drm/i915: Do NOT skip the first 4k of stolen memory for pre-allocated buffers v2

Hans de Goede · jwrdegoede · commit 011f22eb545a · 2018-04-21T10:05:38.000+02:00
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 d435376 ("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 (see note below). 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). Some notes about the mapping of the BIOS framebuffer: v1 led to some discussion if the assumption of the intel_display.c code that the firmware framebuffer is a linear mapping of the stolen memory starting at offset 0 is still correct, because that would mean that the GOP does not implement the WaSkipStolenMemoryFirstPage workaround. To verify this the following code was added at the end of i915_gem_object_create_stolen_for_preallocated() : pr_err("first ggtt entry before bind: 0x%016llx\n", readq(dev_priv->ggtt.gsm)); ret = i915_vma_bind(vma, HAS_LLC(dev_priv) ? I915_CACHE_LLC : I915_CACHE_NONE, PIN_UPDATE); pr_err("i915_vma_bind ret %d\n", ret); pr_err("first ggtt entry after bind: 0x%016llx\n", readq(dev_priv->ggtt.gsm)); Which prints the mapping of the first page, then does a vma_bind() to force update the mapping with our linear view of the framebuffer and then prints the mapping of the first page again. On an Asrock B150M Pro4S/D3 mainboard with i5-6500 CPU this prints: [ 1.651141] first ggtt entry before bind: 0x0000000078c00001 [ 1.651151] i915_vma_bind ret 0 [ 1.651152] first ggtt entry after bind: 0x0000000078c00083 And "sudo cat /proc/iomem | grep Stolen" gives: 78c00000-88bfffff : Graphics Stolen Memory There are no visual changes with this patch (BIOS vendor logo still stays in place when we inherit the BIOS framebuffer), so the vma_bind() does not impact which memory is being scanned out. The address of the first ggtt entry matches with the start of stolen and the i915_vma_bind call only changes the first gtt entry's flags, or-ing in _PAGE_RW (BIT(1)) and PPAT_CACHED (BIT(7)), which perfectly matches what we would expect based on gen8_pte_encode()'s behavior. So it seems that the GOP indeed does NOT implement the wa and the i915's code assuming a linear mapping at the start of stolen for the BIOS fb still holds true for gen8+. I've also tested this on a Cherry Trail based device (a GPD Win) with identical results (the flags are 0x1b after the vma_bind on CHT, which matches with I915_CACHE_NONE). Changed in v2: No code changes, extended the commit message with the verification that the intel_display.c BIOS framebuffer mapping is still correct. Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> Signed-off-by: Hans de Goede <hdegoede@redhat.com> Link: https://patchwork.freedesktop.org/patch/msgid/20180420095933.16442-1-hdegoede@redhat.com
diff --git 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;
 }
