drm/i915: Flush WC GGTT only on required platforms

gen8_ggtt_invalidate() is only needed for limited set of platforms
where GGTT is mapped as WC. This was added as way to fix WC based GGTT in
commit 0f9b91c7 ("drm/i915: flush system agent TLBs on SNB") and
there are no reference in HW docs that forces us to use this on non-WC
backed GGTT.

This can also cause unwanted side-effects on XE_HP platforms where
GFX_FLSH_CNTL_GEN6 is not valid anymore.

v2: Add a func to detect wc ggtt detection (Ville)
v3: Improve commit log and add reference commit (Daniel)

Fixes: d2eae8e9 ("drm/i915/dg2: Drop force_probe requirement")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Jonathan Cavitt <jonathan.cavitt@intel.com>
Cc: John Harrison <john.c.harrison@intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: <stable@vger.kernel.org> # v6.2+
Suggested-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231018093815.1349-1-nirmoy.das@intel.com

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

@@ -195,6 +195,21 @@ void gen6_ggtt_invalidate(struct i915_ggtt *ggtt)
 	spin_unlock_irq(&uncore->lock);
 }
 
+static bool needs_wc_ggtt_mapping(struct drm_i915_private *i915)
+{
+	/*
+	 * On BXT+/ICL+ writes larger than 64 bit to the GTT pagetable range
+	 * will be dropped. For WC mappings in general we have 64 byte burst
+	 * writes when the WC buffer is flushed, so we can't use it, but have to
+	 * resort to an uncached mapping. The WC issue is easily caught by the
+	 * readback check when writing GTT PTE entries.
+	 */
+	if (!IS_GEN9_LP(i915) && GRAPHICS_VER(i915) < 11)
+		return true;
+
+	return false;
+}
+
 static void gen8_ggtt_invalidate(struct i915_ggtt *ggtt)
 {
 	struct intel_uncore *uncore = ggtt->vm.gt->uncore;
@@ -202,8 +217,12 @@ static void gen8_ggtt_invalidate(struct i915_ggtt *ggtt)
 	/*
 	 * Note that as an uncached mmio write, this will flush the
 	 * WCB of the writes into the GGTT before it triggers the invalidate.
+	 *
+	 * Only perform this when GGTT is mapped as WC, see ggtt_probe_common().
 	 */
-	intel_uncore_write_fw(uncore, GFX_FLSH_CNTL_GEN6, GFX_FLSH_CNTL_EN);
+	if (needs_wc_ggtt_mapping(ggtt->vm.i915))
+		intel_uncore_write_fw(uncore, GFX_FLSH_CNTL_GEN6,
+				      GFX_FLSH_CNTL_EN);
 }
 
 static void guc_ggtt_ct_invalidate(struct intel_gt *gt)
@@ -1139,17 +1158,11 @@ static int ggtt_probe_common(struct i915_ggtt *ggtt, u64 size)
 	GEM_WARN_ON(pci_resource_len(pdev, GEN4_GTTMMADR_BAR) != gen6_gttmmadr_size(i915));
 	phys_addr = pci_resource_start(pdev, GEN4_GTTMMADR_BAR) + gen6_gttadr_offset(i915);
 
-	/*
-	 * On BXT+/ICL+ writes larger than 64 bit to the GTT pagetable range
-	 * will be dropped. For WC mappings in general we have 64 byte burst
-	 * writes when the WC buffer is flushed, so we can't use it, but have to
-	 * resort to an uncached mapping. The WC issue is easily caught by the
-	 * readback check when writing GTT PTE entries.
-	 */
-	if (IS_GEN9_LP(i915) || GRAPHICS_VER(i915) >= 11)
-		ggtt->gsm = ioremap(phys_addr, size);
-	else
+	if (needs_wc_ggtt_mapping(i915))
 		ggtt->gsm = ioremap_wc(phys_addr, size);
+	else
+		ggtt->gsm = ioremap(phys_addr, size);
+
 	if (!ggtt->gsm) {
 		drm_err(&i915->drm, "Failed to map the ggtt page table\n");
 		return -ENOMEM;