drm/i915/selftests: Skip poisoning SET_PREDICATE_RESULT on dg2

When predication is enabled all commands baring a few (such as MI_BB_END)
are nop'ed. If we accidentally enable predication while poisoning the
context, not only is the rest of the poisoning skipped (thus disabling
the test), but the closing instructions of the poison request are
nop'ed. Not only do we then not signal the waiting context, but we even
prevent re-enabling arbitration and the GPU will not perform a context
switch at the end of the request.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Suggested-by: CQ Tang <cq.tang@intel.com>
Signed-off-by: Chris Wilson <chris.p.wilson@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220425152317.4275-3-ramalingam.c@intel.com

drivers/gpu/drm/i915/gt/intel_engine_regs.h

@@ -193,6 +193,7 @@
 #define RING_TIMESTAMP_UDW(base)		_MMIO((base) + 0x358 + 4)
 #define RING_CONTEXT_STATUS_PTR(base)		_MMIO((base) + 0x3a0)
 #define RING_CTX_TIMESTAMP(base)		_MMIO((base) + 0x3a8) /* gen8+ */
+#define RING_PREDICATE_RESULT(base)		_MMIO((base) + 0x3b8)
 #define RING_FORCE_TO_NONPRIV(base, i)		_MMIO(((base) + 0x4D0) + (i) * 4)
 #define   RING_FORCE_TO_NONPRIV_ADDRESS_MASK	REG_GENMASK(25, 2)
 #define   RING_FORCE_TO_NONPRIV_ACCESS_RW	(0 << 28)    /* CFL+ & Gen11+ */

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

@@ -945,6 +945,19 @@ create_user_vma(struct i915_address_space *vm, unsigned long size)
 	return vma;
 }
 
+static u32 safe_poison(u32 offset, u32 poison)
+{
+	/*
+	 * Do not enable predication as it will nop all subsequent commands,
+	 * not only disabling the tests (by preventing all the other SRM) but
+	 * also preventing the arbitration events at the end of the request.
+	 */
+	if (offset == i915_mmio_reg_offset(RING_PREDICATE_RESULT(0)))
+		poison &= ~REG_BIT(0);
+
+	return poison;
+}
+
 static struct i915_vma *
 store_context(struct intel_context *ce, struct i915_vma *scratch)
 {
@@ -1154,7 +1167,9 @@ static struct i915_vma *load_context(struct intel_context *ce, u32 poison)
 		*cs++ = MI_LOAD_REGISTER_IMM(len);
 		while (len--) {
 			*cs++ = hw[dw];
-			*cs++ = poison;
+			*cs++ = safe_poison(hw[dw] & get_lri_mask(ce->engine,
+								  MI_LRI_LRM_CS_MMIO),
+					    poison);
 			dw += 2;
 		}
 	} while (dw < PAGE_SIZE / sizeof(u32) &&