[PATCH] drm/i915: Do NOT skip the first 4k of stolen memory for pre-allocated buffers

[Hans de Goede]

Before this commit the WaSkipStolenMemoryFirstPage workaround code was
skipping the first 4k by passing 4096 as start of the address range passed
to drm_mm_init(). This means that calling drm_mm_reserve_node() to try and
reserve the firmware framebuffer so that we can inherit it would always
fail, as the firmware framebuffer starts at address 0.

Commit d43537610470 ("drm/i915: skip the first 4k of stolen memory on
everything >= gen8") says in its commit message: "This is confirmed to fix
Skylake screen flickering issues (probably caused by the fact that we
initialized a ring in the first page of stolen, but I didn't 100% confirm
this theory)."

Which suggests that it is safe to use the first page for a linear
framebuffer as the firmware is doing.

This commit always passes 0 as start to drm_mm_init() and works around
WaSkipStolenMemoryFirstPage in i915_gem_stolen_insert_node_in_range()
by insuring the start address passed by to drm_mm_insert_node_in_range()
is always 4k or more. All entry points to i915_gem_stolen.c go through
i915_gem_stolen_insert_node_in_range(), so that any newly allocated
objects such as ring-buffers will not be allocated in the first 4k.

The one exception is i915_gem_object_create_stolen_for_preallocated()
which directly calls drm_mm_reserve_node() which now will be able to
use the first 4k.

This fixes the i915 driver no longer being able to inherit the firmware
framebuffer on gen8+, which fixes the video output changing from the
vendor logo to a black screen as soon as the i915 driver is loaded
(on systems without fbcon).

Signed-off-by: Hans de Goede <hdegoede at redhat.com>
---
 drivers/gpu/drm/i915/i915_gem_stolen.c | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_stolen.c b/drivers/gpu/drm/i915/i915_gem_stolen.c
index af915d041281..ad949cc30928 100644
--- a/drivers/gpu/drm/i915/i915_gem_stolen.c
+++ b/drivers/gpu/drm/i915/i915_gem_stolen.c
@@ -51,6 +51,10 @@ int i915_gem_stolen_insert_node_in_range(struct drm_i915_private *dev_priv,
 	if (!drm_mm_initialized(&dev_priv->mm.stolen))
 		return -ENODEV;
 
+	/* WaSkipStolenMemoryFirstPage:bdw+ */
+	if (INTEL_GEN(dev_priv) >= 8 && start < 4096)
+		start = 4096;
+
 	mutex_lock(&dev_priv->mm.stolen_lock);
 	ret = drm_mm_insert_node_in_range(&dev_priv->mm.stolen, node,
 					  size, alignment, 0,
@@ -343,7 +347,6 @@ int i915_gem_init_stolen(struct drm_i915_private *dev_priv)
 {
 	resource_size_t reserved_base, stolen_top;
 	resource_size_t reserved_total, reserved_size;
-	resource_size_t stolen_usable_start;
 
 	mutex_init(&dev_priv->mm.stolen_lock);
 
@@ -435,17 +438,11 @@ int i915_gem_init_stolen(struct drm_i915_private *dev_priv)
 			 (u64)resource_size(&dev_priv->dsm) >> 10,
 			 ((u64)resource_size(&dev_priv->dsm) - reserved_total) >> 10);
 
-	stolen_usable_start = 0;
-	/* WaSkipStolenMemoryFirstPage:bdw+ */
-	if (INTEL_GEN(dev_priv) >= 8)
-		stolen_usable_start = 4096;
-
 	dev_priv->stolen_usable_size =
-		resource_size(&dev_priv->dsm) - reserved_total - stolen_usable_start;
+		resource_size(&dev_priv->dsm) - reserved_total;
 
 	/* Basic memrange allocator for stolen space. */
-	drm_mm_init(&dev_priv->mm.stolen, stolen_usable_start,
-		    dev_priv->stolen_usable_size);
+	drm_mm_init(&dev_priv->mm.stolen, 0, dev_priv->stolen_usable_size);
 
 	return 0;
 }
-- 
2.17.0.rc2