drm/i915/execlists: Trim irq handler

I noticed that gcc was spilling the CSB to the stack, so rearrange the
code to be more compact. Spilling in this function is slightly more
interesting due to the mmio reads acting as memory barriers and so
end up flushing the stack spills. Still miniscule to having to do at
least the pair of uncached reads :(

function                                     old     new   delta
intel_lrc_irq_handler                       1039     878    -161

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/20170325201053.21306-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/intel_lrc.c

@@ -538,7 +538,7 @@ static void intel_lrc_irq_handler(unsigned long data)
 			dev_priv->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_PTR(engine));
 		u32 __iomem *buf =
 			dev_priv->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_BUF_LO(engine, 0));
-		unsigned int csb, head, tail;
+		unsigned int head, tail;
 
 		/* The write will be ordered by the uncached read (itself
 		 * a memory barrier), so we do not need another in the form
@@ -551,17 +551,14 @@ static void intel_lrc_irq_handler(unsigned long data)
 		 * is set and we do a new loop.
 		 */
 		__clear_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted);
-		csb = readl(csb_mmio);
-		head = GEN8_CSB_READ_PTR(csb);
-		tail = GEN8_CSB_WRITE_PTR(csb);
-		if (head == tail)
-			break;
+		head = readl(csb_mmio);
+		tail = GEN8_CSB_WRITE_PTR(head);
+		head = GEN8_CSB_READ_PTR(head);
+		while (head != tail) {
+			unsigned int status;
 
-		if (tail < head)
-			tail += GEN8_CSB_ENTRIES;
-		do {
-			unsigned int idx = ++head % GEN8_CSB_ENTRIES;
-			unsigned int status = readl(buf + 2 * idx);
+			if (++head == GEN8_CSB_ENTRIES)
+				head = 0;
 
 			/* We are flying near dragons again.
 			 *
@@ -580,11 +577,12 @@ static void intel_lrc_irq_handler(unsigned long data)
 			 * status notifier.
 			 */
 
+			status = readl(buf + 2 * head);
 			if (!(status & GEN8_CTX_STATUS_COMPLETED_MASK))
 				continue;
 
 			/* Check the context/desc id for this event matches */
-			GEM_DEBUG_BUG_ON(readl(buf + 2 * idx + 1) !=
+			GEM_DEBUG_BUG_ON(readl(buf + 2 * head + 1) !=
 					 port[0].context_id);
 
 			GEM_BUG_ON(port[0].count == 0);
@@ -602,10 +600,9 @@ static void intel_lrc_irq_handler(unsigned long data)
 
 			GEM_BUG_ON(port[0].count == 0 &&
 				   !(status & GEN8_CTX_STATUS_ACTIVE_IDLE));
-		} while (head < tail);
+		}
 
-		writel(_MASKED_FIELD(GEN8_CSB_READ_PTR_MASK,
-				     GEN8_CSB_WRITE_PTR(csb) << 8),
+		writel(_MASKED_FIELD(GEN8_CSB_READ_PTR_MASK, head << 8),
 		       csb_mmio);
 	}