[Intel-gfx] [PATCH v3 4/5] drm/i915/execlists: Read the context-status buffer from the HWSP

[Chris Wilson]

The engine provides a mirror of the CSB in the HWSP. If we use the
cacheable reads from the HWSP, we can shave off a few mmio reads per
context-switch interrupt (which are quite frequent!). Just removing a
couple of mmio is not enough to actually reduce any latency, but a small
reduction in overall cpu usage.

Much appreciation for Ben dropping the bombshell that the CSB was in the
HWSP and for Michel in digging out the details.

v2: Don't be lazy, add the defines for the indices.
v3: Include the HWSP in debugfs/i915_engine_info
v4: Check for GVT-g, it currently depends on intercepting CSB mmio

Suggested-by: Ben Widawsky <benjamin.widawsky at intel.com>
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry at intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Mika Kuoppala <mika.kuoppala at intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
Cc: Zhenyu Wang <zhenyuw at linux.intel.com>
Cc: Zhi Wang <zhi.a.wang at intel.com>
Acked-by: Michel Thierry <michel.thierry at intel.com>
---
 drivers/gpu/drm/i915/i915_debugfs.c     |  7 +++++--
 drivers/gpu/drm/i915/intel_lrc.c        | 16 +++++++++++-----
 drivers/gpu/drm/i915/intel_ringbuffer.h |  2 ++
 3 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
index 620c9218d1c1..5fd01c14a3ec 100644
--- a/drivers/gpu/drm/i915/i915_debugfs.c
+++ b/drivers/gpu/drm/i915/i915_debugfs.c
@@ -3384,6 +3384,7 @@ static int i915_engine_info(struct seq_file *m, void *unused)
 			   upper_32_bits(addr), lower_32_bits(addr));
 
 		if (i915.enable_execlists) {
+			const u32 *hws = &engine->status_page.page_addr[I915_HWS_CSB_BUF0_INDEX];
 			u32 ptr, read, write;
 			unsigned int idx;
 
@@ -3404,10 +3405,12 @@ static int i915_engine_info(struct seq_file *m, void *unused)
 				write += GEN8_CSB_ENTRIES;
 			while (read < write) {
 				idx = ++read % GEN8_CSB_ENTRIES;
-				seq_printf(m, "\tExeclist CSB[%d]: 0x%08x, context: %d\n",
+				seq_printf(m, "\tExeclist CSB[%d]: 0x%08x [0x%08x in hwsp], context: %d [%d in hwsp]\n",
 					   idx,
 					   I915_READ(RING_CONTEXT_STATUS_BUF_LO(engine, idx)),
-					   I915_READ(RING_CONTEXT_STATUS_BUF_HI(engine, idx)));
+					   hws[idx * 2],
+					   I915_READ(RING_CONTEXT_STATUS_BUF_HI(engine, idx)),
+					   hws[idx * 2 + 1]);
 			}
 
 			rcu_read_lock();
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 3469badedbe0..5b721f65d232 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -547,10 +547,17 @@ static void intel_lrc_irq_handler(unsigned long data)
 	while (test_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted)) {
 		u32 __iomem *csb_mmio =
 			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));
+		/* The HWSP contains a (cacheable) mirror of the CSB */
+		const u32 *buf =
+			&engine->status_page.page_addr[I915_HWS_CSB_BUF0_INDEX];
 		unsigned int head, tail;
 
+		/* However GVT emulation depends upon intercepting CSB mmio */
+		if (unlikely(intel_vgpu_active(dev_priv))) {
+			buf = (u32 * __force)
+				(dev_priv->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_PTR(engine)));
+		}
+
 		/* The write will be ordered by the uncached read (itself
 		 * a memory barrier), so we do not need another in the form
 		 * of a locked instruction. The race between the interrupt
@@ -590,13 +597,12 @@ static void intel_lrc_irq_handler(unsigned long data)
 			 * status notifier.
 			 */
 
-			status = readl(buf + 2 * head);
+			status = 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 * head + 1) !=
-					 port->context_id);
+			GEM_DEBUG_BUG_ON(buf[2 * head + 1] != port->context_id);
 
 			rq = port_unpack(port, &count);
 			GEM_BUG_ON(count == 0);
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index d33c93444c0d..2c55cfa14fb5 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -496,6 +496,8 @@ intel_write_status_page(struct intel_engine_cs *engine, int reg, u32 value)
 #define I915_GEM_HWS_SCRATCH_INDEX	0x40
 #define I915_GEM_HWS_SCRATCH_ADDR (I915_GEM_HWS_SCRATCH_INDEX << MI_STORE_DWORD_INDEX_SHIFT)
 
+#define I915_HWS_CSB_BUF0_INDEX		0x10
+
 struct intel_ring *
 intel_engine_create_ring(struct intel_engine_cs *engine, int size);
 int intel_ring_pin(struct intel_ring *ring,
-- 
2.13.2