[Intel-gfx] [PATCH 3/3] drm/i915: Enable render context support for gen4 (Broadwater to Cantiga)

[Chris Wilson]

Broadwater and the rest of gen4  do support being able to saving and
reloading context specific registers between contexts, providing isolation
of the basic GPU state (as programmable by userspace). This allows
userspace to assume that the GPU retains their state from one batch to the
next, minimising the amount of state it needs to reload and manually save
across batches.

v2: CONSTANT_BUFFER woes

Running through piglit turned up an interesting issue, a GPU hang inside
the context load. The context image includes the CONSTANT_BUFFER command
that loads an address into a on-gpu buffer, and the context load was
executing that immediately. However, since it was reading from the GTT
there is no guarantee that the GTT retains the same configuration as
when the context was saved, resulting in stray reads and a GPU hang.

Having tried issuing a CONSTANT_BUFFER (to disable the command) from the
ring before saving the context to no avail, we resort to patching out
the instruction inside the context image before loading.

This does impose that gen4 always reissues CONSTANT_BUFFER commands on
each batch, but due to the use of a shared GTT that was and will remain
a requirement.

v3: ECOSPKD to the rescue

Ville found the magic bit in the ECOSPKD to disable saving and restoring
the CONSTANT_BUFFER from the context image, thereby completely avoiding
the GPU hangs from chasing invalid pointers. This appears to be the
default behaviour for gen5, and so we just need to tweak gen4 to match.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Kenneth Graunke <kenneth at whitecape.org>
---
 drivers/gpu/drm/i915/i915_reg.h         |  3 +++
 drivers/gpu/drm/i915/intel_engine_cs.c  |  2 +-
 drivers/gpu/drm/i915/intel_ringbuffer.c | 14 ++++++++++++++
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index b74824f0b5b1..5815703ac35f 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -2665,6 +2665,9 @@ enum i915_power_well_id {
 # define MODE_IDLE					(1 << 9)
 # define STOP_RING					(1 << 8)
 
+#define ECOSPKD		_MMIO(0x21d0)
+# define CONSTANT_BUFFER_SR_DISABLE BIT(4)
+
 #define GEN6_GT_MODE	_MMIO(0x20d0)
 #define GEN7_GT_MODE	_MMIO(0x7008)
 #define   GEN6_WIZ_HASHING(hi, lo)			(((hi) << 9) | ((lo) << 7))
diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c b/drivers/gpu/drm/i915/intel_engine_cs.c
index fc8be2fcb4e6..f9db2e0bca12 100644
--- a/drivers/gpu/drm/i915/intel_engine_cs.c
+++ b/drivers/gpu/drm/i915/intel_engine_cs.c
@@ -211,6 +211,7 @@ __intel_engine_context_size(struct drm_i915_private *dev_priv, u8 class)
 			return round_up(GEN6_CXT_TOTAL_SIZE(cxt_size) * 64,
 					PAGE_SIZE);
 		case 5:
+		case 4:
 			/*
 			 * There is a discrepancy here between the size reported
 			 * by the register and the size of the context layout
@@ -227,7 +228,6 @@ __intel_engine_context_size(struct drm_i915_private *dev_priv, u8 class)
 					 cxt_size * 64,
 					 cxt_size - 1);
 			return round_up(cxt_size * 64, PAGE_SIZE);
-		case 4:
 		case 3:
 		case 2:
 		/* For the special day when i810 gets merged. */
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 2d2e33cd3fae..26b276ed00b3 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -832,6 +832,20 @@ static int init_render_ring(struct intel_engine_cs *engine)
 {
 	struct drm_i915_private *dev_priv = engine->i915;
 
+	/*
+	 * Disable CONSTANT_BUFFER before it is loaded from the context
+	 * image. For as it is loaded, it is executed and the stored
+	 * address may no longer be valid, leading to a GPU hang.
+	 *
+	 * This imposes the requirement that userspace reload their
+	 * CONSTANT_BUFFER on every batch, fortunately a requirement
+	 * they are already accustomed to from before contexts were
+	 * enabled.
+	 */
+	if (IS_GEN(dev_priv, 4))
+		I915_WRITE(ECOSPKD,
+			   _MASKED_BIT_ENABLE(CONSTANT_BUFFER_SR_DISABLE));
+
 	/* WaTimedSingleVertexDispatch:cl,bw,ctg,elk,ilk,snb */
 	if (IS_GEN_RANGE(dev_priv, 4, 6))
 		I915_WRITE(MI_MODE, _MASKED_BIT_ENABLE(VS_TIMER_DISPATCH));
-- 
2.20.1