |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~

> drivers/gpu/drm/i915/i915_pmu.c:288:17: note: 'flags' was declared here

>   288 |   unsigned long flags;

>       |                 ^~~~~

> 

> Split out the part between the locks into a separate function

> for readability and to let the compiler figure out what the

> logic actually is.

> 

> Fixes: d79e1bd676f0 ("drm/i915/pmu: Only use exclusive mmio access for gen7")

> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

> ---

> I have no idea why I see three separate issues like this pop up in i915,

> there are not a lot of them elsewhere.


gcc v8:
add/remove: 1/0 grow/shrink: 0/1 up/down: 99/-164 (-65)
Function                                     old     new   delta
engine_sample                                  -      99     +99
i915_sample                                  871     707    -164

Which is compelling. Is gcc 9 simliar?

Given the above reduction, I find it hard to argue with.
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

-Chris
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Patch

diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
index e991a707bdb7..962ded9ce73f 100644
--- a/drivers/gpu/drm/i915/i915_pmu.c
+++ b/drivers/gpu/drm/i915/i915_pmu.c
@@ -269,12 +269,48 @@  static bool exclusive_mmio_access(const struct drm_i915_private *i915)
 	return IS_GEN(i915, 7);
 }
 
+static void engine_sample(struct intel_engine_cs *engine, unsigned int period_ns)
+{
+	struct intel_engine_pmu *pmu = &engine->pmu;
+	bool busy;
+	u32 val;
+
+	val = ENGINE_READ_FW(engine, RING_CTL);
+	if (val == 0) /* powerwell off => engine idle */
+		return;
+
+	if (val & RING_WAIT)
+		add_sample(&pmu->sample[I915_SAMPLE_WAIT], period_ns);
+	if (val & RING_WAIT_SEMAPHORE)
+		add_sample(&pmu->sample[I915_SAMPLE_SEMA], period_ns);
+
+	/* No need to sample when busy stats are supported. */
+	if (intel_engine_supports_stats(engine))
+		return;
+
+	/*
+	 * While waiting on a semaphore or event, MI_MODE reports the
+	 * ring as idle. However, previously using the seqno, and with
+	 * execlists sampling, we account for the ring waiting as the
+	 * engine being busy. Therefore, we record the sample as being
+	 * busy if either waiting or !idle.
+	 */
+	busy = val & (RING_WAIT_SEMAPHORE | RING_WAIT);
+	if (!busy) {
+		val = ENGINE_READ_FW(engine, RING_MI_MODE);
+		busy = !(val & MODE_IDLE);
+	}
+	if (busy)
+		add_sample(&pmu->sample[I915_SAMPLE_BUSY], period_ns);
+}
+
 static void
 engines_sample(struct intel_gt *gt, unsigned int period_ns)
 {
 	struct drm_i915_private *i915 = gt->i915;
 	struct intel_engine_cs *engine;
 	enum intel_engine_id id;
+	unsigned long flags;
 
 	if ((i915->pmu.enable & ENGINE_SAMPLE_MASK) == 0)
 		return;
@@ -283,53 +319,17 @@  engines_sample(struct intel_gt *gt, unsigned int period_ns)
 		return;
 
 	for_each_engine(engine, gt, id) {
-		struct intel_engine_pmu *pmu = &engine->pmu;
-		spinlock_t *mmio_lock;
-		unsigned long flags;
-		bool busy;
-		u32 val;
-
 		if (!intel_engine_pm_get_if_awake(engine))
 			continue;
 
-		mmio_lock = NULL;
-		if (exclusive_mmio_access(i915))
-			mmio_lock = &engine->uncore->lock;
-
-		if (unlikely(mmio_lock))
-			spin_lock_irqsave(mmio_lock, flags);
-
-		val = ENGINE_READ_FW(engine, RING_CTL);
-		if (val == 0) /* powerwell off => engine idle */
-			goto skip;
-
-		if (val & RING_WAIT)
-			add_sample(&pmu->sample[I915_SAMPLE_WAIT], period_ns);
-		if (val & RING_WAIT_SEMAPHORE)
-			add_sample(&pmu->sample[I915_SAMPLE_SEMA], period_ns);
-
-		/* No need to sample when busy stats are supported. */
-		if (intel_engine_supports_stats(engine))
-			goto skip;
-
-		/*
-		 * While waiting on a semaphore or event, MI_MODE reports the
-		 * ring as idle. However, previously using the seqno, and with
-		 * execlists sampling, we account for the ring waiting as the
-		 * engine being busy. Therefore, we record the sample as being
-		 * busy if either waiting or !idle.
-		 */
-		busy = val & (RING_WAIT_SEMAPHORE | RING_WAIT);
-		if (!busy) {
-			val = ENGINE_READ_FW(engine, RING_MI_MODE);
-			busy = !(val & MODE_IDLE);
+		if (exclusive_mmio_access(i915)) {
+			spin_lock_irqsave(&engine->uncore->lock, flags);
+			engine_sample(engine, period_ns);
+			spin_unlock_irqrestore(&engine->uncore->lock, flags);
+		} else {
+			engine_sample(engine, period_ns);
 		}
-		if (busy)
-			add_sample(&pmu->sample[I915_SAMPLE_BUSY], period_ns);
 
-skip:
-		if (unlikely(mmio_lock))
-			spin_unlock_irqrestore(mmio_lock, flags);
 		intel_engine_pm_put_async(engine);
 	}
 }