[PATCH v3] drm/i915: Fallback to using unmappable memory for scanout

[Chris Wilson <chris@chris-wilson.co.uk>]

The existing ABI says that scanouts are pinned into the mappable region
so that legacy clients (e.g. old Xorg or plymouthd) can write directly
into the scanout through a GTT mapping. However if the surface does not
fit into the mappable region, we are better off just trying to fit it
anywhere and hoping for the best. (Any userspace that is cappable of
using ginormous scanouts is also likely not to rely on pure GTT
updates.) In the future, there may even be a kernel mediated method for
the legacy clients.

v2: Skip fence pinning when not mappable.
v3: Add a comment to explain the possible rammifactions of not being
    able to use fences for unmappable scanouts.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Satyanantha, Rama Gopal M <rama.gopal.m.satyanantha@intel.com>
Cc: Deepak S <deepak.s@linux.intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
---
 drivers/gpu/drm/i915/i915_gem.c      |  7 ++++++-
 drivers/gpu/drm/i915/intel_display.c | 34 ++++++++++++++++++++++++----------
 drivers/gpu/drm/i915/intel_fbc.c     |  4 ++++
 3 files changed, 34 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index cdba1597c905..3383aa0aab5e 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4042,12 +4042,17 @@ i915_gem_object_pin_to_display_plane(struct drm_i915_gem_object *obj,
 
 	/* As the user may map the buffer once pinned in the display plane
 	 * (e.g. libkms for the bootup splash), we have to ensure that we
-	 * always use map_and_fenceable for all scanout buffers.
+	 * always use map_and_fenceable for all scanout buffers. However,
+	 * it may simply be too big to fit into mappable, in which case
+	 * put it anyway and hope that userspace can cope (but always first
+	 * try to preserve the existing ABI).
 	 */
 	ret = i915_gem_object_ggtt_pin(obj, view, 0, alignment,
 				       view->type == I915_GGTT_VIEW_NORMAL ?
 				       PIN_MAPPABLE : 0);
 	if (ret)
+		ret = i915_gem_obj_ggtt_pin(obj, alignment, 0);
+	if (ret)
 		goto err_unpin_display;
 
 	i915_gem_object_flush_cpu_write_domain(obj);
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index dcb2b6ed3a65..2a566aef58a6 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -2394,16 +2394,29 @@ intel_pin_and_fence_fb_obj(struct drm_plane *plane,
 	if (ret)
 		goto err_interruptible;
 
-	/* Install a fence for tiled scan-out. Pre-i965 always needs a
-	 * fence, whereas 965+ only requires a fence if using
-	 * framebuffer compression.  For simplicity, we always install
-	 * a fence as the cost is not that onerous.
-	 */
-	ret = i915_gem_object_get_fence(obj);
-	if (ret)
-		goto err_unpin;
+	if (obj->map_and_fenceable) {
+		/* Install a fence for tiled scan-out. Pre-i965 always needs a
+		 * fence, whereas 965+ only requires a fence if using
+		 * framebuffer compression.  For simplicity, we always, when
+		 * possible, install a fence as the cost is not that onerous.
+		 *
+		 * If we fail to fence the tiled scanout, then either the
+		 * modeset will reject the change (which is highly unlikely as
+		 * the affected systems, all but one, do not have unmappable
+		 * space) or we will not be able to enable full powersaving
+		 * techniques (also likely not to apply due to various limits
+		 * FBC and the like impose on the size of the buffer, which
+		 * presumably we violated anyway with this unmappable buffer).
+		 * Anyway, it is presumably better to stumble onwards with
+		 * something and try to run the system in a "less than optimal"
+		 * mode that matches the user configuration.
+		 */
+		ret = i915_gem_object_get_fence(obj);
+		if (ret)
+			goto err_unpin;
 
-	i915_gem_object_pin_fence(obj);
+		i915_gem_object_pin_fence(obj);
+	}
 
 	dev_priv->mm.interruptible = true;
 	intel_runtime_pm_put(dev_priv);
@@ -2429,7 +2442,8 @@ static void intel_unpin_fb_obj(struct drm_framebuffer *fb,
 	ret = intel_fill_fb_ggtt_view(&view, fb, plane_state);
 	WARN_ONCE(ret, "Couldn't get view from plane state!");
 
-	i915_gem_object_unpin_fence(obj);
+	if (obj->map_and_fenceable)
+		i915_gem_object_unpin_fence(obj);
 	i915_gem_object_unpin_from_display_plane(obj, &view);
 }
 
diff --git a/drivers/gpu/drm/i915/intel_fbc.c b/drivers/gpu/drm/i915/intel_fbc.c
index 9fcf446e95f5..7ee1baade095 100644
--- a/drivers/gpu/drm/i915/intel_fbc.c
+++ b/drivers/gpu/drm/i915/intel_fbc.c
@@ -578,6 +578,10 @@ void intel_fbc_update(struct drm_device *dev)
 
 	/* The use of a CPU fence is mandatory in order to detect writes
 	 * by the CPU to the scanout and trigger updates to the FBC.
+	 *
+	 * Note that is possible for a tiled surface to be unmappable (and
+	 * so have no fence associated with it) due to aperture constaints
+	 * at the time of pinning.
 	 */
 	if (obj->tiling_mode != I915_TILING_X ||
 	    obj->fence_reg == I915_FENCE_REG_NONE) {
-- 
2.1.4

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