On platforms capable of allowing 8K (7680 x 4320) modes, pinning 2 or
more framebuffers/scanout buffers results in only one that is mappable/
fenceable. Therefore, pageflipping between these 2 FBs where only one
is mappable/fenceable creates latencies large enough to miss alternate
vblanks thereby producing less optimal framerate.
This mainly happens because when i915_gem_object_pin_to_display_plane()
is called to pin one of the FB objs, the associated vma is identified
as misplaced and therefore i915_vma_unbind() is called which unbinds and
evicts it. This misplaced vma gets subseqently pinned only when
i915_gem_object_ggtt_pin_ww() is called without the mappable flag. This
results in a latency of ~10ms and happens every other vblank/repaint cycle.
Testcase:
Running Weston and weston-simple-egl on an Alderlake_S (ADLS) platform
with a 8K@60 mode results in only ~40 FPS. Since upstream Weston submits
a frame ~7ms before the next vblank, the latencies seen between atomic
commit and flip event are 7, 24 (7 + 16.66), 7, 24..... suggesting that
it misses the vblank every other frame.
Here is the ftrace snippet that shows the source of the ~10ms latency:
i915_gem_object_pin_to_display_plane() {
0.102 us | i915_gem_object_set_cache_level();
i915_gem_object_ggtt_pin_ww() {
0.390 us | i915_vma_instance();
0.178 us | i915_vma_misplaced();
i915_vma_unbind() {
__i915_active_wait() {
0.082 us | i915_active_acquire_if_busy();
0.475 us | }
intel_runtime_pm_get() {
0.087 us | intel_runtime_pm_acquire();
0.259 us | }
__i915_active_wait() {
0.085 us | i915_active_acquire_if_busy();
0.240 us | }
__i915_vma_evict() {
ggtt_unbind_vma() {
gen8_ggtt_clear_range() {
10507.255 us | }
10507.689 us | }
10508.516 us | }
v2: Instead of using bigjoiner checks, determine whether a scanout
buffer is too big by checking to see if it is possible to map
two of them into the ggtt.
Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
Cc: Manasi Navare <manasi.d.navare@xxxxxxxxx>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx>
---
drivers/gpu/drm/i915/i915_gem.c | 48 ++++++++++++++++++++++++++-------
drivers/gpu/drm/i915/i915_vma.c | 2 +-
2 files changed, 40 insertions(+), 10 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 981e383d1a5d..0050c7e4bb51 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -866,6 +866,44 @@ static void discard_ggtt_vma(struct i915_vma *vma)
spin_unlock(&obj->vma.lock);
}
+static bool i915_gem_obj_too_big(struct drm_i915_gem_object *obj)
+{
+ struct drm_i915_private *i915 = to_i915(obj->base.dev);
+ struct i915_ggtt *ggtt = &i915->ggtt;
+ struct drm_mm_node *hole;
+ u64 hole_start, hole_end;
+ u64 fence_size;
+
+ /*
+ * If the required space is larger than the available
+ * aperture, we will not able to find a slot for the
+ * object and unbinding the object now will be in
+ * vain. Worse, doing so may cause us to ping-pong
+ * the object in and out of the Global GTT and
+ * waste a lot of cycles under the mutex.
+ */
+ if (obj->base.size > ggtt->mappable_end)
+ return true;
+
+ fence_size = i915_gem_fence_size(i915, obj->base.size,
+ i915_gem_object_get_tiling(obj),
+ i915_gem_object_get_stride(obj));
+
+ /*
+ * Assuming this object is a large scanout buffer, we try to find
+ * out if there is room to map at-least two of them. There could
+ * be space available to map one but to be consistent, we try to
+ * avoid mapping/fencing any of them.
+ */
+ drm_mm_for_each_hole(hole, &ggtt->vm.mm, hole_start, hole_end) {
+ if (hole_end - hole_start > 2 * fence_size &&
+ hole_start + 2 * fence_size < ggtt->mappable_end)
+ return false;
+ }
+
+ return true;
+}
+
struct i915_vma *
i915_gem_object_ggtt_pin_ww(struct drm_i915_gem_object *obj,
struct i915_gem_ww_ctx *ww,
@@ -879,15 +917,7 @@ i915_gem_object_ggtt_pin_ww(struct drm_i915_gem_object *obj,
if (flags & PIN_MAPPABLE &&
(!view || view->type == I915_GGTT_VIEW_NORMAL)) {
- /*
- * If the required space is larger than the available
- * aperture, we will not able to find a slot for the
- * object and unbinding the object now will be in
- * vain. Worse, doing so may cause us to ping-pong
- * the object in and out of the Global GTT and
- * waste a lot of cycles under the mutex.
- */
- if (obj->base.size > ggtt->mappable_end)
+ if (i915_gem_obj_too_big(obj))
return ERR_PTR(-E2BIG);
/*
diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
index 90546fa58fc1..551644dbfa8a 100644
--- a/drivers/gpu/drm/i915/i915_vma.c
+++ b/drivers/gpu/drm/i915/i915_vma.c
@@ -977,7 +977,7 @@ int i915_vma_pin_ww(struct i915_vma *vma, struct i915_gem_ww_ctx *ww,
if (err)
goto err_active;
- if (i915_is_ggtt(vma->vm))
+ if (i915_is_ggtt(vma->vm) && flags & PIN_MAPPABLE)
__i915_vma_set_map_and_fenceable(vma);
}
--
2.31.1
