[PATCH v2] drm/i915: Add interface to reserve fence registers for vGPU

[changbin.du@intel.com]

From: Changbin Du <changbin.du@intel.com>

In the past, vGPU alloc fence registers by walking through mm.fence_list
to find fence which pin_count = 0 and vma is empty. vGPU may not find
enough fence registers this way. Because a fence can be bind to vma even
though it is not in using. We have found such failure many times these
days.

An option to resolve this issue is that we can force-remove fence from
vma in this case.

This patch added two new api to the fence management code:
 - i915_reserve_fence() will try to find a free fence from fence_list
   and force-remove vma if need.
 - i915_unreserve_fence() reclaim a reserved fence after vGPU has
   finished.

With this change, the fence management is more clear to work with vGPU.
GVTg do not need remove fence from fence_list in private.

v2: (Chris)
  - Rename the new api for symmetry.
  - Add safeguard to ensure at least 1 fence remained for host display.

Signed-off-by: Changbin Du <changbin.du@intel.com>
---
 drivers/gpu/drm/i915/gvt/aperture_gm.c    | 26 ++++++++----------
 drivers/gpu/drm/i915/i915_drv.h           |  3 +++
 drivers/gpu/drm/i915/i915_gem_fence_reg.c | 44 +++++++++++++++++++++++++++++++
 3 files changed, 58 insertions(+), 15 deletions(-)

diff --git a/drivers/gpu/drm/i915/gvt/aperture_gm.c b/drivers/gpu/drm/i915/gvt/aperture_gm.c
index ca3d192..7c9ec4f 100644
--- a/drivers/gpu/drm/i915/gvt/aperture_gm.c
+++ b/drivers/gpu/drm/i915/gvt/aperture_gm.c
@@ -173,8 +173,8 @@ static void free_vgpu_fence(struct intel_vgpu *vgpu)
 	_clear_vgpu_fence(vgpu);
 	for (i = 0; i < vgpu_fence_sz(vgpu); i++) {
 		reg = vgpu->fence.regs[i];
-		list_add_tail(&reg->link,
-			      &dev_priv->mm.fence_list);
+		i915_unreserve_fence(reg);
+		vgpu->fence.regs[i] = NULL;
 	}
 	mutex_unlock(&dev_priv->drm.struct_mutex);
 
@@ -187,24 +187,19 @@ static int alloc_vgpu_fence(struct intel_vgpu *vgpu)
 	struct drm_i915_private *dev_priv = gvt->dev_priv;
 	struct drm_i915_fence_reg *reg;
 	int i;
-	struct list_head *pos, *q;
 
 	intel_runtime_pm_get(dev_priv);
 
 	/* Request fences from host */
 	mutex_lock(&dev_priv->drm.struct_mutex);
-	i = 0;
-	list_for_each_safe(pos, q, &dev_priv->mm.fence_list) {
-		reg = list_entry(pos, struct drm_i915_fence_reg, link);
-		if (reg->pin_count || reg->vma)
-			continue;
-		list_del(pos);
+
+	for (i = 0; i < vgpu_fence_sz(vgpu); i++) {
+		reg = i915_reserve_fence(dev_priv);
+		if (IS_ERR(reg))
+			goto out_free_fence;
+
 		vgpu->fence.regs[i] = reg;
-		if (++i == vgpu_fence_sz(vgpu))
-			break;
 	}
-	if (i != vgpu_fence_sz(vgpu))
-		goto out_free_fence;
 
 	_clear_vgpu_fence(vgpu);
 
@@ -212,13 +207,14 @@ static int alloc_vgpu_fence(struct intel_vgpu *vgpu)
 	intel_runtime_pm_put(dev_priv);
 	return 0;
 out_free_fence:
+	gvt_vgpu_err("Failed to alloc fences\n");
 	/* Return fences to host, if fail */
 	for (i = 0; i < vgpu_fence_sz(vgpu); i++) {
 		reg = vgpu->fence.regs[i];
 		if (!reg)
 			continue;
-		list_add_tail(&reg->link,
-			      &dev_priv->mm.fence_list);
+		i915_unreserve_fence(reg);
+		vgpu->fence.regs[i] = NULL;
 	}
 	mutex_unlock(&dev_priv->drm.struct_mutex);
 	intel_runtime_pm_put(dev_priv);
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 7587ef5..763a720 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -3650,6 +3650,9 @@ i915_vm_to_ppgtt(struct i915_address_space *vm)
 /* i915_gem_fence_reg.c */
 int __must_check i915_vma_get_fence(struct i915_vma *vma);
 int __must_check i915_vma_put_fence(struct i915_vma *vma);
+struct drm_i915_fence_reg *
+i915_reserve_fence(struct drm_i915_private *dev_priv);
+void i915_unreserve_fence(struct drm_i915_fence_reg *fence);
 
 void i915_gem_revoke_fences(struct drm_i915_private *dev_priv);
 void i915_gem_restore_fences(struct drm_i915_private *dev_priv);
diff --git a/drivers/gpu/drm/i915/i915_gem_fence_reg.c b/drivers/gpu/drm/i915/i915_gem_fence_reg.c
index 5fe2cd8..429ce5f 100644
--- a/drivers/gpu/drm/i915/i915_gem_fence_reg.c
+++ b/drivers/gpu/drm/i915/i915_gem_fence_reg.c
@@ -360,6 +360,50 @@ i915_vma_get_fence(struct i915_vma *vma)
 }
 
 /**
+ * i915_reserve_fence - Reserve a fence for vGPU
+ * @dev_priv: i915 device private
+ *
+ * This function walks the fence regs looking for a free one and remove
+ * it from the fence_list. It is used to reserve fence for vGPU to use.
+ */
+struct drm_i915_fence_reg *
+i915_reserve_fence(struct drm_i915_private *dev_priv)
+{
+	struct drm_i915_fence_reg *fence;
+	int ret;
+
+	/* Host at least need one fence available for display engine. */
+	if (unlikely(dev_priv->mm.fence_list.prev ==
+		     dev_priv->mm.fence_list.next))
+		return ERR_PTR(-ENOSPC);
+
+	fence = fence_find(dev_priv);
+	if (IS_ERR(fence))
+		return fence;
+
+	if (fence->vma) {
+		/* Force-remove fence from VMA */
+		ret = fence_update(fence, NULL);
+		if (ret)
+			return ERR_PTR(ret);
+	}
+
+	list_del(&fence->link);
+	return fence;
+}
+
+/**
+ * i915_unreserve_fence - Reclaim a reserved fence
+ * @fence: the fence reg
+ *
+ * This function add a reserved fence register from vGPU to the fence_list.
+ */
+void i915_unreserve_fence(struct drm_i915_fence_reg *fence)
+{
+	list_add_tail(&fence->link, &fence->i915->mm.fence_list);
+}
+
+/**
  * i915_gem_revoke_fences - revoke fence state
  * @dev_priv: i915 device private
  *
-- 
2.7.4

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