drm/i915: Enable render context support for gen4 (Broadwater to Cantiga)

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: ECOSKPD to the rescue

Ville found the magic bit in the ECOSKPD 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.

v4: Fix spelling of ECOSKPD and discover it already exists
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190419172720.5462-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_engine_cs.c

@@ -213,6 +213,7 @@ __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
@@ -229,7 +230,6 @@ __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. */

drivers/gpu/drm/i915/gt/intel_ringbuffer.c

@@ -834,6 +834,20 @@ static int rcs_resume(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(ECOSKPD,
+			   _MASKED_BIT_ENABLE(ECO_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));

drivers/gpu/drm/i915/i915_reg.h

@@ -2870,6 +2870,7 @@ enum i915_power_well_id {
 #define GFX_FLSH_CNTL_GEN6	_MMIO(0x101008)
 #define   GFX_FLSH_CNTL_EN	(1 << 0)
 #define ECOSKPD		_MMIO(0x21d0)
+#define   ECO_CONSTANT_BUFFER_SR_DISABLE REG_BIT(4)
 #define   ECO_GATING_CX_ONLY	(1 << 3)
 #define   ECO_FLIP_DONE		(1 << 0)