drm/i915: alloc/free the FBC CFB during enable/disable

One of the problems with the current code is that it frees the CFB and
releases its drm_mm node as soon as we flip FBC's enable bit. This is
bad because after we disable FBC the hardware may still use the CFB
for the rest of the frame, so in theory we should only release the
drm_mm node one frame after we disable FBC. Otherwise, a stolen memory
allocation done right after an FBC disable may result in either
corrupted memory for the new owner of that memory region or corrupted
screen/underruns in case the new owner changes it while the hardware
is still reading it. This case is not exactly easy to reproduce since
we currently don't do a lot of stolen memory allocations, but I see
patches on the mailing list trying to expose stolen memory to user
space, so races will be possible.

I thought about three different approaches to solve this, and they all
have downsides.

The first approach would be to simply use multiple drm_mm nodes and
freeing the unused ones only after a frame has passed. The problem
with this approach is that since stolen memory is rather small,
there's a risk we just won't be able to allocate a new CFB from stolen
if the previous one was not freed yet. This could happen in case we
quickly disable FBC from pipe A and decide to enable it on pipe B, or
just if we change pipe A's fb stride while FBC is enabled.

The second approach would be similar to the first one, but maintaining
a single drm_mm node and keeping track of when it can be reused. This
would remove the disadvantage of not having enough space for two
nodes, but would create the new problem where we may not be able to
enable FBC at the point intel_fbc_update() is called, so we would have
to add more code to retry updating FBC after the time has passed. And
that can quickly get too complex since we can get invalidate, flush,
disable and other calls in the middle of the wait.

Both solutions above - and also the current code - have the problem
that we unnecessarily free+realloc FBC during invalidate+flush
operations even if the CFB size doesn't change.

The third option would be to move the allocation/deallocation to
enable/disable. This makes sure that the pipe is always disabled when
we allocate/deallocate the CFB, so there's no risk that the FBC
hardware may read or write to the memory right after it is freed from
drm_mm. The downside is that it is possible for user space to change
the buffer stride without triggering a disable/enable - only
deactivate/activate -, so we'll have to handle this case somehow - see
igt's kms_frontbuffer_tracking test, fbc-stridechange subtest. It
could be possible to implement a way to free+alloc the CFB during said
stride change, but it would involve a lot of book-keeping - exactly as
mentioned above - just for on case, so for now I'll keep it simple and
just deactivate FBC. Besides, we may not even need to disable FBC
since we do CFB over-allocation.

Note from Chris: "Starting a fullscreen client that covers a single
monitor in a multi-monitor setup will trigger a change in stride on
one of the CRTCs (the monitors will be flipped independently).". It
shouldn't be a huge problem if we lose FBC on multi-monitor setups
since these setups already have problems reaching deep PC states
anyway.

v2: Rebase after changing the patch order.
v3:
  - Remove references to the stride change case being "uncommon" and
    paste Chris' example.
  - Rebase after a change in a previous patch.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/

drivers/gpu/drm/i915/intel_fbc.c

@@ -64,6 +64,46 @@ static unsigned int get_crtc_fence_y_offset(struct intel_crtc *crtc)
 	return crtc->base.y - crtc->adjusted_y;
 }
 
+/*
+ * For SKL+, the plane source size used by the hardware is based on the value we
+ * write to the PLANE_SIZE register. For BDW-, the hardware looks at the value
+ * we wrote to PIPESRC.
+ */
+static void intel_fbc_get_plane_source_size(struct intel_crtc *crtc,
+					    int *width, int *height)
+{
+	struct intel_plane_state *plane_state =
+			to_intel_plane_state(crtc->base.primary->state);
+	int w, h;
+
+	if (intel_rotation_90_or_270(plane_state->base.rotation)) {
+		w = drm_rect_height(&plane_state->src) >> 16;
+		h = drm_rect_width(&plane_state->src) >> 16;
+	} else {
+		w = drm_rect_width(&plane_state->src) >> 16;
+		h = drm_rect_height(&plane_state->src) >> 16;
+	}
+
+	if (width)
+		*width = w;
+	if (height)
+		*height = h;
+}
+
+static int intel_fbc_calculate_cfb_size(struct intel_crtc *crtc,
+					struct drm_framebuffer *fb)
+{
+	struct drm_i915_private *dev_priv = crtc->base.dev->dev_private;
+	int lines;
+
+	intel_fbc_get_plane_source_size(crtc, NULL, &lines);
+	if (INTEL_INFO(dev_priv)->gen >= 7)
+		lines = min(lines, 2048);
+
+	/* Hardware needs the full buffer stride, not just the active area. */
+	return lines * fb->pitches[0];
+}
+
 static void i8xx_fbc_deactivate(struct drm_i915_private *dev_priv)
 {
 	u32 fbc_ctl;
@@ -558,11 +598,17 @@ again:
 	}
 }
 
-static int intel_fbc_alloc_cfb(struct drm_i915_private *dev_priv, int size,
-			       int fb_cpp)
+static int intel_fbc_alloc_cfb(struct intel_crtc *crtc)
 {
+	struct drm_i915_private *dev_priv = crtc->base.dev->dev_private;
+	struct drm_framebuffer *fb = crtc->base.primary->state->fb;
 	struct drm_mm_node *uninitialized_var(compressed_llb);
-	int ret;
+	int size, fb_cpp, ret;
+
+	WARN_ON(drm_mm_node_allocated(&dev_priv->fbc.compressed_fb));
+
+	size = intel_fbc_calculate_cfb_size(crtc, fb);
+	fb_cpp = drm_format_plane_cpp(fb->pixel_format, 0);
 
 	ret = find_compression_threshold(dev_priv, &dev_priv->fbc.compressed_fb,
 					 size, fb_cpp);
@@ -639,65 +685,6 @@ void intel_fbc_cleanup_cfb(struct drm_i915_private *dev_priv)
 	mutex_unlock(&dev_priv->fbc.lock);
 }
 
