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

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.

v3: (Chris)
  - Add struct_mutex lock assertion.
  - Only count for unpinned fence.

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>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/1504512061-5892-1-git-send-email-changbin.du@intel.com
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

1 files changed, 51 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/i915_gem_fence_reg.c b/drivers/gpu/drm/i915/i915_gem_fence_reg.c
index 5fe2cd8c8f28..2783d63bd1ad 100644
--- a/drivers/gpu/drm/i915/i915_gem_fence_reg.c
+++ b/drivers/gpu/drm/i915/i915_gem_fence_reg.c
@@ -360,6 +360,57 @@ 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 count;
+	int ret;
+
+	lockdep_assert_held(&dev_priv->drm.struct_mutex);
+
+	/* Keep at least one fence available for the display engine. */
+	count = 0;
+	list_for_each_entry(fence, &dev_priv->mm.fence_list, link)
+		count += !fence->pin_count;
+	if (count <= 1)
+		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)
+{
+	lockdep_assert_held(&fence->i915->drm.struct_mutex);
+
+	list_add(&fence->link, &fence->i915->mm.fence_list);
+}
+
+/**
  * i915_gem_revoke_fences - revoke fence state
  * @dev_priv: i915 device private
  *