[PATCH v2 1/3] drm/vram: Add kmap ref-counting to GEM VRAM objects

[Thomas Zimmermann]

The kmap and kunmap operations of GEM VRAM buffers can now be called
in interleaving pairs. The first call to drm_gem_vram_kmap() maps the
buffer's memory to kernel address space and the final call to
drm_gem_vram_kunmap() unmaps the memory. Intermediate calls to these
functions increment or decrement a reference counter.

This change allows for keeping buffer memory mapped for longer and
minimizes the amount of changes to TLB, page tables, etc.

Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
Cc: Davidlohr Bueso <dave at stgolabs.net>
---
 drivers/gpu/drm/drm_gem_vram_helper.c | 74 ++++++++++++++++++++-------
 include/drm/drm_gem_vram_helper.h     | 19 +++++++
 2 files changed, 75 insertions(+), 18 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_vram_helper.c b/drivers/gpu/drm/drm_gem_vram_helper.c
index fd751078bae1..6c7912092876 100644
--- a/drivers/gpu/drm/drm_gem_vram_helper.c
+++ b/drivers/gpu/drm/drm_gem_vram_helper.c
@@ -26,7 +26,11 @@ static void drm_gem_vram_cleanup(struct drm_gem_vram_object *gbo)
 	 * TTM buffer object in 'bo' has already been cleaned
 	 * up; only release the GEM object.
 	 */
+
+	WARN_ON(gbo->kmap_use_count);
+
 	drm_gem_object_release(&gbo->bo.base);
+	mutex_destroy(&gbo->kmap_lock);
 }
 
 static void drm_gem_vram_destroy(struct drm_gem_vram_object *gbo)
@@ -100,6 +104,8 @@ static int drm_gem_vram_init(struct drm_device *dev,
 	if (ret)
 		goto err_drm_gem_object_release;
 
+	mutex_init(&gbo->kmap_lock);
+
 	return 0;
 
 err_drm_gem_object_release:
@@ -283,6 +289,34 @@ int drm_gem_vram_unpin(struct drm_gem_vram_object *gbo)
 }
 EXPORT_SYMBOL(drm_gem_vram_unpin);
 
+static void *drm_gem_vram_kmap_locked(struct drm_gem_vram_object *gbo,
+				      bool map, bool *is_iomem)
+{
+	int ret;
+	struct ttm_bo_kmap_obj *kmap = &gbo->kmap;
+
+	if (gbo->kmap_use_count > 0)
+		goto out;
+
+	if (kmap->virtual || !map)
+		goto out;
+
+	ret = ttm_bo_kmap(&gbo->bo, 0, gbo->bo.num_pages, kmap);
+	if (ret)
+		return ERR_PTR(ret);
+
+out:
+	if (!kmap->virtual) {
+		if (is_iomem)
+			*is_iomem = false;
+		return NULL; /* not mapped; don't increment ref */
+	}
+	++gbo->kmap_use_count;
+	if (is_iomem)
+		return ttm_kmap_obj_virtual(kmap, is_iomem);
+	return kmap->virtual;
+}
+
 /**
  * drm_gem_vram_kmap() - Maps a GEM VRAM object into kernel address space
  * @gbo:	the GEM VRAM object
@@ -304,40 +338,44 @@ void *drm_gem_vram_kmap(struct drm_gem_vram_object *gbo, bool map,
 			bool *is_iomem)
 {
 	int ret;
-	struct ttm_bo_kmap_obj *kmap = &gbo->kmap;
-
-	if (kmap->virtual || !map)
-		goto out;
+	void *virtual;
 
-	ret = ttm_bo_kmap(&gbo->bo, 0, gbo->bo.num_pages, kmap);
+	ret = mutex_lock_interruptible(&gbo->kmap_lock);
 	if (ret)
 		return ERR_PTR(ret);
+	virtual = drm_gem_vram_kmap_locked(gbo, map, is_iomem);
+	mutex_unlock(&gbo->kmap_lock);
 
-out:
-	if (!is_iomem)
-		return kmap->virtual;
-	if (!kmap->virtual) {
-		*is_iomem = false;
-		return NULL;
-	}
-	return ttm_kmap_obj_virtual(kmap, is_iomem);
+	return virtual;
 }
 EXPORT_SYMBOL(drm_gem_vram_kmap);
 
-/**
- * drm_gem_vram_kunmap() - Unmaps a GEM VRAM object
- * @gbo:	the GEM VRAM object
- */
-void drm_gem_vram_kunmap(struct drm_gem_vram_object *gbo)
+static void drm_gem_vram_kunmap_locked(struct drm_gem_vram_object *gbo)
 {
 	struct ttm_bo_kmap_obj *kmap = &gbo->kmap;
 
+	if (WARN_ON_ONCE(!gbo->kmap_use_count))
+		return;
+	if (--gbo->kmap_use_count > 0)
+		return;
+
 	if (!kmap->virtual)
 		return;
 
 	ttm_bo_kunmap(kmap);
 	kmap->virtual = NULL;
 }
+
+/**
+ * drm_gem_vram_kunmap() - Unmaps a GEM VRAM object
+ * @gbo:	the GEM VRAM object
+ */
+void drm_gem_vram_kunmap(struct drm_gem_vram_object *gbo)
+{
+	mutex_lock(&gbo->kmap_lock);
+	drm_gem_vram_kunmap_locked(gbo);
+	mutex_unlock(&gbo->kmap_lock);
+}
 EXPORT_SYMBOL(drm_gem_vram_kunmap);
 
 /**
diff --git a/include/drm/drm_gem_vram_helper.h b/include/drm/drm_gem_vram_helper.h
index ac217d768456..8c08bc87b788 100644
--- a/include/drm/drm_gem_vram_helper.h
+++ b/include/drm/drm_gem_vram_helper.h
@@ -34,11 +34,30 @@ struct vm_area_struct;
  * backed by VRAM. It can be used for simple framebuffer devices with
  * dedicated memory. The buffer object can be evicted to system memory if
  * video memory becomes scarce.
+ *
+ * GEM VRAM objects perform reference counting for pin and mapping
+ * operations. So a buffer object that has been pinned N times with
+ * drm_gem_vram_pin() must be unpinned N times with
+ * drm_gem_vram_unpin(). The same applies to pairs of
+ * drm_gem_vram_kmap() and drm_gem_vram_kunmap().
  */
 struct drm_gem_vram_object {
 	struct ttm_buffer_object bo;
 	struct ttm_bo_kmap_obj kmap;
 
+	/**
+	 * @kmap_lock: Protects the kmap address and use count
+	 */
+	struct mutex kmap_lock;
+
+	/**
+	 * @kmap_use_count:
+	 *
+	 * Reference count on the virtual address.
+	 * The address are un-mapped when the count reaches zero.
+	 */
+	unsigned int kmap_use_count;
+
 	/* Supported placements are %TTM_PL_VRAM and %TTM_PL_SYSTEM */
 	struct ttm_placement placement;
 	struct ttm_place placements[2];
-- 
2.23.0