-/*
- * For SKL+, the plane source size used by the hardware is based on the value we
- * write to the PLANE_SIZE register. For BDW-, the hardware looks at the value
- * we wrote to PIPESRC.
- */
-static void intel_fbc_get_plane_source_size(struct intel_crtc *crtc,
-					    int *width, int *height)
-{
-	struct intel_plane_state *plane_state =
-			to_intel_plane_state(crtc->base.primary->state);
-	int w, h;
-
-	if (intel_rotation_90_or_270(plane_state->base.rotation)) {
-		w = drm_rect_height(&plane_state->src) >> 16;
-		h = drm_rect_width(&plane_state->src) >> 16;
-	} else {
-		w = drm_rect_width(&plane_state->src) >> 16;
-		h = drm_rect_height(&plane_state->src) >> 16;
-	}
-
-	if (width)
-		*width = w;
-	if (height)
-		*height = h;
-}
-
-static int intel_fbc_calculate_cfb_size(struct intel_crtc *crtc)
-{
-	struct drm_i915_private *dev_priv = crtc->base.dev->dev_private;
-	struct drm_framebuffer *fb = crtc->base.primary->fb;
-	int lines;
-
-	intel_fbc_get_plane_source_size(crtc, NULL, &lines);
-	if (INTEL_INFO(dev_priv)->gen >= 7)
-		lines = min(lines, 2048);
-
-	/* Hardware needs the full buffer stride, not just the active area. */
-	return lines * fb->pitches[0];
-}
-
-static int intel_fbc_setup_cfb(struct intel_crtc *crtc)
-{
-	struct drm_i915_private *dev_priv = crtc->base.dev->dev_private;
-	struct drm_framebuffer *fb = crtc->base.primary->fb;
-	int size, cpp;
-
-	size = intel_fbc_calculate_cfb_size(crtc);
-	cpp = drm_format_plane_cpp(fb->pixel_format, 0);
-
-	if (drm_mm_node_allocated(&dev_priv->fbc.compressed_fb) &&
-	    size <= dev_priv->fbc.compressed_fb.size * dev_priv->fbc.threshold)
-		return 0;
-
-	/* Release any current block */
-	__intel_fbc_cleanup_cfb(dev_priv);
-
-	return intel_fbc_alloc_cfb(dev_priv, size, cpp);
-}
-
 static bool stride_is_valid(struct drm_i915_private *dev_priv,
 			    unsigned int stride)
 {
@@ -853,8 +840,19 @@ static void __intel_fbc_update(struct intel_crtc *crtc)
 		goto out_disable;
 	}
 
-	if (intel_fbc_setup_cfb(crtc)) {
-		set_no_fbc_reason(dev_priv, "not enough stolen memory");
+	/* It is possible for the required CFB size change without a
+	 * crtc->disable + crtc->enable since it is possible to change the
+	 * stride without triggering a full modeset. Since we try to
+	 * over-allocate the CFB, there's a chance we may keep FBC enabled even
+	 * if this happens, but if we exceed the current CFB size we'll have to
+	 * disable FBC. Notice that it would be possible to disable FBC, wait
+	 * for a frame, free the stolen node, then try to reenable FBC in case
+	 * we didn't get any invalidate/deactivate calls, but this would require
+	 * a lot of tracking just for a specific case. If we conclude it's an
+	 * important case, we can implement it later. */
+	if (intel_fbc_calculate_cfb_size(crtc, fb) >
+	    dev_priv->fbc.compressed_fb.size * dev_priv->fbc.threshold) {
+		set_no_fbc_reason(dev_priv, "CFB requirements changed");
 		goto out_disable;
 	}
 
@@ -907,7 +905,6 @@ out_disable:
 		DRM_DEBUG_KMS("unsupported config, deactivating FBC\n");
 		__intel_fbc_deactivate(dev_priv);
 	}
-	__intel_fbc_cleanup_cfb(dev_priv);
 }
 
 /*
@@ -1020,6 +1017,11 @@ void intel_fbc_enable(struct intel_crtc *crtc)
 		goto out;
 	}
 
+	if (intel_fbc_alloc_cfb(crtc)) {
+		set_no_fbc_reason(dev_priv, "not enough stolen memory");
+		goto out;
+	}
+
 	DRM_DEBUG_KMS("Enabling FBC on pipe %c\n", pipe_name(crtc->pipe));
 	dev_priv->fbc.no_fbc_reason = "FBC enabled but not active yet\n";
 
@@ -1047,6 +1049,8 @@ static void __intel_fbc_disable(struct drm_i915_private *dev_priv)
 
 	DRM_DEBUG_KMS("Disabling FBC on pipe %c\n", pipe_name(crtc->pipe));
 
+	__intel_fbc_cleanup_cfb(dev_priv);
+
 	dev_priv->fbc.enabled = false;
 	dev_priv->fbc.crtc = NULL;
 }