5 files changed, 41 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index 343e79a44abd..23e9a86cbc2a 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -441,6 +441,9 @@ static int i915_getparam_ioctl(struct drm_device *dev, void *data,
 	case I915_PARAM_CS_TIMESTAMP_FREQUENCY:
 		value = 1000 * INTEL_INFO(dev_priv)->cs_timestamp_frequency_khz;
 		break;
+	case I915_PARAM_MMAP_GTT_COHERENT:
+		value = INTEL_INFO(dev_priv)->has_coherent_ggtt;
+		break;
 	default:
 		DRM_DEBUG("Unknown parameter %d\n", param->param);
 		return -EINVAL;
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index fcc73a6ab503..8b52cb768a67 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -802,6 +802,11 @@ void i915_gem_flush_ggtt_writes(struct drm_i915_private *dev_priv)
 	 * that was!).
 	 */
 
+	wmb();
+
+	if (INTEL_INFO(dev_priv)->has_coherent_ggtt)
+		return;
+
 	i915_gem_chipset_flush(dev_priv);
 
 	intel_runtime_pm_get(dev_priv);
diff --git a/drivers/gpu/drm/i915/i915_pci.c b/drivers/gpu/drm/i915/i915_pci.c
index 6a4d1388ad2d..e443fe44da3a 100644
--- a/drivers/gpu/drm/i915/i915_pci.c
+++ b/drivers/gpu/drm/i915/i915_pci.c
@@ -74,6 +74,7 @@
 	.unfenced_needs_alignment = 1, \
 	.ring_mask = RENDER_RING, \
 	.has_snoop = true, \
+	.has_coherent_ggtt = false, \
 	GEN_DEFAULT_PIPEOFFSETS, \
 	GEN_DEFAULT_PAGE_SIZES, \
 	CURSOR_OFFSETS
@@ -110,6 +111,7 @@ static const struct intel_device_info intel_i865g_info = {
 	.has_gmch_display = 1, \
 	.ring_mask = RENDER_RING, \
 	.has_snoop = true, \
+	.has_coherent_ggtt = true, \
 	GEN_DEFAULT_PIPEOFFSETS, \
 	GEN_DEFAULT_PAGE_SIZES, \
 	CURSOR_OFFSETS
@@ -117,6 +119,7 @@ static const struct intel_device_info intel_i865g_info = {
 static const struct intel_device_info intel_i915g_info = {
 	GEN3_FEATURES,
 	PLATFORM(INTEL_I915G),
+	.has_coherent_ggtt = false,
 	.cursor_needs_physical = 1,
 	.has_overlay = 1, .overlay_needs_physical = 1,
 	.hws_needs_physical = 1,
@@ -178,6 +181,7 @@ static const struct intel_device_info intel_pineview_info = {
 	.has_gmch_display = 1, \
 	.ring_mask = RENDER_RING, \
 	.has_snoop = true, \
+	.has_coherent_ggtt = true, \
 	GEN_DEFAULT_PIPEOFFSETS, \
 	GEN_DEFAULT_PAGE_SIZES, \
 	CURSOR_OFFSETS
@@ -220,6 +224,7 @@ static const struct intel_device_info intel_gm45_info = {
 	.has_hotplug = 1, \
 	.ring_mask = RENDER_RING | BSD_RING, \
 	.has_snoop = true, \
+	.has_coherent_ggtt = true, \
 	/* ilk does support rc6, but we do not implement [power] contexts */ \
 	.has_rc6 = 0, \
 	GEN_DEFAULT_PIPEOFFSETS, \
@@ -243,6 +248,7 @@ static const struct intel_device_info intel_ironlake_m_info = {
 	.has_hotplug = 1, \
 	.has_fbc = 1, \
 	.ring_mask = RENDER_RING | BSD_RING | BLT_RING, \
+	.has_coherent_ggtt = true, \
 	.has_llc = 1, \
 	.has_rc6 = 1, \
 	.has_rc6p = 1, \
@@ -287,6 +293,7 @@ static const struct intel_device_info intel_sandybridge_m_gt2_info = {
 	.has_hotplug = 1, \
 	.has_fbc = 1, \
 	.ring_mask = RENDER_RING | BSD_RING | BLT_RING, \
