From a6b0a141289db58d4934345fa2617602bf859a44 Mon Sep 17 00:00:00 2001
From: Chris Wilson <chris@chris-wilson.co.uk>
Date: Wed, 15 Mar 2017 22:22:59 +0000
Subject: [PATCH] drm/i915/breadcrumbs: Tweak commentary
Tvrtko spotted a stale reference to b->lock (now b->rb_lock) so review
the comments and try to improve them in passing.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170315222259.1469-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
---
drivers/gpu/drm/i915/intel_breadcrumbs.c | 25 ++++++++++++++++--------
1 file changed, 17 insertions(+), 8 deletions(-)
diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
index cb6985acc542c..ba986edee3127 100644
--- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
@@ -85,7 +85,7 @@ static void intel_breadcrumbs_hangcheck(unsigned long data)
return;
}
- /* We keep the hangcheck time alive until we disarm the irq, even
+ /* We keep the hangcheck timer alive until we disarm the irq, even
* if there are no waiters at present.
*
* If the waiter was currently running, assume it hasn't had a chance
@@ -110,12 +110,11 @@ static void intel_breadcrumbs_fake_irq(unsigned long data)
struct intel_engine_cs *engine = (struct intel_engine_cs *)data;
struct intel_breadcrumbs *b = &engine->breadcrumbs;
- /*
- * The timer persists in case we cannot enable interrupts,
+ /* The timer persists in case we cannot enable interrupts,
* or if we have previously seen seqno/interrupt incoherency
- * ("missed interrupt" syndrome). Here the worker will wake up
- * every jiffie in order to kick the oldest waiter to do the
- * coherent seqno check.
+ * ("missed interrupt" syndrome, better known as a "missed breadcrumb").
+ * Here the worker will wake up every jiffie in order to kick the
+ * oldest waiter to do the coherent seqno check.
*/
spin_lock_irq(&b->irq_lock);
@@ -290,7 +289,12 @@ static inline void __intel_breadcrumbs_finish(struct intel_breadcrumbs *b,
GEM_BUG_ON(b->irq_wait == wait);
/* This request is completed, so remove it from the tree, mark it as
- * complete, and *then* wake up the associated task.
+ * complete, and *then* wake up the associated task. N.B. when the
+ * task wakes up, it will find the empty rb_node, discern that it
+ * has already been removed from the tree and skip the serialisation
+ * of the b->rb_lock and b->irq_lock. This means that the destruction
+ * of the intel_wait is not serialised with the interrupt handler
+ * by the waiter - it must instead be serialised by the caller.
*/
rb_erase(&wait->node, &b->waiters);
RB_CLEAR_NODE(&wait->node);
@@ -397,6 +401,11 @@ static bool __intel_engine_add_wait(struct intel_engine_cs *engine,
}
if (completed) {
+ /* Advance the bottom-half (b->irq_wait) before we wake up
+ * the waiters who may scribble over their intel_wait
+ * just as the interrupt handler is dereferencing it via
+ * b->irq_wait.
+ */
if (!first) {
struct rb_node *next = rb_next(completed);
GEM_BUG_ON(next == &wait->node);
@@ -653,7 +662,7 @@ void intel_engine_enable_signaling(struct drm_i915_gem_request *request)
/* Note that we may be called from an interrupt handler on another
* device (e.g. nouveau signaling a fence completion causing us
* to submit a request, and so enable signaling). As such,
- * we need to make sure that all other users of b->lock protect
+ * we need to make sure that all other users of b->rb_lock protect
* against interrupts, i.e. use spin_lock_irqsave.
*/
--
2.30.2