[Intel-gfx] [RFC 46/60] drm/i915: Provide a way to disable PCIe relaxed write ordering

[Matthew Auld]

From: Abdiel Janulgue <abdiel.janulgue at linux.intel.com>

For performance writes over PCIe may not be strictly ordered by default.
This provides an option to expose a kernel configuration option to disable
relaxed ordering and turn on strict ordering instead for debug purposes.

Signed-off-by: Abdiel Janulgue <abdiel.janulgue at linux.intel.com>
Signed-off-by: Stuart Summers <stuart.summers at intel.com>
Cc: Matthew Auld <matthew.auld at intel.com>
---
 drivers/gpu/drm/i915/Kconfig.debug         | 11 +++++++++++
 drivers/gpu/drm/i915/intel_memory_region.c | 12 ++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/drivers/gpu/drm/i915/Kconfig.debug b/drivers/gpu/drm/i915/Kconfig.debug
index 206882e154bc..114240e890aa 100644
--- a/drivers/gpu/drm/i915/Kconfig.debug
+++ b/drivers/gpu/drm/i915/Kconfig.debug
@@ -221,3 +221,14 @@ config DRM_I915_DEBUG_RUNTIME_PM
 	  driver loading, suspend and resume operations.
 
 	  If in doubt, say "N"
+
+config DRM_I915_PCIE_STRICT_WRITE_ORDERING
+	bool "Enable PCIe strict ordering "
+	depends on DRM_I915
+	default n
+	help
+	  Relaxed ordering in writes is enabled by default to improve system
+	  performance. Strict ordering can be selected instead to assist in
+	  debugging.
+
+	  If in doubt, say "N".
diff --git a/drivers/gpu/drm/i915/intel_memory_region.c b/drivers/gpu/drm/i915/intel_memory_region.c
index eeea520f97ba..d564b596efda 100644
--- a/drivers/gpu/drm/i915/intel_memory_region.c
+++ b/drivers/gpu/drm/i915/intel_memory_region.c
@@ -280,6 +280,18 @@ int intel_memory_regions_hw_probe(struct drm_i915_private *i915)
 {
 	int err, i;
 
+	/* All platforms currently have system memory */
+	GEM_BUG_ON(!HAS_REGION(i915, REGION_SMEM));
+
+	if (IS_DGFX(i915)) {
+		if (IS_ENABLED(CONFIG_DRM_I915_PCIE_STRICT_WRITE_ORDERING))
+			pcie_capability_clear_word(i915->drm.pdev, PCI_EXP_DEVCTL,
+						   PCI_EXP_DEVCTL_RELAX_EN);
+		else
+			pcie_capability_set_word(i915->drm.pdev, PCI_EXP_DEVCTL,
+						 PCI_EXP_DEVCTL_RELAX_EN);
+	}
+
 	for (i = 0; i < ARRAY_SIZE(i915->mm.regions); i++) {
 		struct intel_memory_region *mem = ERR_PTR(-ENODEV);
 		u16 type, instance;
-- 
2.26.2