+	.has_coherent_ggtt = true, \
 	.has_llc = 1, \
 	.has_rc6 = 1, \
 	.has_rc6p = 1, \
@@ -347,6 +354,7 @@ static const struct intel_device_info intel_valleyview_info = {
 	.has_aliasing_ppgtt = 1,
 	.has_full_ppgtt = 1,
 	.has_snoop = true,
+	.has_coherent_ggtt = false,
 	.ring_mask = RENDER_RING | BSD_RING | BLT_RING,
 	.display_mmio_offset = VLV_DISPLAY_BASE,
 	GEN_DEFAULT_PAGE_SIZES,
@@ -441,6 +449,7 @@ static const struct intel_device_info intel_cherryview_info = {
 	.has_full_ppgtt = 1,
 	.has_reset_engine = 1,
 	.has_snoop = true,
+	.has_coherent_ggtt = false,
 	.display_mmio_offset = VLV_DISPLAY_BASE,
 	GEN_DEFAULT_PAGE_SIZES,
 	GEN_CHV_PIPEOFFSETS,
@@ -517,6 +526,7 @@ static const struct intel_device_info intel_skylake_gt4_info = {
 	.has_full_48bit_ppgtt = 1, \
 	.has_reset_engine = 1, \
 	.has_snoop = true, \
+	.has_coherent_ggtt = false, \
 	.has_ipc = 1, \
 	GEN9_DEFAULT_PAGE_SIZES, \
 	GEN_DEFAULT_PIPEOFFSETS, \
diff --git a/drivers/gpu/drm/i915/intel_device_info.h b/drivers/gpu/drm/i915/intel_device_info.h
index 633f9fbf72ea..07e8364d1a8c 100644
--- a/drivers/gpu/drm/i915/intel_device_info.h
+++ b/drivers/gpu/drm/i915/intel_device_info.h
@@ -106,6 +106,7 @@ enum intel_platform {
 	func(has_resource_streamer); \
 	func(has_runtime_pm); \
 	func(has_snoop); \
+	func(has_coherent_ggtt); \
 	func(unfenced_needs_alignment); \
 	func(cursor_needs_physical); \
 	func(hws_needs_physical); \
diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index 7f5634ce8e88..a4446f452040 100644
--- a/include/uapi/drm/i915_drm.h
+++ b/include/uapi/drm/i915_drm.h
@@ -529,6 +529,28 @@ typedef struct drm_i915_irq_wait {
  */
 #define I915_PARAM_CS_TIMESTAMP_FREQUENCY 51
 
+/*
+ * Once upon a time we supposed that writes through the GGTT would be
+ * immediately in physical memory (once flushed out of the CPU path). However,
+ * on a few different processors and chipsets, this is not necessarily the case
+ * as the writes appear to be buffered internally. Thus a read of the backing
+ * storage (physical memory) via a different path (with different physical tags
+ * to the indirect write via the GGTT) will see stale values from before
+ * the GGTT write. Inside the kernel, we can for the most part keep track of
+ * the different read/write domains in use (e.g. set-domain), but the assumption
+ * of coherency is baked into the ABI, hence reporting its true state in this
+ * parameter.
+ *
+ * Reports true when writes via mmap_gtt are immediately visible following an
+ * lfence to flush the WCB.
+ *
+ * Reports false when writes via mmap_gtt are indeterminately delayed in an in
+ * internal buffer and are _not_ immediately visible to third parties accessing
+ * directly via mmap_cpu/mmap_wc. Use of mmap_gtt as part of an IPC
+ * communications channel when reporting false is strongly disadvised.
+ */
+#define I915_PARAM_MMAP_GTT_COHERENT	52
+
 typedef struct drm_i915_getparam {
 	__s32 param;
 	/*