drm/i915: Add interface to reserve fence registers for vGPU

Commit Message

Du, Changbin Aug. 30, 2017, 8:54 a.m. UTC

From: Changbin Du <[e-mail]>

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_one_fence() will try to find a free fence from fence_list
   and force-remove vma if need.
 - i915_giveback_reserved_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.

Signed-off-by: Changbin Du <[e-mail]>
---
 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 | 39 +++++++++++++++++++++++++++++++
 3 files changed, 53 insertions(+), 15 deletions(-)

Comments

Chris Wilson Aug. 30, 2017, 9:27 a.m. UTC | #1

Quoting [e-mail] (2017-08-30 09:54:21)
> This patch added two new api to the fence management code:
>  - i915_reserve_one_fence() will try to find a free fence from fence_list
>    and force-remove vma if need.
>  - i915_giveback_reserved_fence() reclaim a reserved fence after vGPU has
>    finished.

Symmetry: reserve_fence, unreserve_fence.

We need a safeguard here so that the host is able to always able allocate
to allocate a fence for the display engine. (That requirement should be
quite soft for modern hw, nevertheless it should be in the design to
prevent overuse from leading to an unusable system.)
-Chris

Du, Changbin Aug. 31, 2017, 3:21 a.m. UTC | #2

Hi chris,

On Wed, Aug 30, 2017 at 10:27:18AM +0100, Chris Wilson wrote:
> Quoting [e-mail] (2017-08-30 09:54:21)
> > This patch added two new api to the fence management code:
> >  - i915_reserve_one_fence() will try to find a free fence from fence_list
> >    and force-remove vma if need.
> >  - i915_giveback_reserved_fence() reclaim a reserved fence after vGPU has
> >    finished.
> 
> Symmetry: reserve_fence, unreserve_fence.
>
ok, I will rename the functions.

> We need a safeguard here so that the host is able to always able allocate
> to allocate a fence for the display engine. (That requirement should be
> quite soft for modern hw, nevertheless it should be in the design to
> prevent overuse from leading to an unusable system.)
> -Chris
Yes, agree. Is there any other components always need a fence ready? otherwise,
I will add a safeguard to ensure at least 1 fence register remained for host.

Patch

diff --git a/drivers/gpu/drm/i915/gvt/aperture_gm.c b/drivers/gpu/drm/i915/gvt/aperture_gm.c
index ca3d192..29f1bf7 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_giveback_reserved_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_one_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_giveback_reserved_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..c505b44 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_one_fence(struct drm_i915_private *dev_priv);
+void i915_giveback_reserved_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..63e4e94 100644
--- a/drivers/gpu/drm/i915/i915_gem_fence_reg.c
+++ b/drivers/gpu/drm/i915/i915_gem_fence_reg.c
@@ -360,6 +360,45 @@  i915_vma_get_fence(struct i915_vma *vma)
 }
 
 /**
+ * i915_reserve_one_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_one_fence(struct drm_i915_private *dev_priv)
+{
+	struct drm_i915_fence_reg *fence;
+	int ret;
+
+	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_giveback_reserved_fence - Reclaim a reserved fence
+ * @fence: the fence reg
+ *
+ * This function add a reserved fence register from vGPU to the fence_list.
+ */
+void i915_giveback_reserved_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
  *